Method and device for estimating internal state of battery

Abstract

A method executable by a processor 10 and for estimating the internal state of a lithium ion secondary battery, wherein the processor 10 measures a charge characteristic of the lithium ion secondary battery and a discharge characteristic of the lithium ion secondary battery, and estimates the deterioration state of the positive electrode of the lithium ion secondary battery on the basis of the difference in behavior between the charge characteristic and the discharge characteristic.

Description

Method and apparatus for estimating the internal state of a battery

[0001] This invention relates to a method and apparatus for estimating the internal state of a battery.

[0002] Conventional battery state estimation methods for estimating the degree of degradation of secondary batteries have been known. For example, the estimation method described in Patent Document 1 calculates the estimated integrated current value ΔAh1' that flowed through the secondary battery 101 from the time OCV1 was measured (start of integration) to the time OCV2 was measured (end of integration) in a degraded secondary battery 101, the estimated integrated current value ΔAh2' that flowed through the secondary battery 101 from the time OCV2 was measured (start of integration) to the time OCV3 was measured (end of integration) and the estimated integrated current value ΔAh3' that flowed through the secondary battery 101 from the time OCV3 was measured (start of integration) to the time OCV4 was measured (end of integration), and calculates the positive electrode capacity degradation coefficient α, OCP deviation amount β, and capacity deviation amount δ based on the error between the measured integrated current value ΔAhi and the estimated integrated current value ΔAhi' (where the subscript i is a natural number from 1 to 3).

[0003] Japanese Patent Publication No. 2021-44068

[0004] As with the battery state estimation method described in Patent Document 1 above, calculating multiple estimated current integral values ​​(ΔAh1', ΔAh2', ΔAh3') has the problem that it takes time to acquire the battery characteristics of the secondary battery and it takes a long time to estimate the degradation state of the positive electrode.

[0005] The problem that this invention aims to solve is to provide an internal state estimation method and an internal state estimation device that can estimate the degradation state of a positive electrode in a short amount of time.

[0006] This invention solves the above problem by measuring the charging characteristics and discharge characteristics of a lithium-ion secondary battery, and estimating the degradation state of the positive electrode of the lithium-ion secondary battery based on the difference in the behavior of the charging characteristics and discharge characteristics.

[0007] According to the present invention, the degradation state of the positive electrode can be estimated in a short time.

[0008] Figure 1 is a block diagram showing the internal state estimation system according to the first embodiment. Figure 2 is a conceptual diagram showing the movement of lithium ions before and after positive electrode degradation. Figure 3A is a graph showing the characteristics of charge / discharge current and voltage when a secondary battery is charged and discharged in a predetermined charge / discharge sequence. Figure 3B is a graph showing the characteristics of DC resistance on the discharge side and DC resistance on the charge side. Figure 4 is a flowchart showing the procedure of the internal state estimation method by the internal state estimation system according to the first embodiment. Figure 5 is a graph showing the correlation between the DC resistance ratio (Rd / Rc) and the positive electrode degradation state. Figure 6 is a flowchart showing the procedure of the internal state estimation method by the internal state estimation system according to a modified example of this embodiment. Figure 7 is a graph showing the correlation between the DC resistance ratio (Rd / Rc) and the positive electrode degradation state. Figure 8 is a graph showing the characteristics of the voltage change range on the discharge side (ΔVd) and the voltage change range on the charge side (ΔVc). Figure 9 is a graph showing the correlation between the voltage change ratio (ΔVd / ΔVc) and the positive electrode degradation state. Figure 10 is a graph showing the correlation between the voltage change ratio (ΔVd / ΔVc) and the positive electrode degradation state. Figure 11 is a graph showing the characteristics of the imaginary component value of AC impedance with respect to frequency. Figure 12 is a graph showing the ratio of the imaginary component to SOC (Z"d / Z"c). Figure 13 is a graph showing the difference of the imaginary component to SOC (Z"d-Z"c). Figure 14 is a flowchart showing the procedure of the internal state estimation method according to the third embodiment. Figure 15 is a graph showing the correlation between the ratio of the imaginary component of AC impedance (Z"d / Z"c) and the positive electrode degradation state. Figure 16 is a graph showing the correlation between the difference of the imaginary component of AC impedance (Z"d-Z"c) and the positive electrode degradation state. Figure 17 is a graph showing the correlation between the ratio of the real component of AC impedance (Z'd / Z'c) and the positive electrode degradation state. Figure 18 is a graph showing the correlation between the difference in the real components of the AC impedance (Z'd - Z'c) and the positive electrode degradation state. Figure 19 is a graph of the AC current-voltage response characteristics. Figure 20 is a graph showing the correlation between the positive electrode degradation state and the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p). Figure 21 is a flowchart showing the procedure of the internal state estimation method according to the fourth embodiment. Figure 22 is a flowchart showing the procedure of the internal state estimation method according to modification 6 of the fourth embodiment.Figure 23 is a graph showing the correlation between the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) and the positive electrode degradation state.

[0009] 《First Embodiment》 The internal state estimation system for a lithium-ion secondary battery according to this embodiment will be described based on the drawings. Figure 1 is a block diagram showing the internal state estimation system according to this embodiment. The internal state estimation system is a system for estimating the degradation state of the positive electrode of a secondary battery 1. For example, when recycling the positive electrode active material contained in the positive electrode, it is desirable to understand the degradation state of the positive electrode. The internal state estimation system according to this embodiment may be used, as an example, as a system for estimating the degradation state of the positive electrode in order to carry out positive electrode recycling with high efficiency.

[0010] As shown in Figure 1, the internal state estimation system comprises a secondary battery 1, a DC-DC converter 2, a voltage sensor 3, a current sensor 4, and a controller 5. The secondary battery 1 includes a group of batteries connected together. The secondary battery 1 is a lithium-ion secondary battery. An example of this type of secondary battery 1 is one in which an active material having multiple charge-discharge regions in which the charge-discharge potential changes in steps with the insertion and removal of lithium ions is used as the negative electrode active material. A graphite-based active material containing a graphite structure is suitable as such an active material having multiple charge-discharge regions in which the charge-discharge potential changes in steps with the insertion and removal of lithium ions. The positive electrode active material is not particularly limited, and known positive electrode active materials for lithium-ion secondary batteries, such as lithium-transition metal composite oxides having a layered rock salt type structure, can be used. In addition to the positive and negative electrodes, the secondary battery 1 also has an electrolyte, a separator, and a tab.

[0011] The DC-DC converter 2 is a power conversion device that converts the voltage input from the secondary battery 1 into a predetermined voltage and outputs power to a load such as a motor. The DC-DC converter 2 also converts the voltage input from a load such as a motor or a charging device into a predetermined voltage and outputs power to the secondary battery 1. This DC-DC converter 2 is controlled by a controller 5. The secondary battery 1 is connected to the input side of the DC-DC converter 2, and the load and charging device are electrically connected to the output side of the DC-DC converter 2.

[0012] The voltage sensor 3 is a sensor for detecting the voltage between the terminals of the secondary battery 1. The voltage sensor 3 is connected between the wiring connected to the positive and negative terminals of the secondary battery 1. The current sensor 4 is a sensor for detecting the input and output current of the secondary battery 1. The current sensor 4 is connected to the wiring connected to either the positive or negative terminal of the secondary battery 1. The voltage sensor 3 and the current sensor 4 detect the state of the battery and output the detected values ​​to the controller 5.

[0013] The controller 5 is a battery control unit (BCU). The controller 5 controls the charging and discharging of the secondary battery 1 and estimates the battery resistance of the secondary battery 1 based on the detected voltage detected by the voltage sensor 3 and / or the detected current detected by the current sensor 4. The controller 5 is composed of a processor 10 such as a CPU and a memory 20 such as ROM or RAM. The processor 10 is a processing unit that processes each step included in the internal state estimation method by executing a program stored in the memory 20. The processor 10 has a charge / discharge control unit 11 and a degradation state estimation unit 12 as functional blocks for estimating the internal state of the secondary battery 1. The processor 10 may also include functional blocks other than the charge / discharge control unit 11 and the degradation state estimation unit 12.

[0014] The charge / discharge control unit 11 controls the charging and discharging of the secondary battery 1. In the present embodiment, as will be described later, the charging and / or discharging of the secondary battery 1 is performed to estimate the deterioration state of the positive electrode of the secondary battery 1. A charge / discharge current for estimating the internal state of the positive electrode is set, and the charge / discharge control unit 11 outputs a control command to the DC / DC converter 2 so that the charge / discharge current of the secondary battery 1 becomes the set current value. In the following description, the charge / discharge current is a general term for the charging current and the discharging current and is at least one of the charging current and the discharging current.

[0015] The deterioration state estimation unit 12 estimates the internal state of the positive electrode of the secondary battery 1. When estimating the internal state of the positive electrode, the deterioration state estimation unit 12 outputs a control command to the charge / discharge control unit 11 so that the secondary battery 1 is charged and discharged in a charge / discharge sequence. The deterioration state estimation unit 12 measures the charging characteristics and the discharging characteristics of the secondary battery 1 during the charging and discharging of the secondary battery 1 and / or during the rest period after the charging and discharging. The deterioration state estimation unit 12 estimates the deterioration state of the positive electrode of the secondary battery 1 based on the difference in the behavior of the measured charging characteristics and discharging characteristics.

[0016] The memory 20 stores programs processed by the processor 10, data indicating the battery characteristics of the secondary battery 1, and the like.

[0017] Incidentally, according to the inventor's findings, the difference in the behavior of the charging characteristics and the discharging characteristics when the positive electrode deteriorates can be explained as follows. FIG. 2 is a conceptual diagram showing the movement of lithium ions before and after the deterioration of the positive electrode. (a) is a conceptual diagram of the movement of lithium ions before deterioration, and (b) is a conceptual diagram of the movement of lithium ions after deterioration. In the example of FIG. 2, lithium nickel manganese cobalt composite oxide (NMC) is used as the positive electrode as an example of the positive electrode active material. The phenomenon at the time of deterioration of the positive electrode described below is not limited to NMC and appears similarly at the time of deterioration of a positive electrode containing other positive electrode active materials.

[0018] First, when the positive electrode active material deteriorates, the crystal structure changes on a portion of the NMC particle surface, forming a so-called insulating layer (a region with extremely high electrical resistance). As shown in Figure 2(a), before deterioration, no insulating layer is formed on the surface of the NMC particles. Therefore, during charging and discharging, lithium ions can move smoothly between the electrolyte and the active material particles without being obstructed by the surface of the NMC particles. On the other hand, as shown in Figure 2(b), when the positive electrode deteriorates, an insulating layer (M) is formed on the surface of the NMC particles. During discharge, lithium ions in the electrolyte move into the active material particles, avoiding the insulating layer (M), and the average distance traveled by lithium ions in the electrolyte increases. During charging, lithium ions in the active material move into the electrolyte, bypassing the insulating layer, and the average distance traveled by lithium ions in the active material increases. When the average distance traveled by lithium ions differs between charging and discharging, the increase in lithium ion transport resistance differs between the electrolyte and the positive electrode active material, which manifests as a difference in the behavior of charging and discharging characteristics during deterioration. Therefore, in this embodiment, based on the positive electrode degradation state estimation mechanism described above, the degradation state of the positive electrode of the secondary battery 1 is estimated by identifying the amount of increase in lithium ion transport resistance from the difference in the behavior of the charging characteristics and the discharging characteristics.

[0019] The controller 5 identifies the difference in the behavior of the secondary battery 1's charging and discharging characteristics from the difference between the DC resistance value on the discharge side (Rd) and the DC resistance value on the charging side (Rc), and estimates the degradation state of the positive electrode based on the difference between the DC resistance value on the discharge side (Rd) and the DC resistance value on the charging side (Rc). As will be described later, the controller 5 identifies the difference between the DC resistance value on the discharge side (Rd) and the DC resistance value on the charging side (Rc) by calculating the DC resistance ratio (Rd / Rc) between the DC resistance value on the discharge side (Rd) and the DC resistance value on the charging side (Rc).

[0020] Next, we will explain how the degradation state of the positive electrode is reflected in the difference between the DC resistance on the discharge side and the DC resistance on the charge side. Figure 3A is a graph showing the characteristics of the charge / discharge current and the voltage characteristics of the secondary battery 1 when the secondary battery 1 is charged and discharged in a predetermined charge / discharge sequence. (a) shows the characteristics before positive electrode degradation, and (b) shows the characteristics after positive electrode degradation. Graph a １ and a ２shows the characteristics of the charge and discharge current, graph b １ and b ２ shows the voltage characteristics. Note that the current on the vertical axis is the absolute value. The charge and discharge sequence is a sequence that repeats discharge and charge the same number of times with the same current value and charge and discharge time. In the example of FIG. 3A, discharge and charge are performed once each.

[0021] When the discharge of the secondary battery 1 starts, a discharge current flows as shown in graph a １ and a ２ As shown in. When the secondary battery 1 discharges for a certain period of time (corresponding to the time of the first rectangular wave in the characteristics of graph a １ and a ２ ), the voltage of the secondary battery 1 becomes lower due to the discharge. The voltage change due to discharge is ΔVd shown in FIG. 3A １ , ΔVd ２ . After the discharge stops and a rest time is provided, when the charging of the secondary battery 1 starts, a charging current flows as shown in graph a １ and a ２ . In the example of FIG. 3A, the charging current has the same current value as the discharge current, but it may have a different current value. When the secondary battery 1 charges for a certain period of time (corresponding to the time of the second rectangular wave in the characteristics of graph a １ and a ２ ), the voltage of the secondary battery 1 becomes higher due to the charging. The voltage change due to charging is ΔVc shown in FIG. 3A １ , ΔVc ２ . Both the voltage change due to discharge (ΔVd) and the voltage change due to charging (ΔVc) are larger after the positive electrode deterioration than before the positive electrode deterioration. However, the change in ΔVd and ΔVc before and after the positive electrode deterioration is different, and the rate of increase of ΔVd is larger than the rate of increase of ΔVc. Therefore, it appears as the difference between the discharge-side DC resistance value ΔVd / Id and the charge-side DC resistance value ΔVc / Ic.

[0022] FIG. 3B is a graph showing the characteristics of the discharge-side DC resistance and the charge-side DC resistance when the secondary battery 1 is charged and discharged with a predetermined charge and discharge sequence. (a) shows the characteristics before the positive electrode deterioration, and (b) shows the characteristics after the positive electrode deterioration. Graph c １ and c ２ show the characteristics of the discharge-side DC resistance, and graph d １and d ２ This shows the characteristics of the DC current on the charging side.

[0023] As shown in Figure 3B(a), the DC resistance on the discharge side and the DC resistance on the charge side remain approximately the same in magnitude with respect to the charge-discharge time before positive electrode degradation. Before positive electrode degradation, the DC resistance value on the discharge side and the DC resistance value on the charge side are approximately equal. On the other hand, as shown in Figure 3B(b), after positive electrode degradation, the DC resistance on the discharge side and the DC resistance on the charge side change with different characteristics with respect to the charge-discharge time, and the DC resistance value on the discharge side becomes larger than the DC resistance value on the charge side. In other words, the degradation state of the positive electrode can be estimated based on the difference between the DC resistance on the discharge side and the DC resistance on the charge side (hereinafter also referred to as the "difference in charge-discharge DC resistance").

[0024] Next, the method for estimating the degradation state of the positive electrode will be explained with reference to Figure 4. Figure 4 is a flowchart showing the procedure for the internal state estimation method according to this embodiment. Each control flow in the flowchart shown in Figure 4 is executed by the processor 10. In the explanation of each control flow, the specific time is just an example and can be any time.

[0025] In step S1, after connecting the secondary battery 1 to the load and charging device via the DC-DC converter 2, the secondary battery 1 is charged for a predetermined time (t ０ ), put into a paused state. For a predetermined time (t ０ ) is a pre-set time, for example, a time of 1 minute or more is set. The charge / discharge control unit 11 uses the voltage sensor 3 to detect the voltage of the secondary battery 1 during the idle period.

[0026] In step S2, the charge / discharge control unit 11 discharges the battery at a discharge current (Id) and measures the discharge characteristics. The discharge current (Id) is a constant current with a predetermined C rate. The charge / discharge control unit 11 measures the discharge characteristics by detecting the voltage of the secondary battery 1 during discharge. The discharge time is set to 5 seconds or more.

