Method for estimating the degradation of secondary batteries and apparatus for estimating the degradation of secondary batteries

Abstract

In estimating the degradation state of the positive electrode of a secondary battery installed in a vehicle, the method for estimating the degradation state is appropriately selected according to the vehicle's usage conditions, and the accuracy of the obtained positive electrode state estimate is ensured. This allows for more accurate estimation of the degradation state at all stages, from the time of manufacture to the end of use of the secondary battery. [Solution] The method comprises the steps of: selecting either a first positive electrode state estimation method or a second positive electrode state estimation method for estimating the state of the positive electrode of a secondary battery based on the state of a vehicle equipped with the secondary battery to be subjected to degradation estimation; confirming the reliability of the estimated positive electrode state of the secondary battery obtained by the first positive electrode state estimation method or the second positive electrode state estimation method, which indicates the accuracy of the degradation state estimation; and determining whether or not to correct the estimated positive electrode state value based on the confirmed level of reliability.

Description

[[Technical Field]]

[0001] Embodiments of the present invention relate to a degradation estimation method for a secondary battery and a degradation estimation apparatus for a secondary battery. [[Background Art]]

[0002] Patent Literature 1 discloses a battery state estimation apparatus capable of estimating the degree of degradation of a secondary battery. In a specific method, based on actually measured current integrated values measured in an actual secondary battery for which the battery state estimation apparatus estimates degradation, a positive electrode capacity degradation coefficient α, an OCP deviation amount β, and a capacity deviation amount δ are calculated with high accuracy.

[0003] Further, based on the positive electrode capacity degradation coefficient α, the OCP deviation amount β, and the capacity deviation amount δ calculated with high accuracy, and initial characteristic data stored in a storage unit, degradation characteristic data, which are the relationship between capacity and the open circuit potential of the positive electrode and the relationship between capacity and the open circuit potential of the negative electrode in a degraded secondary battery, are estimated. Therefore, it is described that degradation characteristic data can be estimated with high accuracy. [[Prior Art Document]] [[Patent Document]]

[0004] [[Patent Document 1]] Japanese Patent Laid-Open No.2021-044068 [[Summary of the Invention]] [[Problem to be Solved by the Invention]]

[0005] Here, in the battery state estimation apparatus disclosed in the above-mentioned Patent Document 1, in order to accurately identify the positive electrode capacity degradation coefficient α and the like, the voltage of the battery at a plurality of points is measured to estimate OCV (Open Circuit Voltage). As an acquisition condition for the OCV, it is required that a state in which the current value of the secondary battery is equal to or less than a predetermined value continues for a predetermined period of time.

[0006] However, in vehicles equipped with secondary batteries, such as electric vehicles currently in use, the current value changes, for example, during rapid charging or while driving. Therefore, it is possible that the current value may take a value greater than the "predetermined value" mentioned above.

[0007] In such cases, if the degradation state of a secondary battery is estimated using the method disclosed in Patent Document 1, there may be situations where the degradation state cannot be estimated. The occurrence of such situations means, for example, that there may be a period from the time of manufacture to the end of use when the degradation state cannot be determined. As a result, this could lead to a decrease in the overall accuracy of the degradation state estimation.

[0008] The present invention was made to solve the above problems, and the object of the present invention is to provide a secondary battery degradation estimation method and a secondary battery degradation estimation device that can estimate the degradation state of the positive electrode of a secondary battery mounted in a vehicle with higher accuracy at all stages from the time of manufacture to the end of use of the secondary battery, by appropriately selecting a method for estimating the degradation state according to the usage state of the vehicle and ensuring the reliability of the obtained positive electrode state estimate. [Means for solving the problem]

[0009] The method for estimating the degradation of a secondary battery in this embodiment includes the steps of: selecting either a first positive electrode state estimation method or a second positive electrode state estimation method for estimating the state of the positive electrode of a secondary battery based on the state of the vehicle on which the secondary battery to be degraded is installed; confirming the reliability of the estimated positive electrode state of the secondary battery obtained by the first positive electrode state estimation method or the second positive electrode state estimation method, which indicates the accuracy of the degradation state estimation; and determining whether or not to correct the estimated positive electrode state value based on the level of the confirmed reliability.