[0027] In step S3, the degradation state estimation unit 12 calculates the DC resistance value (Rd) from the voltage change amount and discharge current (Id) at a certain time after the start of discharge of the secondary battery 1, based on the discharge characteristics. Specifically, the degradation state estimation unit 12 calculates the voltage change amount ΔVd from the difference between voltage [Vd(-1)] and voltage [Vd(5)]. Vd(-1) is the voltage detected 1 second before the start of discharge, as detected in the control flow of step S1, and Vd(5) is the voltage detected 5 seconds after the start of discharge in step S2. In other words, the degradation state estimation unit 12 sets the period from 1 second before the start of discharge to 5 seconds after the start of discharge as a certain time, and calculates the voltage change amount as the voltage change amount during that certain time. The degradation state estimation unit 12 calculates the DC resistance value (Rd) by dividing the discharge current by the voltage change amount [Rd = ΔVd / Id]. In the control flow of step S3, the voltage [Vd(-1)] corresponds to the voltage immediately before the start of discharge. The voltage change during the elapsed time (5 seconds) from the start of discharge corresponds to the voltage change at a certain time after the start of discharge of the secondary battery 1.

[0028] In step S4, the charge / discharge control unit 11 stops discharging and discharges the secondary battery 1 for a predetermined time (t ０ ), and put into a pause state. The charge / discharge control unit 11 uses the voltage sensor 3 to detect the voltage of the secondary battery 1 during the pause period. Note that the timing for stopping the discharge may be before step S3.

[0029] In step S5, the charge / discharge control unit 11 charges the battery with a charging current (Ic) and measures the charging characteristics. The charging current (Ic) is a constant current with a predetermined C rate. The charge / discharge control unit 11 measures the charging characteristics by detecting the voltage of the secondary battery 1 during charging. The charging time is set to 5 seconds or more.

[0030] In step S6, the degradation state estimation unit 12 calculates the DC resistance value (Rc) from the voltage change amount and charging current (Ic) at a certain time after the start of charging of the secondary battery 1, based on the charging characteristics. Specifically, the degradation state estimation unit 12 calculates the voltage change amount ΔVc from the difference between voltage [Vc(-1)] and voltage [Vc(5)]. Vc(-1) is the voltage detected 1 second before the start of charging, as detected in the control flow of step S4. Vc(5) is the voltage detected 5 seconds after the start of charging in step S5. In other words, the degradation state estimation unit 12 sets the period from 1 second before the start of charging to 5 seconds after the start of charging as a certain time, and calculates the voltage change amount as the voltage change amount during that certain time. The degradation state estimation unit 12 calculates the DC resistance value (Rc) by dividing the voltage change amount by the charging current [Rc = ΔVc / Ic]. In the control flow of step S6, the voltage [Vc(-1)] corresponds to the voltage immediately before the start of charging. The change in voltage during the elapsed time (5 seconds) from the start of charging corresponds to the change in voltage at a certain time after the start of charging of the secondary battery 1.

[0031] In step S7, the degradation state estimation unit 12 estimates the degradation state of the positive electrode based on the difference between the DC resistance value (Rd) and the DC resistance value (Rc). Specifically, the degradation state estimation unit 12 calculates the DC resistance ratio (Rd / Rc) as a parameter indicating the difference between the DC resistance value (Rd) and the DC resistance value (Rc). The degradation state estimation unit 12 then compares the calculated DC resistance ratio (Rd / Rc) with a predetermined DC resistance ratio threshold, and determines that the positive electrode has degraded if the DC resistance ratio (Rd / Rc) is greater than or equal to the DC resistance ratio threshold. On the other hand, if the DC resistance ratio (Rd / Rc) is less than the DC resistance ratio threshold, the degradation state estimation unit 12 determines that the positive electrode has not degraded. In the embodiment shown in Figure 4, the charging characteristics are measured after the discharge characteristics, but the degradation state of the positive electrode can be similarly estimated even if the discharge characteristics are measured after the charging characteristics.

[0032] Figure 5 is a graph showing the correlation between the difference in charge / discharge DC resistance and the positive electrode degradation state (degree of positive electrode degradation) in secondary battery 1. The characteristics shown in Figure 5 can be obtained from a prototype of secondary battery 1, which will be described in the embodiment described later. The positive electrode degradation state represents the rate of capacity reduction of the positive electrode (degree of positive electrode degradation). The difference in charge / discharge DC resistance is shown by the DC resistance ratio (Rd / Rc), which is the ratio of the DC resistance value on the discharge side (Rd) to the DC resistance value on the charge side (Rc). As shown in Figure 5, when the positive electrode degradation state is less than about 10%, the DC resistance ratio (Rd / Rc) is close to 1, and the DC resistance on the discharge side and the DC resistance on the charge side have almost the same resistance value. On the other hand, when the positive electrode degradation state is greater than 10%, the difference between the DC resistance on the discharge side and the DC resistance on the charge side becomes larger, and the DC resistance ratio (Rd / Rc) becomes greater than 1. For example, when the degradation of the positive electrode exceeds 14%, the DC resistance ratio (Rd / Rc) will exceed 1.04.

[0033] As shown in Figure 5, the greater the degradation of the positive electrode, the greater the DC resistance ratio (Rd / Rc). In the example in Figure 5, the DC resistance ratio threshold is set to 1.04. When the DC resistance ratio (Rd / Rc) is greater than or equal to the DC resistance ratio threshold (1.04), the degradation state estimation unit 12 determines that the positive electrode has degraded. When the DC resistance ratio (Rd / Rc) is less than the DC resistance ratio threshold (1.04), the degradation state estimation unit 12 determines that the positive electrode has not degraded. Thus, the degradation state estimation unit 12 estimates the degradation state of the positive electrode based on the difference between the DC resistance value (Rd) and the DC resistance value (Rc).

[0034] In the control flow of step S7, the degradation state estimation unit 12 may also estimate the degradation state of the positive electrode using battery characteristic data pre-stored in the memory 20. The battery characteristic data is data that represents the correlation between the DC resistance ratio (Rd / Rc) and the degradation state of the positive electrode. The correlation can be determined experimentally, for example, as shown in Figure 5. The degradation state estimation unit 12 acquires the battery characteristic data from the memory 20. Then, the degradation state estimation unit 12 calculates the DC resistance ratio (Rd / Rc), refers to the battery characteristic data, and estimates the degradation state of the positive electrode corresponding to the calculated DC resistance ratio (Rd / Rc).

[0035] For example, suppose secondary battery 1 has the battery characteristics shown in Figure 5, and the DC resistance ratio (Rd / Rc) calculated in the control flow of step S7 is 1.10. In this case, in the graph shown in Figure 5, the positive electrode degradation state corresponding to the DC resistance ratio (Rd / Rc = 1.10) is approximately 19%. As a result, the degradation state estimation unit 12 can quantitatively estimate the degradation state of the positive electrode based on the correlation between the DC resistance ratio (Rd / Rc) and the degradation state of the positive electrode, which are pre-stored in the memory 20.

[0036] As described above, the internal state estimation method and internal state estimation device according to this embodiment measure the charging characteristics and discharge characteristics of the secondary battery 1 and estimate the degradation state of the positive electrode of the secondary battery 1 based on the difference between the charging characteristics and discharge characteristics. When the positive electrode degrades, an insulating layer is formed on a part of the surface of the positive electrode active material particles. When the secondary battery 1 is charged and discharged in a degraded state, lithium ions move while avoiding the insulating layer as they move between the positive electrode active material and the electrolyte. Therefore, the transport resistance of lithium ions differs between the movement of lithium ions within the positive electrode active material and the movement of lithium ions within the electrolyte. It has been newly discovered that this difference in transport resistance causes a difference in the behavior of the charging characteristics and discharge characteristics. In other words, based on this knowledge of the inventors, the internal state estimation method and internal state estimation device according to this embodiment estimate the degradation state of the positive electrode of the secondary battery 1 by identifying the amount of increase in lithium ion transport resistance from the difference in the behavior of the charging characteristics and discharge characteristics. This makes it possible to estimate the degradation state of the positive electrode in a short time.

[0037] In this embodiment, the processor 10 calculates a DC resistance value (Rd) based on the discharge characteristics and a DC resistance value (Rc) based on the charging characteristics, and estimates the degradation state of the positive electrode based on the difference between the DC resistance values ​​(Rd) and (Rc). The relationship between the DC resistance values ​​(Rd) and (Rc) changes before and after the degradation of the positive electrode. In this embodiment, the change in the relationship between the DC resistance values ​​(Rd) and (Rc) is identified from the difference between the DC resistance values ​​(Rd) and (Rc), and then the degradation state of the positive electrode is estimated. This makes it possible to estimate the degradation state of the positive electrode in a short time (for example, from tens of seconds to a few minutes).

[0038] In this embodiment, the processor 10 determines that the positive electrode has deteriorated if the DC resistance ratio (Rd / Rc) is greater than or equal to a predetermined DC resistance ratio threshold. There is a correlation between the deterioration state of the positive electrode and the DC resistance ratio (Rd / Rc). In this embodiment, this correlation can be used to determine whether or not the positive electrode has deteriorated. This allows the deterioration state of the positive electrode to be estimated in a short amount of time.

[0039] In this embodiment, the processor 10 obtains battery characteristic data from the memory 20 that represents the correlation between the DC resistance ratio (Rd / Rc) and the degradation state of the positive electrode, calculates the DC resistance ratio (Rd / Rc), refers to the battery characteristic data, and estimates the degradation state of the positive electrode corresponding to the calculated DC resistance ratio (Rd / Rc). This allows for quantitative estimation of the degradation state of the positive electrode in a short time.

[0040] In this embodiment, the positive electrode includes a positive electrode active material having a layered rock salt structure. For example, a positive electrode active material having a layered rock salt structure changes its crystal structure during degradation, forming an insulating layer on its surface. Because the insulating layer alters the lithium ion transport path during charging and discharging, differences occur between the discharge characteristics and the charging characteristics. This allows for particularly accurate estimation of the degradation state of the positive electrode active material having a layered rock salt structure.

[0041] As a modification 1 of this embodiment, the secondary battery 1 may be charged and discharged using charge and discharge currents with different current values, and the degradation state of the positive electrode may be estimated based on multiple charge and discharge characteristics. Referring to Figure 6, the method for estimating the degradation state of the positive electrode in modification 1 will be explained. Figure 6 is a flowchart showing the procedure for the internal state estimation method according to modification 1 of this embodiment. Each control flow in the flowchart shown in Figure 6 is executed by the processor 10. In the explanation of each control flow, the specific time is just an example and can be any time.

[0042] In step S11, after connecting the secondary battery 1 to the load and charging device via the DC-DC converter 2, the secondary battery 1 is set for a predetermined time (t ０ ), put into a paused state. For a predetermined time (t ０) is a pre-set time, for example, a time of 1 minute or more is set. The charge / discharge control unit 11 uses the voltage sensor 3 to detect the voltage of the secondary battery 1 during the idle period.

[0043] In step S12, the charge / discharge control unit 11 discharges with a first current (Id_1) and measures the discharge characteristics, and charges with a first current (Ic_1) and measures the charge characteristics. In this embodiment, the first current during discharge and the first current during charging have the same current value (C rate), but they may be different current values. It is desirable that the discharge time and the charge time are the same. A pause period is set between the end of discharge and the start of charge. The charge / discharge control unit 11 detects the voltage of the secondary battery 1 during charging and discharging, and during the pause period. The degradation state estimation unit 12 calculates the DC resistance value (Rd_1) based on the discharge characteristics and the DC resistance value (Rc_1) based on the charge characteristics. The method for calculating the DC resistance (Rd_1, Rc_1) is the same as the method for calculating the DC resistance values ​​(Rd, Rc) in steps S3 and S6.

[0044] In step S13, the degradation state estimation unit 12 calculates the DC resistance ratio (Rd_1 / Rc_1) as a parameter indicating the difference between the DC resistance value (Rd_1) and the DC resistance value (Rc_1). The degradation state estimation unit 12 compares the calculated DC resistance ratio (Rd_1 / Rc_1) with the DC resistance ratio threshold (Rd / c_th1) and determines whether the DC resistance ratio (Rd_1 / Rc_1) is less than the DC resistance ratio threshold (Rd / c_th1). The DC resistance ratio threshold (Rd / c_th1) is a predetermined threshold according to the battery characteristics of the secondary battery 1. If the DC resistance ratio (Rd_1 / Rc_1) is greater than or equal to the DC resistance ratio threshold (Rd / c_th1), the control flow processed by the processor 10 proceeds to step S18.

[0045] If the DC resistance ratio (Rd_1 / Rc_1) is less than the DC resistance ratio threshold (Rd / c_th1), in step S14, the charge / discharge control unit 11 discharges with a second current (Id_2) and measures the discharge characteristics, and charges with a second current (Ic_2) and measures the charge characteristics. In this embodiment, the second current during discharge and the second current during charging have the same current value (C rate), but they may be different current values. Also, the second current value is greater than the first current value. It is desirable that the discharge time and the charge time are the same. A pause period is set between the end of discharge and the start of charge. The charge / discharge control unit 11 detects the voltage of the secondary battery 1 during charging and discharging, and during the pause period. The degradation state estimation unit 12 calculates the DC resistance value (Rd_2) based on the discharge characteristics and calculates the DC resistance value (Rc_2) based on the charge characteristics. The method for calculating DC resistance (Rd_2, Rc_2) is the same as the method for calculating DC resistance (Rd_1, Rc_1).

[0046] In step S15, the degradation state estimation unit 12 calculates the DC resistance ratio (Rd_2 / Rc_2) as a parameter indicating the difference between the DC resistance value (Rd_2) and the DC resistance value (Rc_2). The degradation state estimation unit 12 compares the calculated DC resistance ratio (Rd_2 / Rc_2) with the DC resistance ratio threshold (Rd / c_th2) and determines whether the DC resistance ratio (Rd_2 / Rc_2) is less than the DC resistance ratio threshold (Rd / c_th2). The DC resistance ratio threshold (Rd / c_th2) is a predetermined threshold according to the battery characteristics of the secondary battery 1. Note that the DC resistance ratio threshold (Rd / c_th2) may be the same value as the DC resistance ratio threshold (Rd / c_th1). If the DC resistance ratio (Rd_2 / Rc_2) is less than the DC resistance ratio threshold (Rd / c_th2), the control flow processed by the processor 10 proceeds to step S16. If the DC resistance ratio (Rd_2 / Rc_2) is greater than or equal to the DC resistance ratio threshold (Rd / c_th2), the control flow processed by the processor 10 proceeds to step S17.

[0047] In the control flow of steps S16 to S18, the degradation state estimation unit 12 estimates the degradation state of the positive electrode based on the DC resistance ratio (Rd_1 / Rc_1, Rd_2 / Rc_2).

[0048] Referring to Figure 7, the correlation between the DC resistance ratio (Rd_1 / Rc_1, Rd_2 / Rc_2) and the positive electrode degradation state (degree of positive electrode degradation) will be explained. Figure 7 is a graph showing the correlation between the DC resistance ratio and the positive electrode degradation state. Graph e in Figure 7 １ The graph shows the characteristics when secondary battery 1 is charged and discharged with a first current (Id_1, Ic_1), as shown in graph e. ２ This shows the characteristics of secondary battery 1 when it is charged and discharged with a second current (Id_2, Ic_2). The characteristics shown in Figure 7 can be obtained from a prototype of secondary battery 1, which will be described in the embodiment described later.

[0049] When secondary battery 1 is charged and discharged with a first current (Id_1, Ic_1), in the range where the positive electrode degradation state is greater than 14%, the correlation between the DC resistance ratio and the positive electrode degradation state is such that a larger DC resistance ratio indicates a greater positive electrode degradation state. On the other hand, in the range where the positive electrode degradation state is 14% or less, the relationship where a larger DC resistance ratio indicates a greater positive electrode degradation state is not observed. The degradation state estimation unit 12 estimates whether the positive electrode degradation state is in the range greater than 14% based on the charge and discharge characteristics when charged and discharged with the first current (Id_1, Ic_1). Furthermore, when secondary battery 1 is charged and discharged with a second current (Id_2, Ic_2), in the range where the positive electrode degradation state is greater than 10%, the correlation between the DC resistance ratio and the positive electrode degradation state is such that a larger DC resistance ratio indicates a greater positive electrode degradation state. The degradation state estimation unit 12 estimates whether the positive electrode degradation state is greater than 10% but less than 14%, or whether the positive electrode degradation state is 10% or less, based on the charge-discharge characteristics when charged and discharged with the second current (Id_2, Ic_2).

[0050] Referring to Figure 6, if the DC resistance ratio (Rd_2 / Rc_2) is less than the DC resistance ratio threshold (Rd / c_th2), the degradation state estimation unit 12 determines that the positive electrode degradation state is in the range of 10% or less (step S16). If the DC resistance ratio (Rd_2 / Rc_2) is greater than or equal to the DC resistance ratio threshold (Rd / c_th2), the degradation state estimation unit 12 determines that the positive electrode degradation state is greater than 10% but less than 14% (step S17). Also, if the DC resistance ratio (Rd_1 / Rc_1) is greater than or equal to the DC resistance ratio threshold (Rd / c_th1), the degradation state estimation unit 12 estimates that the positive electrode degradation state is greater than 14% (step S18). In the control flow of steps S16 to S18, when estimating the positive electrode degradation state from the comparison result of the DC resistance ratio (Rd_1 / Rc_1, Rd_2 / Rc_2) and the DC resistance ratio threshold (Rd / c_th1, Rd / c_th2), for example, when the secondary battery 1 has the correlation shown in Figure 7, it is preferable to set the DC resistance ratio threshold (Rd / c_th1, Rd / c_th2) to 1.02.

[0051] In Modification 1 of this embodiment, the processor 10 calculates the DC resistance value (Rd_1) based on the discharge characteristics when discharged with a first current (Id_1), calculates the DC resistance value (Rc_1) based on the charging characteristics when charged with a first current (Ic_1), calculates the DC resistance value (Rd_2) based on the discharge characteristics when discharged with a second current (Id_2), calculates the DC resistance value (Rc_2) based on the charging characteristics when charged with a first current (Ic_2), and estimates the degradation state of the positive electrode based on the DC resistance ratio (Rd_1 / Rc_1) and DC resistance ratio (Rd_2 / Rc_2). By calculating the DC resistance ratio (Rd_1 / Rc_1, Rd_2 / Rc_2) at multiple current values, the degradation state of the positive electrode can be quantitatively determined. Furthermore, the DC resistance values ​​(Rd_1, Rc_1) correspond to the "first DC resistance value (Rd_1, Rc_1)" of the present invention, the DC resistance values ​​(Rd_2, Rc_2) correspond to the "second DC resistance value (Rd_2, Rc_2)" of the present invention, and the DC resistance ratio (Rd_1 / Rc_1, Rd_2 / Rc_2) corresponds to the first / second DC resistance ratio (Rd_1 / Rc_1, Rd_2 / Rc_2) of the present invention.

[0052] In the modified example 1, in order to more quantitatively estimate the positive electrode degradation state within the range of 10% or less, the processor 10 may charge and discharge the secondary battery 1 with a current value greater than the second current (Id_2, Ic_2), calculate the DC resistance values ​​(Rd, Rc) based on the charge and discharge characteristics, and estimate the positive electrode degradation state from the comparison result between the DC resistance ratio and a predetermined DC resistance ratio. Furthermore, in order to more quantitatively estimate the positive electrode degradation state within the range of greater than 10% but less than 14%, the processor 10 may charge and discharge the secondary battery 1 with a current value greater than the first current (Id_1, Ic_1) and smaller than the second current (Id_2, Ic_2), calculate the DC resistance values ​​(Rd, Rc) based on the charge and discharge characteristics, and estimate the positive electrode degradation state from the comparison result between the DC resistance ratio and a predetermined DC resistance ratio. Furthermore, in order to more quantitatively estimate the positive electrode degradation state within the range of 14% or more, the processor 10 may charge and discharge the secondary battery 1 with a current value smaller than the first current (Id_1, Ic_1), calculate the DC resistance values ​​(Rd, Rc) based on the charge and discharge characteristics, and estimate the positive electrode degradation state from the comparison result of the DC resistance ratio with a predetermined DC resistance ratio.

[0053] 《Second Embodiment》 In the first embodiment, the difference between the charging and discharging characteristics of the secondary battery 1 was identified from the difference between the DC resistance value (Rd) based on the voltage change before and after the start of discharge and the DC resistance value (Rc) based on the voltage change before and after the start of charging. In this embodiment, however, the difference is identified from the difference between the voltage change range (ΔVd) before and after the cessation of discharge and the voltage change range (ΔVc) before and after the cessation of charging. The configuration of the internal state estimation system and the estimation mechanism for the positive electrode degradation state are the same as in the first embodiment, and their descriptions will be referenced. Furthermore, in the following description, the parts of the degradation state estimation method and degradation state estimation device that differ from the first embodiment will be mainly described, but other parts will refer to the descriptions of the first embodiment as appropriate.

[0054] Referring to Figure 8, we will explain how the degradation state of the positive electrode is reflected in the difference between the voltage change (ΔVd) before and after stopping discharge and the voltage change (ΔVc) before and after stopping charge. Figure 8 is a graph showing the characteristics of the voltage change on the discharge side (ΔVd) and the voltage change on the charge side (ΔVc) when the secondary battery 1 is charged and discharged in a predetermined charge-discharge sequence. (a) shows the characteristics before positive electrode degradation, and (b) shows the characteristics after positive electrode degradation. Graph g １ and g ２ This shows the characteristics of the DC resistance on the discharge side, and graph f １ and f ２ The graph shows the characteristics of the DC current on the charging side. The predetermined charge-discharge sequence is the same as the sequence used to obtain the charge-discharge current characteristics and voltage characteristics shown in Figure 3A in the first embodiment. The time on the horizontal axis of Figure 8 indicates the pause time after charging or discharging in the predetermined charge-discharge sequence. The voltage change on the discharge side (ΔVd) corresponds to the voltage change from the time the secondary battery 1 stops discharging until a certain period of time has elapsed, and the voltage change on the charging side (ΔVc) corresponds to the voltage change from the time the secondary battery 1 stops charging until a certain period of time has elapsed.

[0055] As shown in Figure 8(a), before positive electrode degradation, the voltage change after discharge cessation (ΔVd) and the voltage change after charging cessation (ΔVc) remain roughly the same in magnitude with respect to the rest time. On the other hand, as shown in Figure 8(b), after positive electrode degradation, the voltage change after discharge cessation and the voltage change after charging cessation exhibit different characteristics with respect to the rest time, with the voltage change after discharge cessation becoming larger than the DC resistance value after charging cessation. In other words, the degradation state of the positive electrode can be estimated based on the difference between the voltage change after discharge cessation and the voltage change after charging cessation (hereinafter also referred to as the "difference in voltage change before and after charging / discharging cessation").

[0056] Next, a method for estimating the degradation state of the positive electrode will be explained. The control flow of the estimation method can be explained by replacing the DC resistance (Rd) with the voltage change amount after discharge cessation (ΔVd) and the DC resistance (Rc) with the voltage change amount after charging cessation (ΔVc) with the control flow from step S1 to step S7 of the first embodiment. The control flow of the estimation method of this embodiment will be explained below with reference to the description of the control flow from step S1 to step S7 of the first embodiment and Figure 4.

[0057] Steps S1 and S2 are the same as steps S1 and S2 of the first embodiment. In step S3, the degradation state estimation unit 12 calculates the voltage change amount at a certain time after the discharge of the secondary battery 1 has stopped, based on the discharge characteristics. Specifically, the degradation state estimation unit 12 calculates the voltage change amount from the difference between voltage [Vd(-1)] and voltage [Vd(5)]. Vd(-1) is the voltage detected 1 second before the end of discharge in step S2, and Vd(5) is the voltage detected 5 seconds after the end of discharge in step S2. Note that the discharge characteristics include not only the behavior of the voltage during discharge, but also the behavior of the voltage after the end of discharge. In other words, the discharge characteristics are defined as the behavior of the voltage before, during, and after the end of discharge.

[0058] Steps S4 and S5 are the same as steps S4 and S5 of the first embodiment. In step S6, the degradation state estimation unit 12 calculates the voltage change amount at a certain time after charging of the secondary battery 1 has stopped, based on the charging characteristics. Specifically, the degradation state estimation unit 12 calculates the voltage change amount from the difference between voltage [Vc(-1)] and voltage [Vc(5)]. Vc(-1) is the voltage detected 1 second before the end of charging in step S5, and Vc(5) is the voltage detected 5 seconds after the end of charging in step S5. Note that the charging characteristics include not only the behavior of the voltage during charging, but also the behavior of the voltage after charging is complete. In other words, the charging characteristics are defined as the behavior of the voltage before, during, and after the end of charging.

[0059] In step S7, the degradation state estimation unit 12 estimates the degradation state of the positive electrode based on the difference between the voltage change amount (ΔVd) after discharge cessation and the voltage change amount (ΔVc) after charging cessation. Specifically, the degradation state estimation unit 12 calculates a voltage change ratio (ΔVd / ΔVc) as a parameter indicating the difference between the voltage change amount (ΔVd) and the voltage change amount (ΔVc). The degradation state estimation unit 12 then compares the calculated voltage change ratio (ΔVd / ΔVc) with a predetermined voltage change ratio threshold, and determines that the positive electrode has degraded if the voltage change ratio (ΔVd / ΔVc) is greater than or equal to the voltage change ratio threshold. On the other hand, if the voltage change ratio (ΔVd / ΔVc) is less than the voltage change ratio threshold, the degradation state estimation unit 12 determines that the positive electrode has not degraded.

[0060] Figure 9 is a graph showing the correlation between the difference in voltage change after charging and discharging stops and the positive electrode degradation state. The characteristics shown in Figure 9 can be obtained from a prototype of secondary battery 1, which will be described in the embodiment described later. The positive electrode degradation state represents the rate of capacity reduction of the positive electrode (degree of positive electrode degradation). The difference in voltage change after charging and discharging stops is shown by the voltage change ratio (ΔVd / ΔVc), which is the ratio of the voltage change after discharging stops (ΔVd) to the voltage change after charging stops (ΔVc). As shown in Figure 11, when the positive electrode degradation state is less than about 10%, the voltage change ratio (ΔVd / ΔVc) is close to 1, and the voltage change after discharging stops and the voltage change after charging stops are almost the same. On the other hand, when the positive electrode degradation state is greater than 10%, the difference between the voltage change after discharging stops and the voltage change after charging stops becomes larger, and the voltage change ratio (ΔVd / ΔVc) becomes greater than 1. For example, when the positive electrode degradation exceeds 14%, the voltage change ratio (ΔVd / ΔVc) exceeds 1.04.

[0061] Furthermore, the greater the degradation of the positive electrode, the larger the voltage change ratio (ΔVd / ΔVc). In the example in Figure 9, the voltage change ratio threshold is set to 1.04. When the voltage change ratio (ΔVd / ΔVc) is greater than or equal to the voltage change ratio threshold (1.04), the degradation state estimation unit 12 determines that the positive electrode has deteriorated. When the voltage change ratio (ΔVd / ΔVc) is less than the voltage change ratio threshold (1.04), the degradation state estimation unit 12 determines that the positive electrode has not deteriorated. Thus, the degradation state estimation unit 12 estimates the degradation state of the positive electrode based on the difference between the voltage change after discharge cessation and the voltage change after charging cessation.

[0062] In the control flow of step S7, the degradation state estimation unit 12 may also estimate the degradation state of the positive electrode using battery characteristic data pre-stored in the memory 20. The battery characteristic data is data that represents the correlation between the voltage change ratio (ΔVd / ΔVc) and the degradation state of the positive electrode. The correlation can be determined experimentally. The degradation state estimation unit 12 acquires the battery characteristic data from the memory 20. Then, the degradation state estimation unit 12 calculates the voltage change ratio (ΔVd / ΔVc), refers to the battery characteristic data, and estimates the degradation state of the positive electrode corresponding to the calculated voltage change ratio (ΔVd / ΔVc).

[0063] For example, suppose the secondary battery 1 has characteristics similar to those shown in Figure 9, and the voltage change ratio (ΔVd / ΔVc) calculated in the control flow of step S7 is 1.10. In this case, in the graph shown in Figure 9, the positive electrode degradation state corresponding to the voltage change ratio (ΔVd / ΔVc) is approximately 19%. As a result, the degradation state estimation unit 12 can estimate the degradation state of the positive electrode based on the correlation between the voltage change ratio (ΔVd / ΔVc) and the degradation state of the positive electrode, which are pre-stored in the memory 20.

[0064] As described above, in this embodiment, the processor 10 calculates the voltage change (ΔVd) at a certain time after the discharge of the secondary battery 1 has stopped, based on the discharge characteristics, and calculates the voltage change (ΔVc) at a certain time after the charging of the secondary battery 1 has stopped, based on the charging characteristics, and estimates the degradation state of the positive electrode based on the difference between the voltage change (ΔVd) and the voltage change (ΔVc). This makes it possible to estimate the degradation state of the positive electrode in a short time (for example, from tens of seconds to a few minutes).

[0065] In this embodiment, the processor 10 determines that the positive electrode has deteriorated when the voltage change ratio (ΔVd / ΔVc) is greater than or equal to a predetermined voltage change ratio threshold. There is a correlation between the deterioration state of the positive electrode and the voltage change ratio (ΔVd / ΔVc). In this embodiment, this correlation can be used to determine whether or not the positive electrode has deteriorated. This allows the deterioration state of the positive electrode to be estimated in a short time.

[0066] In this embodiment, the processor 10 obtains battery characteristic data from the memory 20 that represents the correlation between the voltage change ratio (ΔVd / ΔVc) and the degradation state of the positive electrode, calculates the voltage change ratio (ΔVd / ΔVc), and, referring to the battery characteristic data, estimates the degradation state of the positive electrode corresponding to the calculated voltage change ratio (ΔVd / ΔVc). This allows the degradation state of the positive electrode to be estimated in a short time.

[0067] As a second modification of this embodiment, the secondary battery 1 may be charged and discharged with charge and discharge currents of different battery values, and the degradation state of the positive electrode may be estimated based on multiple charge and discharge characteristics. The control flow of the estimation method in the second modification can be explained by replacing the DC resistance (Rd) with the voltage change amount after discharge cessation (ΔVd) and the DC resistance (Rc) with the voltage change amount after charging cessation (ΔVc) with respect to the control flow of the first modification 1 of the first embodiment (steps S11 to S18). The control flow of the estimation method of this embodiment will be explained below with reference to the description of the control flow of the first modification 1 of the first embodiment (steps S11 to S18) and Figure 6.

[0068] Step S11 is the same as step S11 of Modification 1 of the First Embodiment. In step S12, the charge / discharge control unit 11 discharges with a first current (Id_1) and measures the discharge characteristics, and charges with a first current (Ic_1) and measures the charge characteristics. The current value of the first current and the charge / discharge time are the same as in step S12 of Modification 1 of the First Embodiment. The degradation state estimation unit 12 calculates the voltage change amount (ΔVd_1) after the discharge is stopped based on the discharge characteristics, and calculates the voltage change amount (ΔVc_1) after the charge is stopped based on the charge characteristics. The method for calculating the voltage change amounts (ΔVd_1, ΔVc_1) is the same as the method for calculating the voltage change amounts (ΔVd, ΔVc) in steps S3 and S6.

[0069] In step S13, the degradation state estimation unit 12 calculates a voltage change ratio (ΔVd_1 / ΔVc_1) as a parameter indicating the difference between the voltage change after discharge cessation (ΔVd_1) and the voltage change after charge cessation (ΔVc_1). The degradation state estimation unit 12 compares the calculated voltage change ratio (ΔVd_1 / ΔVc_1) with a voltage change ratio threshold (ΔVd / c_th1) and determines whether the voltage change ratio (ΔVd_1 / ΔVc_1) is greater than or equal to the voltage change ratio threshold (ΔVd / c_th1). The voltage change ratio threshold (ΔVd / c_th1) is a predetermined threshold according to the battery characteristics of the secondary battery 1. If the voltage change ratio (ΔVd_1 / ΔVc_1) is greater than or equal to the voltage change ratio threshold (ΔVd / c_th1), the control flow processed by the processor 10 proceeds to step S18.

[0070] If the voltage change ratio (ΔVd_1 / ΔVc_1) is less than the voltage change ratio threshold (ΔVd / c_th1), in step S14, the charge / discharge control unit 11 discharges and charges with a second current (Id_2, Ic_2) and measures the discharge characteristics and charge characteristics. In this embodiment, the second current during discharge and the second current during charging have the same current value (C rate) and are greater than the first current. The current value of the second current and the charge / discharge time are the same as in step S14 of Modification 1 of the first embodiment. A pause period is also set between the end of discharge and the start of charging. The charge / discharge control unit 11 detects the voltage of the secondary battery 1 during charging and discharging and during the pause period. The degradation state estimation unit 12 calculates the voltage change amount (ΔVd_2) based on the discharge characteristics and the voltage change amount (ΔVc_2) based on the charge characteristics. The method for calculating the voltage change range (ΔVd_2, ΔVc_2) is the same as the method for calculating the voltage change range (ΔVd_1, ΔVc_1).