[0010] Furthermore, the secondary battery degradation estimation device in the embodiment includes: a determination unit that determines whether to use the first positive electrode state estimation method or the second positive electrode state estimation method for a secondary battery to be mounted on a vehicle and subject to degradation estimation; a positive electrode state calculation unit that calculates a positive electrode state estimate value indicating the state of the positive electrode of the secondary battery using the selected first positive electrode state estimation method or the second positive electrode state estimation method; and a degradation state estimation accuracy determination unit that checks the reliability of the positive electrode state estimate value calculated by the positive electrode state calculation unit and corrects the positive electrode state estimate value if it is determined that the reliability is high. [Effects of the Invention]

[0011] By employing such a secondary battery degradation estimation method and secondary battery degradation estimation device, the present invention enables more accurate estimation of the degradation state at all stages from the time of manufacture to the end of use of a secondary battery, by appropriately selecting a method for estimating the degradation state according to the usage conditions of the vehicle and ensuring the reliability of the obtained positive electrode state estimate. [Brief explanation of the drawing]

[0012] [Figure 1] This is a block diagram showing the overall configuration of a vehicle including a secondary battery degradation estimation device according to an embodiment of the present invention. [Figure 2] This is a block diagram showing the internal configuration of a secondary battery degradation estimation device according to an embodiment of the present invention. [Figure 3] This is an explanatory diagram illustrating the relationship between the state of a vehicle equipped with a secondary battery that is the subject of degradation estimation in an embodiment of the present invention, and the first positive electrode state estimation method or the second positive electrode state estimation method that may be employed. [Figure 4] This is an explanatory diagram illustrating an example of a method for determining the reliability of the positive electrode state estimate obtained by the first positive electrode state estimation method in an embodiment of the present invention. [Figure 5]This is an explanatory diagram illustrating an example of a method for determining the reliability of the positive electrode state estimate obtained by the second positive electrode state estimation method in an embodiment of the present invention. [Figure 6] This is an explanatory diagram illustrating another example of a method for determining the reliability of the positive electrode state estimate obtained by the second positive electrode state estimation method in an embodiment of the present invention. [Figure 7] This is a flowchart showing the flow of the method for estimating the degradation of a secondary battery in an embodiment of the present invention. [Figure 8] This flowchart shows the flow of verifying the accuracy of a method for estimating the degradation of a secondary battery according to an embodiment of the present invention, in which the first positive electrode state estimation method is used. [Figure 9] This flowchart shows the steps for verifying the accuracy of the positive electrode state estimate obtained by the first positive electrode state estimation method, in an embodiment of the present invention for estimating the degradation of a secondary battery. [Figure 10] This flowchart shows the flow of verifying the accuracy of a method for estimating the degradation of a secondary battery according to an embodiment of the present invention, in which a second positive electrode state estimation method is used. [Modes for carrying out the invention]

[0013] Embodiments of the present invention will be described in detail below with reference to the drawings. Note that the drawings are schematic and may differ from actual ones. Furthermore, the embodiments of the present invention shown below are illustrative examples of devices and methods for realizing the technical concept of the present invention, and the technical concept of the present invention is not limited to the structure, arrangement, etc., of the components described below. The technical concept of the present invention can be modified in various ways within the technical scope defined by the claims described in the patent claims.

[0014] FIG. 1 is a block diagram showing the overall configuration of a vehicle V including a secondary battery degradation estimation apparatus 1 according to an embodiment of the present invention. The secondary battery degradation estimation apparatus 1 is used for estimating the degradation state of a secondary battery. The estimation target is the secondary battery mounted on the vehicle V. Therefore, as the vehicle V according to the embodiment of the present invention, any type of vehicle is acceptable as long as it is equipped with a secondary battery, such as an electric vehicle, a hybrid vehicle, and the like.

[0015] The vehicle V includes a charging port CP, and the secondary battery can be charged by connecting a charger C to the charging port CP. Here, the secondary battery that is the target of degradation state estimation in the embodiment of the present invention is a battery that can be charged and discharged multiple times. Examples thereof include a battery mounted on a vehicle and a storage battery mounted on an electric vehicle and used for providing driving force. Also, although there are various types of "secondary batteries", a lithium-ion battery can be mentioned as an example herein.

[0016] FIG. 1 shows a battery pack BP in which a plurality of secondary batteries are connected in series. Further, a current sensor CS is connected to the battery pack BP, and for example, measures a current charged from the charger C via the charging port CP. The battery pack BP (secondary battery) and the current sensor CS are connected to the secondary battery degradation estimation apparatus 1 (hereinafter appropriately abbreviated as "degradation estimation apparatus 1").

[0017] As described above, the degradation estimation apparatus 1 is responsible for estimating the degradation state of a secondary battery. More specifically, it calculates a value (hereinafter appropriately referred to as "positive electrode state estimated value") that enables estimating the state of the positive electrode for the secondary battery by using either a first positive electrode state estimation method or a second positive electrode state estimation method. Then, it determines the reliability of the calculated positive electrode state estimated value, and determines whether correction of the positive electrode state estimated value is allowable based on the determination result. By using such a method, the degradation state of the secondary battery can be estimated with higher accuracy.

[0018] A degradation estimation device 1 is connected to a BMS (Battery Management System) 2, and further connected to a control device (e.g., a vehicle control unit) 3 that controls the entire vehicle. The degradation state of the secondary battery estimated by the degradation estimation device 1 is transmitted to the battery management system 2 and the vehicle control unit 3, and is notified to a user of the vehicle V, for example.

[0019] Note that the degradation estimation device 1 may estimate the degradation state of the secondary battery under the control of the battery management system 2, or may autonomously perform processing for estimating the degradation state of the secondary battery.

[0020] Further, the degradation state of the secondary battery estimated by the degradation estimation device 1 can be transmitted to the outside of the vehicle V via an in-vehicle communication unit 4 connected to the vehicle control unit 3 and an antenna A. Information indicating the degradation state of the secondary battery of the vehicle V transmitted outside the vehicle is stored in, for example, a server provided on a network, and is used for inspection of the vehicle V, for example.

[0021] Electric power charged into the secondary battery through a charging process is discharged from, for example, the secondary battery (battery pack BP) and transmitted to a drive motor M via an inverter I. The drive motor M is connected to a driving unit such as tires of the vehicle V, and the secondary battery provides driving force to the vehicle V.

[0022] Next, functions of each part constituting the degradation estimation device 1 will be described with reference to the drawings. FIG. 2 is a block diagram showing an internal configuration of the secondary battery degradation estimation device 1 according to an embodiment of the present invention. As shown in FIG. 2, the degradation estimation device 1 includes a determination unit 11, a positive electrode state calculation unit 12, a storage unit 13, a measurement information acquisition unit 14, and a degradation state estimation accuracy determination unit 15.

[0023] Note that Figure 2 only shows the configuration necessary for the method of estimating the deterioration state in the embodiment of the present invention. Therefore, in addition to the parts shown in Figure 2, configurations for performing functions other than those performed by the parts shown in Figure 2 may also be provided.

[0024] Furthermore, the functions of the degradation estimation device 1 described below are realized by the execution of a computer program stored in the memory unit 13 by a determination unit 11 consisting of a processor, a positive electrode state calculation unit 12, and a degradation state estimation accuracy determination unit 15. The processor may be, for example, a CPU (Central Processing Unit) or an MPU (Micro-Processing Unit).

[0025] Furthermore, the degradation estimation device 1 may be formed by dedicated hardware for performing the information processing described below. For example, the degradation estimation device 1 may include functional logic circuits set in a general-purpose semiconductor integrated circuit. The degradation estimation device 1 may also have a programmable logic device (PLD) such as a field-programmable gate array (FPGA).

[0026] The determination unit 11 performs the necessary determinations when a determination is required to estimate the degradation state of the positive electrode of the secondary battery. For example, when calculating the estimated positive electrode state value, which is the premise for estimating the degradation state, it determines whether to adopt the first positive electrode state estimation method or the second positive electrode state estimation method, depending on the condition of the vehicle.

[0027] Then, the positive electrode state calculation unit 12 calculates the positive electrode state estimate using either the first positive electrode state estimation method or the second positive electrode state estimation method selected by the determination unit 11. Here, the first positive electrode state estimation method can be any method that can calculate the positive electrode state estimate.

[0028] In the embodiment of the present invention, Charging Curve Analysis (CCA) is employed as the first positive electrode state estimation method. This CCA is a method that uses the voltage value measured during charging and discharging of a secondary battery, and calculates the estimated positive electrode state based on the difference between the initial open-circuit potential of the positive electrode of the secondary battery and the open-circuit potential of the positive electrode of the secondary battery at the time of measurement.

[0029] More specifically, the voltage value measured during charging and discharging, i.e., the closed-circuit voltage, is corrected using a resistor. The open-circuit voltage obtained by this correction is calculated. Then, based on this open-circuit voltage, the open-circuit potential of the positive electrode is calculated.