[0071] In step S15, the degradation state estimation unit 12 calculates the voltage change ratio (ΔVd_2 / ΔVc_2) as a parameter indicating the difference between the voltage change range (ΔVd_2) and the voltage change range (ΔVc_2). The degradation state estimation unit 12 compares the calculated voltage change ratio (ΔVd_2 / ΔVc_2) with the voltage change ratio threshold (ΔVd / c_th2) and determines whether the voltage change ratio (ΔVd_2 / ΔVc_2) is less than the voltage change ratio threshold (ΔVd / c_th2). The voltage change ratio threshold (ΔVd / c_th2) is a predetermined threshold according to the battery characteristics of the secondary battery 1. Note that the voltage change ratio threshold (ΔVd / c_th2) may be the same value as the voltage change ratio threshold (ΔVd / c_th1). If the voltage change ratio (ΔVd_2 / ΔVc_2) is less than the voltage change ratio threshold (ΔVd / c_th2), the control flow processed by the processor 10 proceeds to step S16. If the voltage change ratio (ΔVd_2 / ΔVc_2) is greater than or equal to the voltage change ratio threshold (ΔVd / c_th2), the control flow processed by the processor 10 proceeds to step S17.

[0072] In the control flow of steps S16 to S18, the degradation state estimation unit 12 estimates the positive electrode degradation state based on the voltage change ratio (ΔVd_1 / ΔVc_1, ΔVd_2 / ΔVc_2) calculated based on different charge and discharge currents.

[0073] Figure 10 is a graph showing the correlation between the difference in voltage change after charging and discharging stops and the state of positive electrode degradation. １ This shows the characteristics of secondary battery 1 when it is charged and discharged with a first current (Id_1, Ic_1), and graph j ２ Figure 10 shows the characteristics when the secondary battery 1 is charged and discharged with a second current (Id_2, Ic_2). The characteristics shown in Figure 10 can be obtained from a prototype of the secondary battery 1 described in the embodiment below. When the secondary battery 1 is charged and discharged with a first current (Id_1, Ic_1), in the range where the positive electrode degradation state is greater than 14%, the correlation between the voltage change ratio and the positive electrode degradation state is such that the greater the voltage change ratio, the greater the positive electrode degradation state. Furthermore, when charged and discharged with a second current (Id_2, Ic_2), in the range where the positive electrode degradation state is greater than 10%, the correlation between the DC voltage change ratio and the positive electrode degradation state is such that the greater the voltage change ratio, the greater the positive electrode degradation state. Therefore, the degradation state estimation unit 12 estimates whether the positive electrode degradation state is in the range greater than 14% based on the charge-discharge characteristics when charged and discharged with the first current (Id_1, Ic_1). The degradation state estimation unit 12 estimates whether the positive electrode degradation state is greater than 10% but less than 14%, or whether the positive electrode degradation state is 10% or less, based on the charge-discharge characteristics when charged and discharged with the second current (Id_2, Ic_2).

[0074] Referring to Figure 6, if the voltage change ratio (ΔVd_2 / ΔVc_2) is less than the voltage change ratio threshold (ΔVd / c_th2), the degradation state estimation unit 12 determines that the positive electrode degradation state is in the range of 10% or less (step S16). If the voltage change ratio (ΔVd_2 / ΔVc_2) is greater than or equal to the voltage change ratio threshold (ΔVd / c_th2), the degradation state estimation unit 12 determines that the positive electrode degradation state is greater than 10% but less than 14% (step S17). Also, if the voltage change ratio (ΔVd_1 / ΔVc_1) is greater than or equal to the voltage change ratio threshold (ΔVd / c_th1), the positive electrode degradation state is estimated to be greater than 14%. In the control flow of steps S16 to S18, when estimating the positive electrode degradation state from the comparison result of the voltage change ratio (ΔVd_1 / ΔVc_1, ΔVd_2 / ΔVc_2) and the voltage change ratio threshold (ΔVd / c_th1, ΔVd / c_th2), for example, if the secondary battery 1 has the correlation shown in Figure 7, the voltage change ratio threshold (ΔVd / c_th1, ΔVd / c_th2) should be set to 1.02.

[0075] In a modified example 2 of this embodiment, the processor 10 calculates the voltage change (ΔVd_1) based on the discharge characteristics when discharging with a first current (Id_1), calculates the voltage change (ΔVc_1) based on the charging characteristics when charging with a first current (Ic_1), calculates the voltage change (ΔVd_2) based on the discharge characteristics when discharging with a second current (Id_2), calculates the voltage change (ΔVc_2) based on the charging characteristics when charging with a first current (Ic_2), and estimates the degradation state of the positive electrode based on the DC resistance ratio (ΔVd_1 / ΔVc_1) and DC resistance ratio (ΔVd_2 / ΔVc_2). By doing so, the degradation state of the positive electrode can be quantitatively grasped by calculating the DC resistance ratio (ΔVd_1 / ΔVc_1, ΔVd_2 / ΔVc_2) at multiple current values. Furthermore, the voltage change range (ΔVd_1, ΔVc_1) corresponds to the "first voltage change range (ΔVd_1, ΔVc_1)" of the present invention, the voltage change range (ΔVd_2, ΔVc_2) corresponds to the "second voltage change range (ΔVd_2, ΔVc_2)" of the present invention, and the voltage change range ratio (ΔVd_1 / ΔVc_1, ΔVd_2 / ΔVc_2) corresponds to the first / second voltage change range ratio (ΔVd_1 / ΔVc_1, ΔVd_2 / ΔVc_2) of the present invention.

[0076] In the modified example 2, in order to more quantitatively estimate the positive electrode degradation state within the range of 10% or less, the processor 10 may charge and discharge the secondary battery 1 with a current value greater than the second current (Id_2, Ic_2), calculate the voltage change range (ΔVd, ΔVc) based on the charge and discharge characteristics, and estimate the positive electrode degradation state from the comparison result between the voltage change range ratio and a predetermined voltage change range ratio threshold. Alternatively, in order to more quantitatively estimate the positive electrode degradation state within the range of greater than 10% but less than 14%, the processor 10 may charge and discharge the secondary battery 1 with a current value greater than the first current (Id_1, Ic_1) and smaller than the second current (Id_2, Ic_2), calculate the voltage change range (ΔVd, ΔVc) based on the charge and discharge characteristics, and estimate the positive electrode degradation state from the comparison result between the voltage change range ratio and a predetermined voltage change range ratio threshold. Furthermore, in order to more quantitatively estimate the positive electrode degradation state within the range of 14% or more, the processor 10 may charge and discharge the secondary battery 1 with a current value smaller than the first current (Id_1, Ic_1), calculate the voltage change range (ΔVd, ΔVc) based on the charge and discharge characteristics, and estimate the positive electrode degradation state from the comparison result between the voltage change range ratio and a predetermined voltage change range ratio threshold.

[0077] <Third Embodiment> In the first embodiment, the difference between the charging and discharging characteristics of the secondary battery 1 was determined by the difference between the DC resistance value (Rd) based on the voltage change before and after the start of discharge and the DC resistance value (Rc) based on the voltage change before and after the start of charging. In this embodiment, however, the difference is determined by the difference between the AC impedance (Zd) during discharge and the AC impedance (Zc) during charging. The configuration of the internal state estimation system and the estimation mechanism for the positive electrode degradation state are the same as in the first embodiment, and their descriptions will be referenced. Furthermore, in the following description, the parts of the degradation state estimation method and degradation state estimation device that differ from the first embodiment will be mainly described, but other parts will refer to the descriptions of the first embodiment as appropriate.

[0078] The AC impedance can be measured by an impedance meter connected to the secondary battery 1. The impedance meter measures the AC impedance (complex impedance) of the secondary battery 1 by applying an AC current as an input signal to the secondary battery 1 and obtaining a response voltage corresponding to the AC signal (AC current), or by applying an AC voltage as an input signal to the secondary battery 1 and obtaining a response current corresponding to the AC voltage. In this embodiment, the measurement method will be described based on the method of applying an AC current and obtaining a response voltage, but the same can be carried out using the measurement method of applying an AC voltage and obtaining a response current. The impedance meter can be arbitrarily selected from those commonly used as general AC impedance measuring devices. For example, the impedance meter may measure the AC impedance of the secondary battery 1 by changing the frequency of the AC current over time using the AC impedance method. It may also be capable of simultaneously applying multiple AC currents of different frequencies. The method of measuring AC impedance in the AC impedance method is not particularly limited. For example, analog methods such as the Lissajous method and the AC bridge method, or digital methods such as the digital Fourier integral method and the fast Fourier transform method with noise application can be appropriately adopted. The impedance meter applies AC signals of multiple frequency values ​​within a predetermined frequency band and measures the real-axis component value (Z') and imaginary-axis component value (-Z'') of the AC impedance for each frequency value. There are no particular restrictions on the type and amplitude of the AC current waveform applied to the battery; they can be set arbitrarily. Examples of waveform types that can be applied include limited waves, square waves, and triangular waves. The measurement result of the AC impedance measured by the impedance meter is sent to the controller 5 as the output of the impedance meter. In addition to the charge / discharge control unit 11 and the degradation state estimation unit 12, the processor 10 also has a function to calculate AC impedance, and uses this AC impedance calculation function to calculate the AC impedance during discharge (Zd) and the AC impedance during charging (Zc), respectively. That is, the processor 10 controls the impedance meter to input an AC voltage or AC current during charging and discharging, obtains the output value of the response current or voltage, and then calculates the AC impedance during charging and discharging.

[0079] Referring to Figure 11, we will explain how the degradation state of the positive electrode is reflected in the difference between the AC impedance during discharge (Zd) and the AC impedance during charging (Zc). During the charging and discharging of secondary battery 1, the AC impedance during discharge and AC impedance during charging are measured using EIS (Electro-chemical Impedance Spectroscopy). Figure 11 is a Bode plot showing the characteristics of the AC impedance. The vertical axis of Figure 11 represents the imaginary component (Z'') of the AC impedance, and the horizontal axis represents the frequency. In Figure 11, circles indicate the measurement results during the discharge of secondary battery 1, and squares indicate the measurement results during the charging of secondary battery 1. Note that "before degradation" corresponds to the degradation state of the secondary battery at SOH (100%), and "after degradation" corresponds to the degradation state of the secondary battery at SOH (81%). Note that the positive electrode capacity after degradation has decreased by 15%.

[0080] As shown in Figure 11, the Boeed plots during charging and discharging differ before and after degradation. Before degradation, the imaginary component value of the AC impedance with respect to frequency is almost the same during charging and discharging. On the other hand, after degradation, a difference occurs in the imaginary component value of the AC impedance with respect to frequency between charging and discharging. In particular, the difference in the imaginary component value of the AC impedance between charging and discharging (hereinafter also referred to as the "difference in the imaginary component of impedance during charging and discharging") becomes larger on the low-frequency side.

[0081] When the Bode plot showing the AC impedance characteristics as shown in Figure 11 is viewed in terms of the ratio of imaginary components during charging and discharging (Z"d / Z"c) and the difference of imaginary components during charging and discharging (Z"d-Z"c), the characteristics shown in Figures 12 and 13 are obtained. Figure 12 is a graph showing the ratio of imaginary components (Z"d / Z"c) with respect to SOC, and Figure 13 is a graph showing the difference of imaginary components (Z"d-Z"c) with respect to frequency. Note that the ratio of imaginary components (Z"d / Z"c) is the value obtained by dividing the imaginary component (Z"d) of the AC impedance (Zd) during charging by the imaginary component (Z"c) of the AC impedance (Zc) during charging, while keeping the AC current in the AC impedance measurement the same. Furthermore, the difference in the imaginary component (Z''d - Z''c) is the value obtained by subtracting the imaginary component (Z''c) of the AC impedance during charging (Zc) from the imaginary component (Z''d) of the AC impedance during discharge (Zd), while keeping the AC current the same in the AC impedance measurement.

[0082] Figure 12 shows the characteristics of the ratio of the imaginary components of the AC impedance (Z"d / Z"c) when an AC current of 2 Hz is input to the secondary battery 1 and the AC impedance is measured. Figure 13 shows the characteristics of the difference in the imaginary components (Z"d - Z"c) based on the measured AC impedance, with the SOC set to 30%. As shown in Figure 12, from SOC (30%) to SOC (70%), the ratio of the imaginary components (Z"d / Z"c) increases as the positive electrode degradation increases. Since the ratio of the imaginary components (Z"d / Z"c) corresponds to the difference in the impedance imaginary components during charging and discharging, the degradation state of the positive electrode can be estimated based on the difference in the impedance imaginary components during charging and discharging.

[0083] Furthermore, as shown in Figure 13, when the frequency of the measurement current (AC current) in the EIS measurement is set to the low frequency band (for example, 10 Hz or less) and the EIS measurement is performed, the greater the degradation of the positive electrode, the larger the difference in the imaginary components of the AC impedance (Zd, Zc) (Z"d - Z"c). Since this difference in imaginary components (Z"d - Z"c) corresponds to the difference in the imaginary components of the impedance during charging and discharging, the degradation state of the positive electrode can be determined based on the difference in the imaginary components of the impedance during charging and discharging.

[0084] Next, the method for estimating the degradation state of the positive electrode will be explained with reference to Figure 14. Figure 14 is a flowchart showing the procedure for the internal state estimation method according to this embodiment. Note that each control flow in the flowchart shown in Figure 14 is executed by the processor 10.

[0085] In step S21, with the secondary battery 1 connected to the load and charging device via the DC-DC converter 2, the charge / discharge control unit 11 performs EIS measurement while discharging the secondary battery 1 with a constant discharge current (Id). In step S22, the processor 10 calculates the AC impedance (Zd) during discharge based on the discharge characteristics. The AC impedance (Zd) is an absolute value.

[0086] In step S23, the charge / discharge control unit 11 performs EIS measurement while charging the secondary battery 1 with a constant charging current (Ic). In step S24, the processor 10 calculates the AC impedance (Zc) during charging based on the charging characteristics. The AC impedance (Zc) is an absolute value.

[0087] In step S25, the degradation state estimation unit 12 estimates the degradation state of the positive electrode based on the difference between the AC impedance (Zd) during discharge and the AC impedance (Zc) during charging. Specifically, the degradation state estimation unit 12 calculates the ratio (Z"d / Z"c) of the imaginary component (Z"d) of the AC impedance (Zd) and the imaginary component (Z"c) of the AC impedance (Zc) as a parameter indicating the difference between the AC impedance (Zd) during discharge and the AC impedance (Zc) during charging. The degradation state estimation unit 12 then compares the calculated ratio (Z"d / Z"c) with a predetermined imaginary component ratio threshold, and determines that the positive electrode has degraded if the calculated ratio (Z"d / Z"c) is greater than or equal to the predetermined imaginary component ratio threshold. On the other hand, if the calculated ratio (Z"d / Z"c) is less than the predetermined imaginary component ratio threshold, the degradation state estimation unit 12 determines that the positive electrode has not degraded.

[0088] Figure 15 is a graph showing the correlation between the difference in the imaginary component of impedance during charging and discharging and the positive electrode degradation state (degree of positive electrode degradation) in secondary battery 1. The characteristics shown in Figure 15 can be obtained from a prototype of secondary battery 1, which will be described in the embodiment described later. The positive electrode degradation state represents the rate of capacity reduction of the positive electrode. The difference in the imaginary component of impedance during charging and discharging is shown by the ratio (Z"d / Z"c) of the imaginary component of the AC impedance on the discharging side (Z"d) to the imaginary component of the AC impedance on the charging side (Z"c). As shown in Figure 15, when the positive electrode degradation state is less than about 14%, the ratio (Z"d / Z"c) is less than 1. On the other hand, when the positive electrode degradation state is greater than 14%, the ratio (Z"d / Z"c) of the imaginary component of the AC impedance on the discharging side and the imaginary component of the AC impedance on the charging side becomes greater than 1. For example, when the positive electrode degradation state is greater than 14%, the ratio (Z"d / Z"c) becomes greater than 1.0.

[0089] As shown in Figure 15, when the positive electrode degradation state (degree of positive electrode degradation) exceeds a predetermined degradation state threshold (for example, 7%), the ratio (Z"d / Z"c) increases as the degradation state of the positive electrode increases. In the example in Figure 15, the imaginary component ratio threshold is set to 1.0. Then, if the ratio (Z"d / Z"c) is greater than or equal to the imaginary component ratio threshold (1.0), the degradation state estimation unit 12 determines that the positive electrode has degraded. If the ratio (Z"d / Z"c) is less than the imaginary component ratio threshold (1.0), the degradation state estimation unit 12 determines that the positive electrode has not degraded. Thus, the degradation state estimation unit 12 estimates the degradation state of the positive electrode based on the difference between the imaginary component of the AC impedance (Z"d) and the imaginary component of the AC impedance (Z"c).