[0030] The positive electrode state calculation unit 12 calculates the open-circuit potential of the positive electrode as appropriate at timings described later. Therefore, the open-circuit potentials of multiple positive electrodes are calculated, and the positive electrode state calculation unit 12 uses these open-circuit potentials of multiple positive electrodes to calculate an estimated positive electrode state. Here, the open-circuit potential of the positive electrode of the secondary battery, for example, at the time of shipment, is referred to as the "initial open-circuit potential." The open-circuit potential of the positive electrode obtained by executing the CCA process can be referred to as the "open-circuit potential at measurement."

[0031] The positive electrode state calculation unit 12 calculates an estimated positive electrode state value based on the difference between the initial open-circuit potential and the open-circuit potential at the time of measurement. In the first positive electrode state estimation method, the estimated positive electrode state value is calculated in this manner, and hereafter, this first positive electrode state estimation method will be referred to as "CCA" as appropriate.

[0032] On the other hand, the second method for estimating the positive electrode state can be any method that can calculate an estimated value of the positive electrode state, but in the embodiment of the present invention, EIS (Electro-Chemical Impedance Spectroscopy) is used.

[0033] This EIS (Electron Indication System) is a method for estimating the state of the positive electrode by measuring the impedance when an alternating current is applied to a secondary battery, and then determining the positive electrode parameters in the equivalent circuit from the impedance and the equivalent circuit of the secondary battery. Hereafter, this second positive electrode state estimation method will be referred to as "EIS" as appropriate.

[0034] When such EIS is employed, an alternating current is applied to each battery cell housed in the battery pack BP to measure its impedance. Therefore, the battery pack BP in the embodiment of the present invention incorporates an AC power supply for performing EIS. The measured impedance value is then input to the degradation state estimation accuracy determination unit 15 as appropriate via the measurement information acquisition unit 14, which will be described later.

[0035] Thus, in the method for estimating the degradation of a secondary battery in the embodiment of the present invention, two calculation methods, CCA and EIS, are employed as methods for calculating the positive electrode state estimate. The reason for employing these two calculation methods is that there may be cases where each method is not suitable for estimating the degradation state of a secondary battery.

[0036] In other words, EIS can accurately estimate the state of the positive electrode after the polarization characteristics of the secondary battery have been resolved while the vehicle is stationary (therefore, a predetermined time (non-operating time based on the relaxation phenomenon of the secondary battery) must have elapsed after the vehicle has stopped). On the other hand, in the case of CCA, for example, if the change in SOC (State of Charge) is large, the state of the positive electrode cannot be accurately estimated unless certain conditions are met, such as a constant current flowing through the secondary battery during charging.

[0037] Therefore, in the secondary battery degradation estimation method in the embodiment of the present invention, the positive electrode state is estimated with greater accuracy by appropriately selecting one of these two methods, taking into account the state of the vehicle on which the secondary battery to be degraded is installed. This point will be further explained with reference to the figures. Figure 3 is an explanatory diagram illustrating the relationship between the state of the vehicle V on which the secondary battery to be degraded is installed in the embodiment of the present invention, and the first positive electrode state estimation method or the second positive electrode state estimation method that may be adopted.

[0038] Figure 3 shows the state of a vehicle V from the perspective of State of Charge (SOC). In the explanatory diagram of Figure 3, the vertical axis represents SOC (%) and the horizontal axis represents time. Although the unit for time on the horizontal axis is not specified, this unit could be "days" or "hours (h)". The thick solid line shows the state of vehicle V. Specifically, it shows states such as vehicle V being stopped, moving, or charging.

[0039] For example, if we observe the state of vehicle V over time, starting from the origin where the vertical and horizontal axes intersect, we see that vehicle V is initially being charged normally. As normal charging takes place, the State of Charge (SOC) value increases. Here, the dashed line shown parallel to the horizontal axis representing time indicates that the secondary battery installed in vehicle V has reached full charge.

[0040] Normal charging naturally takes place while the vehicle V is stationary. However, even after the secondary battery is fully charged, the vehicle V remains stationary for a while, so the State of Charge (SOC) value does not change from the fully charged value. Therefore, the solid line after normal charging is shown parallel to the horizontal axis, overlapping with the dashed line.

[0041] Subsequently, vehicle V transitioned to a running state. As vehicle V runs, the secondary battery discharges, and the SOC value gradually decreases. Then, after a short stop, the running state continued, causing the SOC value to decrease further.

[0042] After this, vehicle V stops and charging begins. This charging is not normal charging but rapid charging. Due to this rapid charging, the State of Charge (SOC) value recovers to a higher value than the SOC value when the vehicle was stopped while driving. After the rapid charging is complete, the vehicle stops briefly before returning to driving mode. However, this driving state is short, and the SOC value is higher than the SOC value at the start of rapid charging.

[0043] After stopping for a while, normal charging begins. This normal charging restores the State of Charge (SOC) value of the secondary battery in vehicle V to full charge, and vehicle V resumes operation.

[0044] Figure 3 shows various states of vehicle V, as well as processes that are executed as needed to estimate the degradation state of the secondary battery installed in vehicle V. Above the dashed line parallel to the horizontal axis, which shows the fully charged state of the secondary battery, the CCA process is shown. Furthermore, the EIS process is executed when vehicle V transitions from a stationary state to the next state, such as when it starts moving or when it starts charging.

[0045] In the state of vehicle V shown in Figure 3, the CCA process is executed 3 times and the EIS process is executed 6 times. For each process, the number of times it was executed and whether the process was performed correctly are indicated by "○" or "×". For example, the first CCA (CCA1) is indicated by "○" because the process was performed correctly. On the other hand, for example, the third EIS (EIS3) is indicated by "×" because the process was not performed correctly.

[0046] In the positive electrode state calculation unit 12, when an estimated positive electrode state is calculated using either the CCA or EIS calculation method, the calculated estimated positive electrode state is stored in the storage unit 13. As described above, in the embodiment of the present invention, when estimating the state of the positive electrode, the CCA or EIS calculation method is used as appropriate. This makes it possible to adopt a calculation method suitable for the state of the vehicle V, thereby enabling a more accurate estimation of the state of the positive electrode.

[0047] The memory unit 13 is composed of, for example, a semiconductor or a magnetic disk. In addition to the positive electrode state estimation value mentioned above, the memory unit 13 may also store various other information, such as measurement information related to the secondary battery acquired by the measurement information acquisition unit 14, which will be described later.

[0048] The measurement information acquisition unit 14 acquires measurement information related to the secondary battery whose degradation state is to be estimated. Specifically, this includes values ​​such as the change in SOC (ΔSOC), the C rate, or the temperature of the environment in which the secondary battery is placed. This measurement information is transmitted from measuring devices such as the current value measured by the current sensor CS or a temperature sensor (not shown in Figures 1 and 2). Alternatively, the measurement information acquisition unit 14 may acquire the information itself.

[0049] The deterioration state estimation accuracy determination unit 15 determines whether the positive electrode state estimate calculated by the positive electrode state calculation unit 12 is calculated with good accuracy, that is, it determines the reliability of the positive electrode state estimate. As described above, the calculation method of CCA or EIS is selected according to the state of the vehicle V. However, even if the positive electrode state estimate is calculated using the selected calculation method, the reliability of the calculated positive electrode state estimate may vary depending on the state in which the vehicle V is placed, even if the selected calculation method is appropriate.

[0050] Therefore, the accuracy of the calculated positive electrode state estimate is maintained and improved by re-evaluating its reliability. If the degradation state estimation accuracy determination unit 15 determines that the reliability is high, the most recently calculated positive electrode state estimate is corrected or overwritten with the newly calculated positive electrode state estimate. Alternatively, the positive electrode state estimate with high reliability is stored in the storage unit 13.

[0051] On the other hand, if the degradation state estimation accuracy determination unit 15 determines that the probability is low, the most recently calculated positive electrode state estimate is not modified. Therefore, in this case, the most recently calculated positive electrode state estimate is maintained.

[0052] The following are some examples of conditions for determining the likelihood of the degradation state estimation accuracy determination unit 15. First, when the positive electrode state estimate is calculated using CCA, the likelihood determination condition is whether or not the change in SOC (ΔSOC) during charging or discharging of the secondary battery is greater than or equal to a predetermined value.