[0090] In the control flow of step S25, the degradation state estimation unit 12 may calculate the difference in the imaginary component of the AC impedance during charging and discharging (Z"d - Z"c) as a parameter indicating the difference between the AC impedance during discharge (Zd) and the AC impedance during charging (Zc). The degradation state estimation unit 12 then compares the calculated difference (Z"d - Z"c) with a predetermined imaginary component difference threshold, and determines that the positive electrode has deteriorated if the difference (Z"d - Z"c) is greater than or equal to the predetermined imaginary component difference threshold. On the other hand, if the difference (Z"d - Z"c) is less than the predetermined imaginary component difference threshold, the degradation state estimation unit 12 determines that the positive electrode has not deteriorated.

[0091] Figure 16 is a graph showing the correlation between the difference in the imaginary component of the charge-discharge impedance and the positive electrode degradation state in secondary battery 1. The characteristics shown in Figure 16 can be obtained from a prototype of secondary battery 1, which will be described in the embodiment described later. The positive electrode degradation state represents the rate of capacity reduction of the positive electrode. As shown in Figure 16, when the positive electrode degradation state exceeds 10%, the difference (Z"d - Z"c) exceeds 0.02.

[0092] As shown in Figure 16, the greater the degradation of the positive electrode, the larger the difference (Z"d - Z"c). In the example in Figure 16, the imaginary component difference threshold is set to 0.02. When the difference (Z"d - Z"c) is greater than or equal to the imaginary component difference threshold (0.02), the degradation state estimation unit 12 determines that the positive electrode has degraded. When the difference (Z"d - Z"c) is less than the imaginary component difference threshold (0.02), the degradation state estimation unit 12 determines that the positive electrode has not degraded. Thus, the degradation state estimation unit 12 estimates the degradation state of the positive electrode based on the difference between the imaginary component of the AC impedance (Z"d) and the imaginary component of the AC impedance (Z"c).

[0093] In the control flow of step S25, the degradation state estimation unit 12 may also estimate the degradation state of the positive electrode using battery characteristic data pre-stored in the memory 20. The battery characteristic data is, for example, data representing the correlation between the ratio (Z"d / Z"c") and the degradation state of the positive electrode. The correlation can be determined experimentally, for example, as shown in Figure 15. The degradation state estimation unit 12 acquires the battery characteristic data from the memory 20. Then, the degradation state estimation unit 12 calculates the ratio (Z"d / Z"c), refers to the battery characteristic data, and estimates the degradation state of the positive electrode corresponding to the calculated ratio (Z"d / Z"c).

[0094] For example, suppose secondary battery 1 has the battery characteristics shown in Figure 15, and the ratio (Z"d / Z"c") calculated in the control flow of step S25 is 1.03. In this case, in the graph shown in Figure 15, the positive electrode degradation state corresponding to the ratio (Z"d / Z"c = 1.03) is approximately 15%. As a result, the degradation state estimation unit 12 can estimate the degradation state of the positive electrode based on the correlation between the ratio (Z"d / Z"c") pre-stored in the memory 20 and the degradation state of the positive electrode.

[0095] The battery characteristic data may also be data representing the correlation between the difference (Z"d - Z"c") and the degradation state of the positive electrode. The correlation shown by the battery characteristic data is, for example, the characteristic shown in Figure 16. The degradation state estimation unit 12 acquires the battery characteristic data from the memory 20. Then, the degradation state estimation unit 12 calculates the difference (Z"d - Z"c), refers to the battery characteristic data, and estimates the degradation state of the positive electrode corresponding to the calculated difference (Z"d - Z"c).

[0096] For example, suppose secondary battery 1 has the battery characteristics shown in Figure 16, and the difference (Z"d - Z"c) calculated in the control flow of step S25 is 0.057. In this case, in the graph shown in Figure 15, the positive electrode degradation state corresponding to the difference (Z"d - Z"c = 0.057) is approximately 15%. Based on this, the degradation state estimation unit 12 can estimate the degradation state of the positive electrode based on the correlation between the difference (Z"d - Z"c) pre-stored in memory 20 and the degradation state of the positive electrode.

[0097] As described above, in this embodiment, the processor 10 calculates the AC impedance (Zd) during discharge and the AC impedance (Zc) during charging of the secondary battery 1, and estimates the degradation state of the positive electrode based on the difference between the AC impedance (Zd) and the AC impedance (Zc). This makes it possible to estimate the degradation state of the positive electrode in a short time (for example, from tens of seconds to a few minutes).

[0098] In this embodiment, the processor 10 determines that the positive electrode has deteriorated if the ratio (Z"d / Z"c") is greater than or equal to a predetermined imaginary component ratio threshold. There is a correlation between the deterioration state of the positive electrode and the ratio (Z"d / Z"c). In this embodiment, this correlation can be used to determine whether or not the positive electrode has deteriorated. This allows the deterioration state of the positive electrode to be estimated in a short amount of time.

[0099] In this embodiment, the processor 10 refers to battery characteristic data, which is stored in the memory 20 in advance and represents the correlation between the ratio (Z"d / Z"c") and the degradation state of the positive electrode, and estimates the degradation state of the positive electrode corresponding to the ratio (Z"d / Z"c) calculated from the AC impedance (Zd) and AC impedance (Zc). This allows the degradation state of the positive electrode to be estimated in a short time.

[0100] In this embodiment, the processor 10 determines that the positive electrode has deteriorated if the difference (Z"d - Z"c") is greater than or equal to a predetermined imaginary component difference threshold. There is a correlation between the deterioration state of the positive electrode and the difference (Z"d - Z"c). In this embodiment, this correlation can be used to determine whether or not the positive electrode has deteriorated. This allows the deterioration state of the positive electrode to be estimated in a short amount of time.

[0101] In this embodiment, the processor 10 obtains battery characteristic data from the memory 20 that represents the correlation between the difference (Z"d - Z"c") and the degradation state of the positive electrode, calculates the difference (Z"d - Z"c), refers to the battery characteristic data, and estimates the degradation state of the positive electrode corresponding to the calculated difference (Z"d - Z"c). This allows the degradation state of the positive electrode to be estimated in a short time.

[0102] In this embodiment, the processor 10 may also set the frequency of the AC current supplied to the secondary battery 1 to a predetermined frequency (for example, 10 Hz or less) in the process of calculating the AC impedance (Zd) and AC impedance (Zc) (control flow of steps S22 and S24). By limiting the frequency of the AC current in the EIS measurement to a frequency band in which the impedance originating from the positive electrode is clearly evident, the EIS measurement can be performed over a narrower range. This makes it possible to measure the degradation state of the positive electrode in a short time.

[0103] As a third modification of this embodiment, the processor 10 may estimate the degradation state of the positive electrode based on the charge / discharge current when the ratio (Z"d / Z"c") or difference (Z"d-Z"c) is greater than or equal to a predetermined threshold. The secondary battery 1 has a correlation between the ratio (Z"d / Z"c") or difference (Z"d-Z"c) and the degradation state of the positive electrode. For example, in the battery characteristics shown in Figure 15, when the ratio (Z"d / Z"c") is 0.95 or greater, a linearity can be obtained in which the greater the ratio (Z"d / Z"c), the greater the degradation state of the positive electrode. In order to estimate the degradation state of the positive electrode using the battery characteristics that exhibit linearity, in the example of Figure 15, the imaginary component ratio threshold can be set to 0.95, and the degradation state of the positive electrode (approximately 7%) for the ratio (Z"d / Z"c = 0.95) can be estimated. In other words, when the processor 10 estimates the positive electrode degradation state from the ratio (Z"d / Z"c) within the range where the battery characteristics are linear, it can estimate the positive electrode degradation state with a lower limit of the positive electrode degradation state (approximately 7%). Furthermore, by increasing the current value of the charge / discharge current when measuring the AC impedance, the positive electrode degradation state corresponding to the lower limit of the range where the battery characteristics are linear can be reduced. Similarly, when estimating the positive electrode degradation state from the difference (Z"d-Z"c), increasing the current value of the charge / discharge current when measuring the AC impedance can reduce the positive electrode degradation state corresponding to the lower limit of the range where the battery characteristics are linear. Therefore, when estimating a smaller positive electrode degradation state, the processor 10 can quantitatively grasp the degradation state of the positive electrode by calculating the ratio (Z"d / Z"c) or the difference (Z"d-Z"c) based on the AC impedance during charge / discharge with a larger current value.

[0104] As a fourth modification of this embodiment, the processor 10 may estimate the positive electrode degradation state based on the ratio (Z'd / Z'c) of the real component (Z'd) of the AC impedance (Zd) and the real component (Z'c) of the AC impedance (Zc). When the positive electrode of the secondary battery 1 degrades, a difference occurs in the real component value of the AC impedance with respect to frequency between charging and discharging. The processor 10 calculates the ratio (Z'd / Z'c) as a parameter indicating the difference between the real component (Z'd) and the real component (Z'c), and estimates the positive electrode degradation state based on the calculated ratio (Z'd / Z'c).

[0105] After executing steps S21 to S24 of the third embodiment, the processor 10 executes the following control flow. In step S25, the degradation state estimation unit 12 calculates the ratio (Z'd / Z'c). The degradation state estimation unit 12 compares the calculated ratio (Z'd / Z'c) with a predetermined real component ratio threshold, and determines that the positive electrode has degraded if the ratio (Z'd / Z'c) is greater than or equal to the predetermined real component ratio threshold. On the other hand, if the ratio (Z'd / Z'c) is less than the predetermined real component ratio threshold, the degradation state estimation unit 12 determines that the positive electrode has not degraded.

[0106] There is a correlation between the ratio (Z'd / Z'c) and the positive electrode degradation state, as shown in Figure 17. As shown in Figure 17, when the positive electrode degradation state is less than approximately 11%, the ratio (Z'd / Z'c) is less than 1. On the other hand, when the positive electrode degradation state is greater than 11%, the ratio (Z'd / Z'c) of the real component of the AC impedance during discharge to the real component of the AC impedance during charging becomes greater than 1. For example, when the positive electrode degradation state is greater than 11%, the ratio (Z'd / Z'c) becomes greater than 1.0.

[0107] In the example in Figure 17, the real component ratio threshold is set to 1.0. Then, if the ratio (Z'd / Z'c) is greater than or equal to the real component ratio threshold (1.0), the degradation state estimation unit 12 determines that the positive electrode has degraded. If the ratio (Z'd / Z'c) is less than the real component ratio threshold (1.0), the degradation state estimation unit 12 determines that the positive electrode has not degraded. In this way, the degradation state estimation unit 12 estimates the degradation state of the positive electrode based on the difference between the real component of the AC impedance (Z'd) and the real component of the AC impedance (Z'c).

[0108] In the control flow of step S25, the degradation state estimation unit 12 may calculate the difference in the real components of the AC impedance during charging and discharging (Z'd - Z'c) as a parameter indicating the difference between the AC impedance during discharge (Zd) and the AC impedance during charging (Zc). The degradation state estimation unit 12 then compares the calculated difference (Z'd - Z'c) with a predetermined real component difference threshold, and determines that the positive electrode has deteriorated if the difference (Z'd - Z'c) is greater than or equal to the predetermined real component difference threshold. On the other hand, if the difference (Z'd - Z'c) is less than the predetermined real component difference threshold, the degradation state estimation unit 12 determines that the positive electrode has not deteriorated.

[0109] Figure 18 is a graph showing the correlation between the difference in the real impedance components of charge and discharge in secondary battery 1 and the positive electrode degradation state (degree of positive electrode degradation). The characteristics shown in Figure 18 can be obtained from a prototype of secondary battery 1, which will be described in the embodiment described later. The positive electrode degradation state represents the rate of capacity reduction of the positive electrode. As shown in Figure 18, when the positive electrode degradation state is about 9%, the difference (Z'd - Z'c) is less than 0.03. On the other hand, when the positive electrode degradation state exceeds 10%, the difference (Z'd - Z'c) becomes greater than 0.03.

[0110] In the example in Figure 18, the real component difference threshold is set to 0.03. Then, if the difference (Z'd - Z'c) is greater than or equal to the real component difference threshold (0.03), the degradation state estimation unit 12 determines that the positive electrode has deteriorated. If the difference (Z'd - Z'c) is less than the real component difference threshold (0.03), the degradation state estimation unit 12 determines that the positive electrode has not deteriorated. In this way, the degradation state estimation unit 12 estimates the degradation state of the positive electrode based on the difference between the real component of the AC impedance (Z'd) and the real component of the AC impedance (Z'c).

[0111] In the modified example 4, the processor 10 may refer to battery characteristic data and estimate the degradation state of the positive electrode corresponding to the calculated ratio (Z'd / Z'c) or difference (Z'd-Z'c). The battery characteristic data may also be data representing the correlation between the ratio (Z'd / Z'c) or difference (Z'd-Z'c) and the degradation state of the positive electrode. The correlation shown by the battery characteristic data is, for example, a characteristic like that shown in Figure 17 or Figure 18. The degradation state estimation unit 12 only needs to acquire the battery characteristic data from the memory 20. As a result, the degradation state estimation unit 12 can estimate the degradation state of the positive electrode based on the correlation between the ratio (Z'd / Z'c) or difference (Z'd-Z'c) stored in the memory 20 and the degradation state of the positive electrode. As a result, the degradation state of the positive electrode can be estimated in a short time.

[0112] Furthermore, as a fifth modification of this embodiment, the processor 10 may estimate the degradation state of the positive electrode based on the charge / discharge current when the ratio (Z'd / Z'c) or difference (Z'd-Z'c) exceeds a predetermined threshold. The embodiment of the fifth modification can be explained by replacing the ratio (Z"d / Z"c) or difference (Z"d-Z"c) in the embodiment of the third modification with the ratio (Z'd / Z'c) or difference (Z'd-Z'c).

[0113] 《Fourth Embodiment》 In the first embodiment, the difference in the behavior of the charging characteristics and discharge characteristics of the secondary battery 1 was identified from the difference between the DC resistance value (Rd) based on the voltage change before and after the start of discharge and the DC resistance value (Rc) based on the voltage change before and after the start of charging. In this embodiment, however, the difference is identified from the amplitude difference between the charging side and the discharge side in the response characteristics of the voltage response to the AC current input, or from the amplitude difference between the charging side and the discharge side in the response characteristics of the current response to the AC voltage input. The configuration of the internal state estimation system and the estimation mechanism of the positive electrode degradation state are the same as in the first embodiment, and their descriptions will be referenced. Furthermore, in the following description, the parts of the degradation state estimation method and degradation state estimation device that differ from the first embodiment will be mainly described, but other parts will refer to the descriptions of the first embodiment as appropriate.

[0114] The charge / discharge control unit 11 of the controller 5 inputs an AC current to the secondary battery 1 and measures the voltage response to the AC current input using the voltage sensor 3. The charge / discharge control unit 11 also inputs an AC voltage to the secondary battery 1 and measures the current response to the AC voltage input using the voltage sensor 3. The charge / discharge control unit 11 only needs to input either an AC current or an AC voltage to the secondary battery 1 and measures either the current response or the voltage response. The response characteristics of the voltage response to an AC current input or the current response to an AC voltage input are AC, and the positive and negative characteristics of the AC correspond to the charging and discharging characteristics of the secondary battery 1, respectively. That is, in the process of measuring the charging and discharging characteristics, the charge / discharge control unit 11 measures the response characteristics of the voltage response to an AC current input (hereinafter also referred to as "AC current-voltage response characteristics") or the response characteristics of the current response to an AC voltage input (hereinafter also referred to as "AC voltage-current response characteristics"). For example, the frequency of the AC current or AC voltage should be set to a low frequency band (e.g., 10 Hz or less).

[0115] Referring to Figure 19, we will explain how the degradation state of the positive electrode is reflected in the amplitude difference between the charging and discharging sides in the AC current-voltage response characteristics, and in the amplitude difference between the charging and discharging sides in the AC voltage-current response characteristics. Figure 19 is a graph of the AC current-voltage response characteristics. The horizontal axis represents time, and the vertical axis represents the charge / discharge current and voltage (battery voltage). The characteristics shown in Figure 19 are those of secondary battery 1 after the positive electrode has degraded, and the SOC is 70%. Also, graph h １ The characteristics of the input AC current are shown in graph h. ２ This shows the voltage response characteristics. Note that the voltage of 0 volts is the voltage before the input of AC current, and corresponds to the open-circuit voltage of secondary battery 1.

[0116] ΔVd_p is the discharge-side amplitude of the AC voltage-current response characteristic and corresponds to the difference between the AC voltage peak value on the discharge side and the open-circuit voltage (OCV). The AC voltage peak value on the discharge side is the lowest voltage in the AC voltage-current response characteristic and is the negative peak voltage. ΔVc_p is the charge-side amplitude of the AC voltage-current response characteristic and corresponds to the difference between the AC voltage peak value on the charge side and the open-circuit voltage (OCV). The AC voltage peak value on the charge side is the highest voltage in the AC voltage-current response characteristic and is the positive peak voltage.