[0053] As mentioned above, CCA is a calculation method that estimates the state of the positive electrode from the difference between the initial open-circuit potential and the open-circuit potential at the time of measurement. Here, regarding the open-circuit potential of the positive electrode, if the change in SOC is small, it becomes difficult to obtain information about the difference in the open-circuit potential of the positive electrode before and after degradation. On the other hand, for example, if the battery is sufficiently charged under normal conditions, the change in SOC will also be large, so it will be possible to obtain sufficient information about the difference in the open-circuit potential of the positive electrode, and as a result, the state of degradation of the positive electrode can be grasped with accuracy.

[0054] Therefore, if the SOC changes significantly due to charging or other factors, the degradation state estimation accuracy determination unit 15 determines that the probability is high. Conversely, if the change in SOC is small, the degradation state estimation accuracy determination unit 15 determines that the probability is low.

[0055] Next, let's consider the criteria for determining certainty using the C rate. The C rate essentially represents the speed of charging and discharging, but the voltage measured during charging and discharging (closed-circuit voltage) can be said to be an overvoltage compared to the open-circuit voltage. This overvoltage is caused by the resistance inside the secondary battery, and the larger the current value, the larger the overvoltage value.

[0056] As mentioned above, when calculating the open-circuit potential of the positive electrode, the open-circuit voltage is used, and this open-circuit voltage is obtained using the closed-circuit voltage. Therefore, as the overvoltage increases, the correction made using resistance to the closed-circuit voltage to calculate the open-circuit voltage also increases. As a result, the accuracy of the open-circuit voltage used when calculating the open-circuit potential of the positive electrode deteriorates.

[0057] Therefore, if the C rate is below a predetermined value, the degradation state estimation accuracy determination unit 15 determines that the probability is high. On the other hand, if the C rate is greater than the predetermined value, the degradation state estimation accuracy determination unit 15 determines that the probability is low. The predetermined value here is, for example, a value obtained in advance through experiments for each secondary battery and is stored in the memory unit 13.

[0058] Furthermore, when determining the likelihood using the C-rate, instead of using the C-rate value directly, fluctuations in the C-rate can also be utilized. For example, when charging a secondary battery, if a large-scale power outage occurs while receiving power from the grid current, stable charging may not be possible. Similarly, charging from storage batteries can also be unstable.

[0059] As mentioned above, the calculation of the open-circuit potential of the positive electrode involves correcting the closed-circuit voltage using a resistor, but if the fluctuation in the C rate is large, the fluctuation in the resistance value will also be large. As a result, it becomes difficult to accurately calculate the open-circuit potential of the positive electrode. Therefore, the degradation state estimation accuracy determination unit 15 determines that the probability is high when the fluctuation in the C rate is below a predetermined value.

[0060] On the other hand, if the fluctuation in the C rate is greater than a predetermined value, the degradation state estimation accuracy determination unit 15 determines that the probability is low, as it can be estimated that the open-circuit potential of the positive electrode has not been calculated accurately. The predetermined value here is, for example, a value obtained in advance through experiments for each secondary battery and is stored in the memory unit 13.

[0061] Furthermore, it is conceivable to use temperature to determine accuracy. That is, if the temperature of the secondary battery, or the temperature of the environment in which the secondary battery is placed, is low, the resistance in the secondary battery will increase. And as the resistance increases, the correction using the resistance in the closed-circuit voltage becomes larger, as mentioned above, and ultimately it becomes difficult to accurately calculate the open-circuit potential of the positive electrode.

[0062] Therefore, the higher the temperature, the more accurately the open-circuit potential of the positive electrode can be calculated. Consequently, the degradation state estimation accuracy determination unit 15 determines that the probability is high when the temperature is above a predetermined value. On the other hand, if the temperature is below the predetermined value, the degradation state estimation accuracy determination unit 15 determines that the probability is low. The predetermined value here is, for example, a value obtained in advance through experiments for each secondary battery and is stored in the memory unit 13.

[0063] As shown in the explanatory diagram in Figure 3, the calculation of the positive electrode state estimate is performed using the CCA calculation method at multiple timings during the charging and discharging of the secondary battery. In this case, for example, when normal charging is sufficiently performed and the change in SOC is large, as in CCA1, the accuracy of the positive electrode state estimate calculated using this CCA calculation method can be judged to be high. For this reason, CCA1 in Figure 3 is marked with a "○".

[0064] On the other hand, the method used to calculate CCA is not suitable when rapid charging is performed, and therefore CCA2 in Figure 3 is marked with an "×". Furthermore, even when normal charging is performed, it is possible that a reasonable value cannot be calculated as an estimate of the positive electrode state. CCA3 in Figure 3 takes this into account and is marked with an "×".

[0065] Specifically, for example, consider a case where normal charging was performed, but the SOC only changed from 0% to 20%. Alternatively, even if the SOC reached 100% through normal charging, if the initial value at which the SOC change began was already relatively high, the probability might be judged as low.

[0066] Therefore, it is possible to combine multiple positive electrode state estimates that have been judged to have low probability to generate a single positive electrode state estimate, and then determine the level of probability of that estimate. This process is considered possible because the deterioration of the positive electrode does not worsen significantly in, for example, a day or a week. Rather, if the combination is appropriate, it is possible to create a state that can be judged to have high probability.

[0067] Specifically, first, the positive electrode state calculation unit 12 obtains the relationship between the open-circuit potential of the positive electrode and the change in SOC obtained by executing the first positive electrode state estimation method, and stores it in the storage unit 13. Then, the degradation state estimation accuracy determination unit 15 determines whether or not it is possible to generate the open-circuit potential characteristics of a single positive electrode by combining the multiple SOC changes obtained.

[0068] If it is determined that it can be generated, the degradation state estimation accuracy determination unit 15 generates an open-circuit potential characteristic of one positive electrode by combining the changes in SOC. It then determines whether the generated open-circuit potential characteristic of the positive electrode is greater than or equal to a preset change in SOC. If this open-circuit potential characteristic of the positive electrode is greater than or equal to the change in SOC, it is determined that the probability is high.

[0069] The process of generating the open-circuit potential characteristics of a single positive electrode by combining these multiple SOC changes is explained below with the help of a diagram. Figure 4 is an explanatory diagram illustrating an example of a method for determining the reliability of the positive electrode state estimate obtained by the first positive electrode state estimation method in an embodiment of the present invention.

[0070] Figure 4 shows three graphs, Figures 4(A) through 4(C). In all of these graphs, the vertical axis represents the open-circuit potential of the positive electrode, and the horizontal axis represents the cathode state of charge (SOC). A thin solid line also shows an example of the change in SOC in a secondary battery. This thin solid line is shown so that it can be compared with the change in SOC shown by the thick solid line or dashed line in Figures 4(A) through 4(C), which will be explained below.

[0071] Furthermore, on the horizontal axis representing cathode SOC, for example, two thin solid lines are shown on the right side. The thinner solid line further to the right shows the change in cathode SOC in a new secondary battery. On the other hand, the thinner solid line on the left shows the change in cathode SOC in a secondary battery that has been used to some extent and has deteriorated.

[0072] As the secondary battery charges, and the State of Charge (SOC) value changes from a small value towards 100%, the cathode SOC value changes towards 0%. This is because lithium ions move from the negative electrode to the positive electrode during charging. Therefore, in the graphs shown in Figures 4(A) to 4(C), a smaller cathode SOC value (closer to the origin on the horizontal axis) indicates that the secondary battery is closer to a fully charged state.

[0073] In other words, as shown in Figures 4(A) to 4(C), the difference between the two thin solid lines representing the change in cathode SOC between a new secondary battery and a degraded secondary battery is large when the cathode SOC value is large, and when the secondary battery's SOC value is small. Therefore, the explanation that a large change in SOC due to charging etc. indicates high probability is based on the premise that a large difference occurs, especially between a new secondary battery and a degraded secondary battery, when the SOC value transitions from a low state to a high state. That is, a large cathode SOC value, i.e., a small SOC value, makes it easier to detect the difference between a new secondary battery and a degraded secondary battery, and thus the probability can be determined to be higher.

[0074] Note that the three graphs in Figures 4(A) through 4(C) merely schematically illustrate the change in the relationship between the open-circuit potential of the positive electrode and the cathode SOC; therefore, the individual values ​​for the open-circuit potential of the positive electrode and the cathode SOC have been omitted.

[0075] Based on this graph, Figure 4(A) shows, for example, a state where the SOC changes from 0% to 20% due to charging, represented by a thick solid line. On the other hand, Figure 4(B) shows a state where the SOC has deteriorated from 10% to 70%, represented by a dashed line.

[0076] The changes in SOC shown in Figures 4(A) and 4(B) are changes calculated using CCA at different time points. As mentioned above, all of these changes are stored in the memory unit 13.