[0117] Before the positive electrode deteriorates (when the positive electrode deterioration is minor), the discharge amplitude (ΔVd_p) and the charge amplitude (ΔVc_p) are approximately the same (ΔVd_p ≈ ΔVc_p). On the other hand, when the positive electrode deteriorates, the lithium ion transport resistance changes between charging and discharging, so as shown in Figure 19, the discharge amplitude (ΔVd_p) becomes larger than the charge amplitude (ΔVc_p) (ΔVd_p > ΔVc_p). In other words, after the positive electrode deteriorates, there is a change in the relationship between the voltage amplitudes on the discharge and charge sides compared to before the deterioration.

[0118] Figure 20 is a graph showing the correlation between the positive electrode degradation state (degree of positive electrode degradation) and the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p). The charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) is the ratio of the discharge-side amplitude (ΔVd_p) to the charge-side amplitude (ΔVc_p). When the AC current-voltage response characteristics are measured and the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) is calculated for secondary batteries 1 with different positive electrode degradation states, the characteristics shown in Figure 20 can be obtained. As shown in Figure 20, there is a correlation between the positive electrode degradation state and the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p), and in the range where the positive electrode degradation state is above a predetermined value (for example, 14%), the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) increases as the positive electrode degradation state increases. Since the positive electrode degradation state and the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) correspond to the amplitude difference between the charging and discharging sides in the AC current-voltage response characteristics, the positive electrode degradation state can be estimated based on the amplitude difference between the charging and discharging sides in the AC current-voltage response characteristics.

[0119] Note that the characteristics shown in Figures 19 and 20 are derived from the AC current-voltage response characteristics. Similarly, for the AC voltage-current response characteristics, the charge-discharge amplitude ratio increases as the positive electrode degradation increases. The charge-discharge amplitude ratio is the ratio of the discharge current amplitude to the charge current amplitude. The discharge current amplitude is the negative peak current value in the AC voltage-current response characteristics, and the charge current amplitude is the positive peak current value in the AC voltage-current response characteristics. Furthermore, the amplitude difference between the charge and discharge sides is not limited to the charge-discharge amplitude ratio, but may also be the difference between the discharge amplitude and the charge amplitude, and this difference may be a voltage difference (ΔVd_p - ΔVc_p) or a current difference.

[0120] Next, the method for estimating the degradation state of the positive electrode will be explained with reference to Figure 21. Figure 21 is a flowchart showing the procedure for the internal state estimation method according to this embodiment. Note that each control flow in the flowchart shown in Figure 21 is executed by the processor 10.

[0121] In step S31, with the secondary battery 1 connected to the charging device included in the load via the DC-DC converter 2, the charge / discharge control unit 11 charges or discharges the secondary battery 1 until the State of Charge (SOC) reaches a predetermined SOC (e.g., 70%). The predetermined SOC can be any value as long as it is an SOC in which a change occurs in the relationship between the voltage amplitudes of the discharge side and the charge side. If the secondary battery 1 is within the SOC range in which a change occurs in the relationship between the voltage amplitudes of the discharge side and the charge side, step S31 is unnecessary. In step S32, the charge / discharge control unit 11 measures the open-circuit voltage (OCV) of the secondary battery 1. In step S33, the charge / discharge control unit 11 inputs an alternating current (e.g., 50 mA, 1 Hz) to the secondary battery 1 in order to measure the charge / discharge characteristics of the secondary battery 1. The charge / discharge control unit 11 measures the alternating current-voltage response characteristics.

[0122] In step S34, the degradation state estimation unit 12 calculates the discharge-side amplitude (ΔVd_p) and the charge-side amplitude (ΔVc_p) in order to identify the amplitude difference between the charge-side and discharge-side in the AC current-voltage response characteristics. In step S35, the degradation state estimation unit 12 calculates the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) by dividing the charge-side amplitude (ΔVc_p) by the discharge-side amplitude (ΔVd_p).

[0123] In step S36, the degradation state estimation unit 12 estimates the degradation state of the positive electrode based on the amplitude difference between the charging side and the discharging side in the AC current-voltage response characteristics. Specifically, the degradation state estimation unit 12 compares the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) with an amplitude ratio threshold, and determines that the positive electrode has degraded if the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) is greater than or equal to the amplitude ratio threshold. On the other hand, if the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) is less than the amplitude ratio threshold, the degradation state estimation unit 12 determines that the positive electrode has not degraded. The degradation state estimation unit 12 may estimate the positive electrode degradation state according to the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p). As shown in Figure 20, the secondary battery 1 has a correlation such that the greater the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p), the greater the positive electrode degradation state. The degradation state estimation unit 12 may estimate the positive electrode degradation state such that the greater the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p), the greater the positive electrode degradation state.

[0124] In the control flow of step S36, the degradation state estimation unit 12 may estimate the degradation state of the positive electrode using battery characteristic data pre-stored in the memory 20. The battery characteristic data is data that represents the correlation between the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) and the degradation state of the positive electrode. The correlation can be determined experimentally. The degradation state estimation unit 12 acquires the battery characteristic data from the memory 20. Then, the degradation state estimation unit 12 calculates the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p), refers to the battery characteristic data, and estimates the degradation state of the positive electrode corresponding to the calculated charge-discharge amplitude ratio (ΔVd_p / ΔVc_p).

[0125] For example, suppose secondary battery 1 has characteristics similar to those shown in Figure 20, and the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) calculated in the control flow of step S35 is 1.060. In this case, in the graph shown in Figure 20, the positive electrode degradation state corresponding to the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) is approximately 15%. As a result, the degradation state estimation unit 12 can estimate the degradation state of the positive electrode based on the correlation between the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) and the degradation state of the positive electrode, which are pre-stored in the memory 20.

[0126] As described above, in this embodiment, the processor 10 measures the response characteristics of the voltage response to an AC current input or the current response to an AC voltage input in the process of measuring the charging and discharging characteristics, and estimates the degradation state of the positive electrode based on the amplitude difference between the charging side and the discharging side in the response characteristics. This makes it possible to estimate the degradation state of the positive electrode in a short time (for example, from tens of seconds to a few minutes).

[0127] In this embodiment, the processor 10 calculates the discharge amplitude (ΔVd_p) and the charge amplitude (ΔVc_p), and determines that the positive electrode has deteriorated if the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) is greater than or equal to a predetermined amplitude ratio threshold. There is a correlation between the deterioration state of the positive electrode and the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p). In this embodiment, this correlation can be used to determine whether or not the positive electrode has deteriorated. This makes it possible to estimate the deterioration state of the positive electrode in a short amount of time.

[0128] In this embodiment, the processor 10 refers to battery characteristic data, which is stored in the memory 20 in advance and represents the correlation between the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) and the degradation state of the positive electrode, and estimates the degradation state of the positive electrode corresponding to the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) calculated from the charge-discharge characteristics of the lithium-ion secondary battery. This allows the degradation state of the positive electrode to be estimated in a short time.

[0129] In this embodiment, the processor 10 sets the frequency of the AC current or AC voltage to below a predetermined frequency threshold. By setting the frequency of the AC current to a frequency band that is less affected by factors other than positive electrode degradation, the degradation state of the positive electrode can be estimated with greater accuracy.

[0130] As a sixth modification of this embodiment, alternating currents with different current values ​​may be input to the secondary battery 1, and the degradation state of the positive electrode may be estimated based on the response characteristics to multiple alternating currents. The control flow of the estimation method of this embodiment will be described below with reference to the control flow of the fourth embodiment (steps S31 to S35) and Figure 21. Figure 22 is a flowchart showing the procedure of the internal state estimation method according to the sixth modification of this embodiment. Each control flow in the flowchart shown in Figure 22 is executed by the processor 10.

[0131] Step S41 is the same as step S31 of the fourth embodiment. Step S42 is the same as step S32. In step S43, the charge / discharge control unit 11 inputs a first AC current (for example, 10 mA, 1 Hz) to the secondary battery 1 in order to measure the charge / discharge characteristics of the secondary battery 1. The charge / discharge control unit 11 measures the AC current-voltage response characteristics. In step S44, the degradation state estimation unit 12 calculates the discharge side amplitude (ΔVd1_p) and the charge side amplitude (ΔVc1_p) in order to identify the amplitude difference between the charge side and the discharge side in the AC current-voltage response characteristics. In step S45, the degradation state estimation unit 12 calculates the charge / discharge amplitude ratio (ΔVd1_p / ΔVc1_p) by dividing the charge side amplitude (ΔVc1_p) by the discharge side amplitude (ΔVd1_p).

[0132] In step S46, the degradation state estimation unit 12 compares the calculated charge-discharge amplitude ratio (ΔVd1_p / ΔVc1_p) with the amplitude ratio threshold (ΔVd / c_th) and determines whether the charge-discharge amplitude ratio (ΔVd1_p / ΔVc1_p) is less than the amplitude ratio threshold (ΔVd / c_th). The amplitude ratio threshold (ΔVd / c_th) is a predetermined threshold according to the battery characteristics of the secondary battery 1. If the charge-discharge amplitude ratio (ΔVd1_p / ΔVc1_p) is greater than or equal to the amplitude ratio threshold (ΔVd / c_th1), the control flow processed by the processor 10 proceeds to step S53.

[0133] If the charge-discharge amplitude ratio (ΔVd1_p / ΔVc1_p) is less than the amplitude ratio (ΔVd / c_th1), in step S47, the charge-discharge control unit 11 inputs a second AC current (for example, 50 mA, 1 Hz) to the secondary battery 1 in order to measure the charge-discharge characteristics of the secondary battery 1. In step S48, the degradation state estimation unit 12 calculates the discharge-side amplitude (ΔVd2_p) and the charge-side amplitude (ΔVc2_p) in order to identify the amplitude difference between the charge-side and discharge-side in the AC current-voltage response characteristics. In step S49, the degradation state estimation unit 12 calculates the charge-discharge amplitude ratio (ΔVd2_p / ΔVc2_p) by dividing the charge-side amplitude (ΔVc2_p) by the discharge-side amplitude (ΔVd2_p).

[0134] In step S50, the degradation state estimation unit 12 compares the calculated charge-discharge amplitude ratio (ΔVd2_p / ΔVc2_p) with the amplitude ratio threshold (ΔVd / c_th2) and determines whether the charge-discharge amplitude ratio (ΔVd2_p / ΔVc2_p) is less than the amplitude ratio threshold (ΔVd / c_th2). If the charge-discharge amplitude ratio (ΔVd2_p / ΔVc2_p) is less than the amplitude ratio threshold (ΔVd / c_th2), the control flow processed by the processor 10 proceeds to step S51. If the charge-discharge amplitude ratio (ΔVd2_p / ΔVc2_p) is greater than or equal to the amplitude ratio threshold (ΔVd / c_th2), the control flow processed by the processor 10 proceeds to step S52.

[0135] In the control flow of steps S51 to S53, the degradation state estimation unit 12 estimates the degradation state of the positive electrode based on the charge-discharge amplitude ratio (ΔVd1_p / ΔVc1_p, ΔVd2_p / ΔVc2_p).

[0136] Referring to Figure 23, the correlation between the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) and the positive electrode degradation state will be explained. Figure 23 is a graph showing the correlation between the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) and the positive electrode degradation state. Graph i in Figure 23 １ This shows the characteristics of the charge-discharge amplitude ratio (ΔVd1_p / ΔVc1_p), and graph i ２ This shows the characteristics of the charge-discharge amplitude ratio (ΔVd²_p / ΔVc²_p). The characteristics shown in Figure 23 can be obtained from a prototype of secondary battery 1, which will be described in the embodiment described later.

[0137] Graph i １ As shown, in the range where the positive electrode degradation state is greater than 14%, the correlation between the charge-discharge amplitude ratio (ΔVd1_p / ΔVc1_p) and the positive electrode degradation state is such that the greater the charge-discharge amplitude ratio (ΔVd1_p / ΔVc1_p), the greater the positive electrode degradation state. On the other hand, in the range where the positive electrode degradation state is 14% or less, the relationship is not such that the greater the charge-discharge amplitude ratio (ΔVd1_p / ΔVc1_p), the greater the positive electrode degradation state. The degradation state estimation unit 12 calculates the charge-discharge amplitude ratio (ΔVd1_p / ΔVc1_p) from the response characteristics of the voltage response to the first AC current, and estimates whether the positive electrode degradation state is in the range greater than 14% according to the comparison result between the charge-discharge amplitude ratio (ΔVd1_p / ΔVc1_p) and the amplitude ratio threshold (ΔVd / c_th1).

[0138] Also, graph i ２ As shown, in the range where the positive electrode degradation state is greater than 10%, the correlation between the charge-discharge amplitude ratio (ΔVd1_p / ΔVc1_p) and the positive electrode degradation state is such that the greater the charge-discharge amplitude ratio (ΔVd1_p / ΔVc1_p), the greater the positive electrode degradation state. The degradation state estimation unit 12 calculates the charge-discharge amplitude ratio (ΔVd2_p / ΔVc2_p) from the response characteristics of the voltage response to the second AC current, and estimates whether the positive electrode degradation state is greater than 10% but less than 14%, or whether the positive electrode degradation state is 10% or less, based on the comparison result between the charge-discharge amplitude ratio (ΔVd2_p / ΔVc2_p) and the amplitude ratio threshold (ΔVd / c_th2).

[0139] In modified example 6, the amplitude ratio threshold (ΔVd / c_th1) and the amplitude ratio threshold (ΔVd / c_th2) are set to 1.045. Referring to Figure 22, if the charge-discharge amplitude ratio (ΔVd2_p / ΔVc2_p) is less than the amplitude ratio threshold (ΔVd / c_th2), the degradation state estimation unit 12 determines that the positive electrode degradation state is in the range of 10% or less (step S51). If the charge-discharge amplitude ratio (ΔVd2_p / ΔVc2_p) is greater than or equal to the amplitude ratio threshold (ΔVd / c_th2), the degradation state estimation unit 12 determines that the positive electrode degradation state is greater than 10% but less than 14% (step S52). Furthermore, if the charge-discharge amplitude ratio (ΔVd1_p / ΔVc1_p) is greater than or equal to the amplitude ratio threshold (ΔVd / c_th1), the degradation state estimation unit 12 estimates that the positive electrode degradation state is greater than 14% (step S53).

[0140] In Modification 6 of this embodiment, the processor 10 calculates the discharge-side amplitude (ΔVd1_p) and the charge-side amplitude (ΔVc1_p) based on the response characteristics measured when a first AC current is input to the secondary battery 1, and calculates the discharge-side amplitude (ΔVd2_p) and the charge-side amplitude (ΔVc2_p) based on the response characteristics measured when a second AC current is input to the secondary battery 1. Based on the charge-discharge amplitude ratio (ΔVd1_p / ΔVc1_p) and the charge-discharge amplitude ratio (ΔVd2_p / ΔVc2_p), the processor 10 estimates the degradation state of the positive electrode. By calculating the charge-discharge amplitude ratio (ΔVd1_p / ΔVc1_p, ΔVd2_p / ΔVc2_p) at current values ​​under multiple conditions, the degradation state of the positive electrode can be quantitatively grasped. Furthermore, the discharge-side amplitude (ΔVd1_p) corresponds to the "first discharge-side amplitude (ΔVd1_p)" of the present invention, the charge-side amplitude (ΔVc1_p) corresponds to the "first charge-side amplitude (ΔVd1_p)" of the present invention, the discharge-side amplitude (ΔVd2_p) corresponds to the "second discharge-side amplitude (ΔVd2_p)" of the present invention, and the charge-side amplitude (ΔVc2_p) corresponds to the "second charge-side amplitude (ΔVd2_p)" of the present invention.

[0141] The present invention will be described below based on more detailed examples, but the present invention is not limited to these examples.

[0142] <<Example 1>> <Preparation of Prototype (Example)> A negative electrode layer was created by using graphite carbon (Gr) as the negative electrode active material and styrene butadiene rubber (SBR) as the binder (binding agent), and coating the resulting slurry onto copper foil. A positive electrode layer was created by using ternary cathode material (NMC) as the positive electrode active material, carbon black as the conductive additive, and PVDF as the binder (binding agent), and coating the resulting slurry onto aluminum foil. A polyethylene (PE) film was used as the separator. A mixed solvent of ethylene carbonate (EC) / diethyl carbonate (DEC) / ethyl methyl carbonate (EMC) was used as the electrolyte, with lithium hexafluoride phosphate (LiPF) added. ６ A solution of ) was used. A test cell (prototype) was created by layering the negative electrode layer, separator, and positive electrode layer, sandwiching them between aluminum laminates, injecting the electrolyte solution into the interior, and sealing the area around the laminates.

[0143] <Measurement Method> [SOC Adjustment] The following charge and discharge procedures were performed to adjust the SOC of the prototype before degradation (SOH: 100%) and the prototype after degradation (SOH: 71%). First, the battery was discharged at a discharge current of 0.2C until the battery voltage reached 2.5V. After discharge, it was charged at a charge current of 0.2C until the voltage corresponding to 50% SOC was reached. After reaching the target voltage corresponding to 50% SOC, the charging was switched to constant voltage charging, and charging was stopped when the charge current dropped to 0.02C. [Resistance Measurement] The DC resistance (Rd, Rc) of the prototype before degradation and the prototype after degradation was measured using the following measurement method. After more than one minute had elapsed since the end of the charging for SOC adjustment as described above, charge and discharge procedures were performed in the following order: "discharge at 18mA for 20 seconds", "rest for 10 minutes", "charge at 18mA for 20 seconds", and "rest for 10 minutes". The absolute value of the voltage difference between 1 second before the start of discharge and 5 seconds after the start of discharge (ΔVd) and the absolute value of the voltage difference between 1 second before the start of charging and 5 seconds after the start of charging (ΔVc) were calculated. Furthermore, the discharge-side DC resistance (Rd) was calculated by dividing ΔVd by the discharge current value, and the charging-side DC resistance (Rc) was calculated by dividing ΔVc by the charging current value.

[0144] <Evaluation of Example 1> When the DC resistances (Rd, Rc) on the charging and discharging sides obtained from the evaluation results of the prototype before degradation were plotted, the characteristics shown in Figure 3B(a) were obtained. Furthermore, when the DC resistances (Rd, Rc) on the charging and discharging sides obtained from the evaluation results of the prototype after degradation were plotted, the characteristics shown in Figure 3B(b) were obtained.

[0145] <<Example 2>> In Example 2, a prototype similar to that in Example 1 was created. The prototype was repeatedly charged and discharged a predetermined number of times to allow the degradation state to progress. Then, the same SOC adjustment as in Example 1 was performed on the prototype, and the discharge-side DC resistance (Rd) and charge-side DC resistance (Rc) were calculated by the same resistance measurement. Furthermore, in Example 2, the DC resistance ratio (Rd / Rc) was calculated by dividing the discharge-side DC resistance (Rd) by the charge-side DC resistance (Rc). <<Evaluation of Example 2>> The DC resistance ratio (Rd / Rc) of the prototype obtained from the measurement method was 1.04. When the characteristics of the DC resistance ratio for the positive electrode degradation state were experimentally determined by repeatedly charging and discharging another secondary battery created using the same method as the prototype to reduce the positive electrode capacity, the characteristics shown in Figure 5 were obtained. Based on the graph in Figure 5, the DC resistance ratio (Rd / Rc = 1.04) of the prototype in Example 2 indicates that the positive electrode degradation state (capacity reduction rate) was approximately 14%. The correlation between the positive electrode degradation state and the DC resistance ratio varies depending on the battery temperature, the charge / discharge current value, and the elapsed time after the start of charge / discharge (how many seconds later the voltage is used to calculate the resistance). Therefore, it is necessary to either understand the relationship under each condition in advance, or calculate the DC resistance using the measurement method described above under conditions that are already known.

[0146] <<Example 3>> In Example 3, a prototype similar to that in Example 1 was created. The prototype was repeatedly charged and discharged a predetermined number of times to allow the degradation state to progress, and then the same SOC adjustment as in Example 1 was performed on the prototype. After the SOC adjustment, the charge and discharge current (7.2 mA) was set different from that used for the resistance measurement in Example 1, and the discharge-side DC resistance (Rd) and charge-side DC resistance (Rc) were calculated using the same method as for the resistance measurement in Example 1, with all other conditions being the same. In Example 3, the DC resistance ratio (Rd / Rc) was calculated by dividing the discharge-side DC resistance (Rd) by the charge-side DC resistance (Rc). （７．２ｍＡ）The DC resistance ratio (Rd / Rc) was calculated. （７．２ｍＡ） After calculating the DC resistance (Rd / Rc), the charge / discharge current (18 mA) was applied to the prototype in the same manner as the resistance measurement in Example 1, and the discharge-side DC resistance (Rd) and charge-side DC resistance (Rc) were calculated using the same method as the resistance measurement in Example 1, with the other conditions being the same. （１８ｍＡ） ) was calculated. <Evaluation of Example 3> For another secondary battery created using the same method as the prototype, the DC resistance ratio characteristics with respect to the positive electrode degradation state were experimentally determined while reducing the positive electrode capacity by repeatedly performing charge and discharge. However, the DC resistance ratio was calculated from the charge and discharge characteristics when charged and discharged with a charge and discharge current (7.2 mA) and when charged and discharged with a charge and discharge current (18 mA). From the battery data obtained experimentally in this way, the characteristics shown in Figure 7 were obtained. Note that graph e in Figure 7 １ The graph shows the characteristics when the prototype is charged and discharged at a current of 7.2 mA. ２ This shows the characteristics when secondary battery 1 is charged with a current value (18 mA). The DC resistance ratio (Rd / Rc) of the prototype obtained from the measurement method is shown. （７．２ｍＡ） The DC resistance ratio (Rd / Rc) of the prototype in Example 3 was less than 1.07. Based on the graph in Figure 7, the DC resistance ratio (Rd / Rc) of the prototype in Example 3 was less than 1.07. （７．２ｍＡ） From <1.07), it was determined that the degradation state of the positive electrode (capacity reduction rate) was less than approximately 20%. Furthermore, the DC resistance ratio (Rd / Rc) of the prototype obtained from the measurement method was （１８ｍＡ） The DC resistance ratio (Rd / Rc) of the prototype in Example 3 was 1.04 or higher. Based on the graph in Figure 7, the DC resistance ratio (Rd / Rc) of the prototype in Example 3 was （１８ｍＡ）>1.04) From this, it was determined that the degradation state of the positive electrode (capacity reduction rate) was greater than approximately 14%. From these two determination results, an evaluation result was obtained that the degradation state of the positive electrode of the prototype was within the range of 14% to 20%. <<Example 4>> In Example 4, a prototype similar to that of Example 1 was created. SOC adjustment was performed on the prototype before degradation (SOH: 100%) and the prototype after degradation (SOH: 71%). The SOC adjustment was the same as in Example 1. <Measurement method> [Voltage change range measurement] The voltage change range (ΔVd, ΔVc) of the prototype before degradation and the prototype after degradation was measured using the following measurement method. After more than one minute had elapsed from the end of charging for the SOC adjustment as described above, charging and discharging were performed in the following order: "discharge at 18mA for 20 seconds", "rest for 10 minutes", "charge at 18mA for 20 seconds", and "rest for 10 minutes". The absolute value of the voltage difference between 1 second before the end of discharge and 5 seconds after the end of discharge (ΔVd), and the absolute value of the voltage difference between 1 second before the end of charging and 5 seconds after the end of charging (ΔVc) were calculated.

[0147] <Evaluation of Example 4> When the voltage change ranges (ΔVd, ΔVc) on the charging and discharging sides obtained from the evaluation results of the prototype before degradation were plotted, the characteristics shown in Figure 8(a) were obtained. Furthermore, when the voltage change ranges (ΔVd, ΔVc) on the charging and discharging sides obtained from the evaluation results of the prototype after degradation were plotted, the characteristics shown in Figure 8(b) were obtained.

[0148] <<Example 5>> In Example 5, a prototype similar to that in Example 4 was created. The prototype was repeatedly charged and discharged a predetermined number of times to allow the degradation state to progress. Then, the same SOC adjustment as in Example 4 was performed on the prototype, and the voltage change on the discharge side (ΔVd) and the voltage change on the charging side (ΔVc) were calculated using the same voltage change measurement method. Furthermore, in Example 5, the voltage change ratio (ΔVd / ΔVc) was calculated by dividing the voltage change (ΔVd) by the voltage change (ΔVc). <Evaluation of Example 5> The voltage change ratio (ΔVd / ΔVc) of the prototype obtained from the measurement method was 1.04. When the characteristics of the voltage change ratio for the positive electrode degradation state were experimentally determined by repeatedly charging and discharging another secondary battery created using the same method as the prototype to reduce the positive electrode capacity, the characteristics shown in Figure 5 were obtained. However, the vertical axis of the characteristics in Figure 5 is the voltage change ratio (ΔVd / ΔVc). Based on the graph in Figure 5 (with the vertical axis representing the voltage change ratio), the voltage change ratio of the prototype in Example 2 (ΔVd / ΔVc = 1.04) indicates that the degradation state of the positive electrode (capacity reduction rate) was approximately 14%. <<Example 6>> In Example 6, a prototype similar to that in Example 1 was created. The prototype was repeatedly charged and discharged a predetermined number of times to allow the degradation state to progress, and then the same SOC adjustment as in Example 4 was performed on the prototype. After the SOC adjustment, a different charge / discharge current (7.2 mA) was used for the prototype compared to the voltage change measurement in Example 4, and other conditions were the same as for the voltage change measurement in Example 4. The voltage change before and after stopping the discharge (ΔVd) and the voltage change before and after stopping the charge (ΔVc) were calculated, respectively. In Example 6, the voltage change ratio (ΔVd / ΔVc) was calculated by dividing the voltage change before and after stopping the discharge (ΔVd) by the voltage change after stopping the charge (ΔVc). （７．２ｍＡ） The voltage change ratio (ΔVd / ΔVc) was calculated. （７．２ｍＡ） After calculating the voltage change ratio (ΔVd / ΔVc), the prototype was subjected to the same charge / discharge current (18mA) as in the resistance measurement in Example 4, and the other conditions were the same as in the voltage change measurement in Example 4. The voltage change ratio (ΔVd / ΔVc) was then calculated, and the voltage change ratio (ΔVd / ΔVc) was calculated. （１８ｍＡ）) was calculated. <Evaluation of Example 6> For another secondary battery made using the same method as the prototype, the characteristics of the voltage change ratio with respect to the positive electrode degradation state were experimentally determined while reducing the positive electrode capacity by repeatedly performing charge and discharge. However, the voltage change ratio was calculated from the charge and discharge characteristics when charged and discharged with a charge and discharge current (7.2 mA) and when charged and discharged with a charge and discharge current (18 mA). From the battery data obtained experimentally in this way, the characteristics shown in Figure 7 were obtained. However, the vertical axis of the characteristics in Figure 5 is the voltage change ratio (ΔVd / ΔVc). The voltage change ratio (ΔVd / ΔVc) of the prototype obtained from the measurement method （７．２ｍＡ） The voltage change ratio (ΔVd / ΔVc) of the prototype in Example 3 was less than 1.07. Based on the graph in Figure 7, the voltage change ratio (ΔVd / ΔVc) of the prototype in Example 3 was less than 1.07. （７．２ｍＡ） From the value (= 1.07), it was determined that the degradation state of the positive electrode (capacity reduction rate) was less than approximately 20%. Furthermore, the voltage change ratio (ΔVd / ΔVc) of the prototype obtained from the measurement method was （１８ｍＡ） The voltage change ratio (ΔVd / ΔVc) of the prototype in Example 3 was 1.04 or higher. Based on the graph in Figure 7, the voltage change ratio (ΔVd / ΔVc) of the prototype in Example 3 was （１８ｍＡ） From the value (= 1.04), it was determined that the degradation state of the positive electrode (capacity reduction rate) was greater than approximately 14%. Based on these two determination results, an evaluation result was obtained that the degradation state of the positive electrode of the prototype was within the range of 14% to 20%.

[0149] <<Example 7>> In Example 7, a prototype similar to that in Example 1 was created.

[0150] <Measurement Method> [SOC Adjustment (1st time)] The following charge and discharge procedures were performed to adjust the SOC of the prototype before degradation (SOH: 100%) and the prototype after degradation (SOH: 81%, 84%). First, the battery was discharged at a discharge current value (0.2C) until the battery voltage reached 2.5V. After discharge, it was charged at a charge current value (0.2C) until the voltage corresponding to 30% SOC was reached. After reaching the target voltage corresponding to 30% SOC, it was switched to constant voltage charging, and charging was stopped when the charge current value dropped to 0.02C. [Charging EIS Measurement] Charging was started at 0.1C, and the AC impedance during charging was measured by measuring EIS during charging. At this time, the frequency was changed from 3Hz to 0.3Hz, and the AC impedance was measured at 6 points, and the current amplitude was set to 10.8mA. After the charging EIS measurement was completed, charging was stopped. [SOC Adjustment (2nd time)] The battery was discharged at a discharge current value (0.2C) until the target voltage corresponding to 30% SOC was reached. After reaching the target voltage corresponding to 30% SOC, the system transitioned to constant voltage discharge, and the discharge was terminated when the charging current value dropped to 0.02C. [Discharge EIS Measurement] Discharge was started at 0.1C, and the AC impedance during discharge was measured by EIS measurement. At this time, the frequency was varied from 3Hz to 0.3Hz, and the AC impedance was measured at six points, while the current amplitude was set to 10.8mA. After the discharge EIS measurement was completed, the discharge was stopped. From the absolute value of the impedance obtained by the EIS measurement, the ratio (Z"d / Z"c") and the difference (Z"d-Z"c") were calculated.

[0151] <Evaluation of Example 7> When the ratio (Z"d / Z"c") and difference (Z"d-Z"c) obtained from the evaluation results of the prototype were plotted, the characteristics shown in Figures 12 and 13 were obtained.

[0152] <<Example 8>> In Example 8, a prototype similar to that in Example 1 was created. The prototype was measured using the measurement method of Example 7, with the following modifications: For SOC adjustment (1st and 2nd time), the target voltage was set to a voltage corresponding to 70% SOC. For charging EIS measurement and discharging EIS measurement, the frequency was set to 1.9 Hz and the AC impedance was measured.

[0153] <Evaluation of Example 8> The imaginary component ratio (Z"d / Z"c) of the AC impedance of the prototype obtained from the measurement method was 1.00. For another secondary battery made using the same method as the prototype, the characteristics of the ratio (Z"d / Z"c) to the positive electrode degradation state were experimentally determined while the positive electrode capacity was reduced by repeatedly charging and discharging, and the characteristics shown in Figure 15 were obtained. Based on the graph in Figure 15, the positive electrode degradation state (capacity reduction rate) of the prototype in Example 8 (Z"d / Z"c = 1.00) was approximately 14%.

[0154] <<Example 9>> In Example 9, a prototype similar to that in Example 1 was created. <Measurement Method> [SOC Adjustment] The following charge and discharge procedure was performed on the deteriorated prototype to adjust the SOC. First, the battery was discharged at a discharge current value (0.2C) until the battery voltage reached 2.5V. After discharge, it was charged at a charge current value (0.2C) until the voltage corresponding to 70% SOC was reached. After reaching the target voltage corresponding to 70% SOC, the charging was switched to constant voltage charging, and charging was stopped when the charge current value dropped to 0.02C. [OCV Measurement] OCV measurement was performed for 20 minutes to obtain the open-circuit voltage (OCV) before AC input. [Calculation of Charge-Discharge Amplitude Ratio] An AC current of 50mA 1Hz was input to the prototype. From the voltage response characteristics to the AC current, the peak AC voltage value on the discharge side and the peak AC voltage value on the charging side were calculated, and the difference in OCV from each peak AC voltage value was taken to calculate the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p).

[0155] <Evaluation of Example 9> When the AC current characteristics and voltage response characteristics obtained from the evaluation results of the prototype were plotted, the characteristics shown in Figure 19 were obtained. For another secondary battery created using the same method as the prototype, the positive electrode capacity was reduced by repeatedly charging and discharging it, and the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) was calculated using the above measurement method, and a threshold lower than the calculated value was set. From the evaluation results of the prototype, the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) was above the threshold, confirming the degradation of the positive electrode. Furthermore, the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) of the prototype obtained from the measurement method was 1.047. Based on the graph in Figure 20, the positive electrode degradation state (capacity reduction rate) of the prototype in Example 9, based on the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p = 1.047), was approximately 14%.

[0156] In Examples 1 to 9 above, the measurement method may be performed at any battery temperature, but accuracy can be improved if the change in battery temperature during measurement is small. Also, if the measurement is performed immediately after rapid charging, it is best to wait until the battery temperature stabilizes.

[0157] Furthermore, the target SOC value in the above SOC adjustment can be set arbitrarily. For example, performing the adjustment within the range of 30% to 70% SOC allows for accurate measurement because the correlation between the parameter showing the difference in behavior between charging and discharging characteristics and the positive electrode degradation state is less affected by other degradation factors. If the secondary battery is already within such an SOC range, the above SOC adjustment may be omitted. In addition to discharging to 2.5V and then charging to adjust the SOC as described above, the SOC can also be adjusted by discharging or charging from a certain SOC to the desired SOC. Also, the constant voltage charging (or constant voltage discharging) performed in the above SOC adjustment may be omitted.

[0158] Furthermore, the above measurement method may start with discharge or with charge. The charging current and discharge current values ​​can be any values, but it is preferable that the charging current and discharge current values ​​be the same. In addition, the larger the current value, the more accurately the degradation state of the positive electrode can be estimated. The pause time in the measurement method, or the pause time before starting the resistance measurement, can be of any length, but by providing a pause time until the voltage change after the pause is as small as possible, the degradation state can be estimated with greater accuracy.

[0159] In the above embodiment, the correlation between the voltage change ratio (ΔVd / ΔVc) or ratio (Z"d / Z"c) and the degree of positive electrode degradation varies depending on the temperature, the charge / discharge current value, and the elapsed time after the charge / discharge has stopped (how many seconds later the voltage is used to calculate ΔV). The relationship under each condition should be understood in advance, and measurements should be taken based on the conditions that have been understood in advance.

[0160] Furthermore, in Examples 7 to 9, the set frequency can be any frequency, but by setting it to, for example, 10 Hz or lower, the correlation of the positive electrode degradation degree is less affected by other degradation factors, and can be estimated with high accuracy.

[0161] In Example 9, the parameter indicating the difference in the behavior of the charging and discharging characteristics of the secondary battery 1 is not limited to the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) as in Example 9, but may also be the difference between the discharge amplitude (ΔVd_p) and the charging amplitude (ΔVc_p) (ΔVd_p - ΔVc_p). Furthermore, the threshold value should be set according to the parameter index and should be determined according to the tolerance of degradation and the accuracy of estimating the degradation state.

[0162] In Example 9, the parameter indicating the difference in the behavior of the charging and discharging characteristics of the secondary battery 1 may be the ratio (Δtd / Δtc) or time difference (Δtd-Δtc) of the time difference (Δtd) between the time corresponding to the peak value of the AC current during discharge and the time corresponding to the peak value of the AC voltage during discharge, and the time difference (Δtc) between the time corresponding to the peak value of the AC current during charging and the time corresponding to the peak value of the AC voltage. The threshold value should be set according to the index of the parameter.

[0163] 1...Secondary battery 2...DC-DC converter 3...Voltage sensor 4...Current sensor 5...Controller 10...Processor 11...Charge / discharge control unit 12...Degradation state estimation unit 20...Memory

Claims

A method for estimating the internal state of a lithium-ion secondary battery, which is executed by a processor, The aforementioned processor, The charging characteristics and discharge characteristics of the lithium-ion secondary battery are measured. An internal state estimation method for estimating the degradation state of the positive electrode of a lithium-ion secondary battery based on the difference in behavior between the charging characteristics and the discharging characteristics. 　 An internal state estimation method according to claim 1, The aforementioned processor, Based on the discharge characteristics, the DC resistance value (Rd) is calculated from the voltage change and discharge current at a certain time after the start of discharge of the lithium-ion secondary battery. Based on the charging characteristics described above, the DC resistance value (Rc) is calculated from the voltage change and charging current at a certain time after the start of charging of the lithium-ion secondary battery. An internal state estimation method for estimating the deterioration state of the positive electrode based on the difference between the DC resistance value (Rd) and the DC resistance value (Rc). 　 An internal state estimation method according to claim 2, The aforementioned processor, An internal state estimation method for determining that the positive electrode has deteriorated when the DC resistance ratio (Rd / Rc), which is the ratio of the DC resistance value (Rd) to the DC resistance value (Rc), is greater than or equal to a predetermined DC resistance ratio threshold. 　 An internal state estimation method according to claim 2, The aforementioned processor, The system refers to battery characteristic data that shows the correlation between the DC resistance ratio (Rd / Rc), which is the ratio of the DC resistance value (Rd) to the DC resistance value (Rc), and the degradation state of the positive electrode, which are stored in memory beforehand. An internal state estimation method for estimating the degradation state of the positive electrode corresponding to the DC resistance ratio (Rd / Rc) calculated from the charge-discharge characteristics of the lithium-ion secondary battery. 　 An internal state estimation method according to claim 2, The aforementioned processor, Based on the discharge characteristics when the lithium-ion secondary battery is discharged with a first current, the DC resistance value (Rd) is calculated as the first DC resistance value (Rd_1), Based on the charging characteristics when the lithium-ion secondary battery is charged with the first current, the DC resistance value (Rc) is calculated as the first DC resistance value (Rc_1), Based on the discharge characteristics when the lithium-ion secondary battery is discharged with a second current, the DC resistance value (Rd) is calculated as the second DC resistance value (Rd_2), Based on the charging characteristics when the lithium-ion secondary battery is charged with the second current, the DC resistance value (Rc) is calculated as the second DC resistance value (Rc_2), An internal state estimation method for estimating the deterioration state of the positive electrode based on a first DC resistance ratio (Rd_1 / Rc_1) which represents the ratio of the DC resistance value (Rd_1) to the DC resistance value (Rc_1), and a second DC resistance ratio (Rd_2 / Rc_2) which represents the ratio of the second DC resistance value (Rd_2) to the second DC resistance value (Rc_2). 　 An internal state estimation method according to claim 1, The aforementioned processor, Based on the discharge characteristics, the voltage change (ΔVd) at a certain time after the discharge of the lithium-ion secondary battery has stopped is calculated. Based on the charging characteristics, the voltage change (ΔVc) at a certain time after the charging of the lithium-ion secondary battery has stopped is calculated. An internal state estimation method for estimating the degradation state of the positive electrode based on the difference between the voltage change range (ΔVd) and the voltage change range (ΔVc). 　 The method for estimating the internal state according to claim 6, The aforementioned processor, An internal state estimation method for determining that the positive electrode has deteriorated when the voltage change ratio (ΔVd / ΔVc), which is the ratio of the voltage change amount (ΔVd) to the voltage change amount (ΔVc), is greater than or equal to a predetermined voltage change ratio threshold. 　 The method for estimating the internal state according to claim 6, The aforementioned processor, The system refers to battery characteristic data that shows the correlation between the voltage change ratio (ΔVd / ΔVc), which is the ratio of the voltage change amount (ΔVd) to the voltage change amount (ΔVc), and the degradation state of the positive electrode, which are stored in memory beforehand. An internal state estimation method for estimating the degradation state of the positive electrode corresponding to the voltage change ratio (ΔVd / ΔVc) calculated from the characteristics of the lithium-ion secondary battery after charging and discharging has stopped. 　 The method for estimating the internal state according to claim 6, The aforementioned processor, Based on the discharge characteristics when the lithium-ion secondary battery is discharged with a first current, the voltage change range (ΔVd) is calculated as the first voltage change range (ΔVd_1). Based on the charging characteristics when the lithium-ion secondary battery is charged with the first current, the voltage change range (ΔVc) is calculated as the first voltage change range (ΔVc_1), Based on the discharge characteristics when the lithium-ion secondary battery is discharged with a second current, the voltage change (ΔVd) is calculated as the second voltage change (ΔVd_2), Based on the charging characteristics when the lithium-ion secondary battery is charged with the second current, the voltage change range (ΔVc) is calculated as the second voltage change range (ΔVc_2), An internal state estimation method for estimating the degradation state of the positive electrode based on a first voltage change ratio (ΔVd_1 / ΔVc_1) which shows the ratio of the voltage change amount (ΔVd_1) to the voltage change amount (ΔVc_1), and a second voltage change ratio (ΔVd_2 / ΔVc_2) which shows the ratio of the voltage change amount (ΔVd_2) to the voltage change amount (ΔVc_2). 　 An internal state estimation method according to claim 1, The aforementioned processor, The AC impedance (Zd) during discharge of the aforementioned lithium-ion secondary battery is calculated, The AC impedance (Zc) during charging of the aforementioned lithium-ion secondary battery is calculated, An internal state estimation method for estimating the degradation state of the positive electrode based on the difference between the AC impedance (Zd) and the AC impedance (Zc). 　 An internal state estimation method according to claim 10, The aforementioned processor, An internal state estimation method for determining that the positive electrode has deteriorated when the ratio (Z"d / Z"c) of the imaginary component (Z"d) of the AC impedance (Zd) and the imaginary component (Z"c) of the AC impedance (Zc) is greater than or equal to a predetermined imaginary component ratio threshold. 　 An internal state estimation method according to claim 10, The aforementioned processor, By referring to battery characteristic data, which is stored in memory beforehand, that shows the correlation between the ratio (Z"d / Z"c") of the imaginary component (Z''d) of the AC impedance (Zd) and the imaginary component (Z''c) of the AC impedance (Zc), and the degradation state of the positive electrode, An internal state estimation method for estimating the degradation state of the positive electrode corresponding to the ratio (Z"d / Z"c) calculated from the AC impedance (Zd) and AC impedance (Zc) of the lithium-ion secondary battery. 　 An internal state estimation method according to claim 11, The aforementioned processor, An internal state estimation method for estimating the degradation state of the positive electrode based on the charge / discharge current when the ratio (Z"d / Z"c) is greater than or equal to the imaginary component ratio threshold. 　 An internal state estimation method according to claim 10, The aforementioned processor, An internal state estimation method for determining that the positive electrode has deteriorated when the difference (Z"d - Z"c) between the imaginary component (Z"d) of the AC impedance (Zd) and the imaginary component (Z"c) of the AC impedance (Zc) is greater than or equal to a predetermined imaginary component difference threshold. 　 An internal state estimation method according to claim 10, The aforementioned processor, By referring to battery characteristic data, which is stored in memory beforehand, that shows the correlation between the difference (Z"d - Z"c) between the imaginary component (Z''d) of the AC impedance (Zd) and the imaginary component (Z''c) of the AC impedance (Zc), and the degradation state of the positive electrode, An internal state estimation method for estimating the degradation state of the positive electrode corresponding to the calculated difference (Z"d - Z"c). 　 An internal state estimation method according to claim 14, The aforementioned processor, An internal state estimation method for estimating the degradation state of the positive electrode based on the charge / discharge current when the difference (Z"d - Z"c) is greater than or equal to the imaginary component difference threshold. 　 An internal state estimation method according to claim 10, The aforementioned processor, An internal state estimation method for determining that the positive electrode has deteriorated when the ratio (Z'd / Z'c) of the real component (Z'd) of the AC impedance (Zd) and the real component (Z'c) of the AC impedance (Zc) is greater than or equal to a predetermined real component ratio threshold. 　 An internal state estimation method according to claim 10, The aforementioned processor has previously stored in memory, Referring to battery characteristic data that shows the correlation between the ratio (Z'd / Z'c) of the real component (Z'd) of the AC impedance (Zd) and the real component (Z'c) of the AC impedance (Zc), An internal state estimation method for estimating the degradation state of the positive electrode corresponding to the ratio (Z'd / Z'c) calculated from the AC impedance (Zd) and AC impedance (Zc) of the lithium-ion secondary battery. 　 An internal state estimation method according to claim 17, The aforementioned processor, An internal state estimation method for estimating the degradation state of the positive electrode based on the charge / discharge current when the ratio (Z'd / Z'c) is greater than or equal to the real component ratio threshold. 　 An internal state estimation method according to claim 10, The aforementioned processor, An internal state estimation method for determining that the positive electrode has deteriorated when the difference (Z'd - Z'c) between the real component (Z'd) of the AC impedance (Zd) and the real component (Z'c) of the AC impedance (Zc) is greater than or equal to a predetermined real component difference threshold. 　 An internal state estimation method according to claim 10, The aforementioned processor has previously stored in memory, An internal state estimation method for a lithium-ion secondary battery, which estimates the degradation state of the positive electrode corresponding to the difference (Z'd - Z'c) calculated from the AC impedance (Zd) and AC impedance (Zc) of the lithium-ion secondary battery, by referring to battery characteristic data that shows the correlation between the difference (Z'd - Z'c) between the real component (Z'd) of the AC impedance (Zd) and the real component (Z'c) of the AC impedance (Zc), and the degradation state of the positive electrode. 　 An internal state estimation method according to claim 20, The aforementioned processor, An internal state estimation method for estimating the degradation state of the positive electrode based on the charge / discharge current when the difference (Z'd - Z'c) is greater than or equal to the threshold for the difference of the real number component. 　 An internal state estimation method according to any one of claims 10 to 22, The aforementioned processor, An internal state estimation method in which, in the step of calculating the AC impedance (Zd) and AC impedance (Zc), the frequency of the AC current flowing through the lithium-ion secondary battery is set to a predetermined frequency or less. 　 An internal state estimation method according to claim 1, The aforementioned processor, In the step of measuring the charging characteristics and the discharge characteristics, the response characteristics of the voltage response to an AC current input, or the current response to an AC voltage input are measured. An internal state estimation method for estimating the degradation state of the positive electrode based on the difference in amplitude between the charging side and the discharging side in the response characteristics. 　 An internal state estimation method according to claim 24, The aforementioned processor, The discharge-side amplitude (ΔVd_p), which is the difference between the peak value of the AC voltage on the discharge side in the response characteristics and the open-circuit voltage (OCV) before AC is input to the lithium-ion secondary battery, is calculated. The charging-side amplitude (ΔVc_p), which is the difference between the peak value of the AC voltage on the charging side in the response characteristics and the open-circuit voltage (OCV), is calculated. An internal state estimation method that determines that the positive electrode has deteriorated when the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p), which is the ratio of the discharge amplitude (ΔVd_p) to the charge amplitude (ΔVc_p), is greater than or equal to a predetermined amplitude ratio threshold. 　 An internal state estimation method according to claim 24, The aforementioned processor has previously stored in memory, The charge-discharge amplitude ratio (ΔVd_p / ΔVc_p), which is the ratio of the discharge amplitude (ΔVd_p) to the charge amplitude (ΔVc_p), is referenced to battery characteristic data that shows the correlation with the degradation state of the positive electrode, and the degradation state of the positive electrode corresponding to the charge-discharge amplitude ratio (ΔVd_p / ΔVc_p) calculated from the charge-discharge characteristics of the lithium-ion secondary battery is estimated. Internal state estimation method. 　 An internal state estimation method according to claim 24, The aforementioned processor, Based on the response characteristics measured when a first alternating current is input to the lithium-ion secondary battery, the first discharge-side amplitude (ΔVd1_p) and the first charge-side amplitude (ΔVc1_p) are calculated. Based on the response characteristics measured when a second alternating current is input to the lithium-ion secondary battery, the second discharge-side amplitude (ΔVd2_p) and the second charge-side amplitude (ΔVc2_p) are calculated. Based on the first charge-discharge amplitude ratio (ΔVd1_p / ΔVc1_p), which represents the ratio of the first discharge-side amplitude (ΔVd1_p) to the first charge-side amplitude (ΔVc1_p), and the second charge-discharge amplitude ratio (ΔVd2_p / ΔVc2_p), which represents the ratio of the second discharge-side amplitude (ΔVd2_p) to the second charge-side amplitude (ΔVc2_p), the degradation state of the positive electrode is estimated. The first discharge-side amplitude (ΔVd1_p) is the difference between the peak value of the AC voltage on the discharge side in the response characteristics and the open-circuit voltage (OCV) before the first AC current is passed through the lithium-ion secondary battery. The first charging-side amplitude (ΔVc1_p) is the difference between the peak value of the AC voltage on the charging side in the response characteristics and the open-circuit voltage (OCV) before the first AC current is passed through the lithium-ion secondary battery. The second discharge-side amplitude (ΔVd2_p) is the difference between the peak value of the AC voltage on the discharge side in the response characteristics and the open-circuit voltage (OCV) before the second AC current is passed through the lithium-ion secondary battery. The internal state estimation method wherein the second charging-side amplitude (ΔVc2_p) is the difference between the peak value of the AC voltage on the charging side in the response characteristics and the open-circuit voltage (OCV) before the second AC current is passed through the lithium-ion secondary battery. 　 An internal state estimation method according to any one of claims 24 to 27, The aforementioned processor, An internal state estimation method for setting the frequency of the AC current or AC voltage to below a predetermined frequency threshold. An internal state estimation method according to any one of claims 1 to 28, The aforementioned positive electrode is a method for estimating the internal state of a positive electrode containing a positive electrode active material having a layered rock salt type structure. 　 Equipped with a controller that estimates the internal state of a lithium-ion secondary battery, The aforementioned controller, The charging characteristics and discharge characteristics of the lithium-ion secondary battery are measured. An internal state estimation device that estimates the degradation state of the positive electrode of a lithium-ion secondary battery based on the difference in behavior between the charging characteristics and the discharge characteristics.

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