[0077] The degradation state estimation accuracy determination unit 15 combines this information stored in the memory unit 13 under certain conditions to generate an open-circuit potential characteristic of a single positive electrode. Figure 4(C) shows an example of the open-circuit potential characteristic of a positive electrode generated by the degradation state estimation accuracy determination unit 15.

[0078] Figure 4(C) shows the open-circuit potential characteristics of the positive electrode generated by combining the solid line shown in Figure 4(A) and the dashed line shown in Figure 4(B). Generating such open-circuit potential characteristics of the positive electrode can improve the accuracy of individual positive electrode state estimates, even if their accuracy is low, and does not reduce the accuracy of the estimates in light of the progression of positive electrode degradation.

[0079] Furthermore, it is possible to periodically determine whether the change in SOC is above a predetermined value. If this determination process is performed, and the change in SOC is above a predetermined value, the reliability of the open-circuit potential characteristics of the positive electrode generated by the degradation state estimation accuracy determination unit 15 is determined to be high, which can contribute to correcting the estimated positive electrode state value.

[0080] The degradation state estimation accuracy determination unit 15 will determine whether a combination is possible among multiple SOC changes. For example, the following conditions can be considered for this determination. For example, conditions such as selecting from information from the past few days, selecting from information where the temperature conditions are the same, and not selecting information when rapid charging is performed can be used. In addition, conditions such as not using or estimating SOC changes at high potentials may be used.

[0081] The above are the criteria for determining the accuracy of the degradation state estimation accuracy determination unit 15 when the CCA calculation method is used to calculate the positive electrode state estimate. Next, we will explain the criteria for determining accuracy when EIS is used.

[0082] The first determination criterion is whether the SOC value when EIS is executed is equal to or greater than a predetermined value. If the degradation state estimation accuracy determination unit 15 determines that it is equal to or greater than the predetermined value, the probability is determined to be high. On the other hand, if it is less than the predetermined value, the probability is determined to be low.

[0083] Figure 5 is an explanatory diagram illustrating an example of a method for determining the accuracy of the positive electrode state estimate obtained by the second positive electrode state estimation method in an embodiment of the present invention. In the explanatory diagram shown in Figure 5, the vertical axis represents the value of OCV [V], and the horizontal axis represents the value of SOC [%]. In other words, it is a diagram showing the characteristics of SOC-OCV in a secondary battery.

[0084] In this diagram, the relationship between SOC and OCV is shown by a solid line. Looking at the movement of this solid line, we can see a point where the SOC-OCV characteristics change abruptly, as shown in the area enclosed by the dashed rectangle. After that, the characteristics change gradually and do not change significantly.

[0085] As mentioned above, when calculating the positive electrode state estimate using EIS, the impedance is measured when an AC current is applied to the secondary battery. In this process, the state of charge (SOC) is estimated before the impedance is measured. Therefore, if the SOC-OCV characteristics change significantly on the low SOC side, as shown in the area enclosed by the dashed rectangle in Figure 5, it will affect the impedance measurement.

[0086] Therefore, the degradation state estimation accuracy determination unit 15 determines that the probability is low if the SOC value at the time of EIS is less than a predetermined value (such as when it falls within the area enclosed by the dashed rectangle in Figure 5). In other words, if it determines that the SOC value is above the predetermined value, the probability is determined to be high. The predetermined value here is, for example, a value obtained in advance through experiments for each secondary battery and is stored in the memory unit 13.

[0087] Furthermore, the temperature of the secondary battery during EIS can also be used as a criterion for determining reliability. In other words, EIS calculates an estimated positive electrode state by measuring impedance, and impedance has the characteristic of being smaller at high temperatures and larger at low temperatures.

[0088] Since the impedance being measured is a very small value, if the already small impedance value becomes even smaller due to the effect of temperature, such as at high temperatures, it may become indistinguishable from noise. In such conditions, it may not be possible to accurately determine the impedance even if it is measured.

[0089] Therefore, if the temperature of the secondary battery during EIS is below a predetermined value, the degradation state estimation accuracy determination unit 15 determines that the probability is high. On the other hand, if the temperature is higher than the predetermined value, the probability is determined to be low.

[0090] Furthermore, the predetermined temperature value here is also related to the temperature characteristics of the secondary battery whose degradation state is being estimated. That is, when the EIS calculation method is performed, an AC current is applied to the battery using an AC power supply as shown in Figure 1. Some secondary batteries have a structure in which multiple battery cells are connected by busbars, for example. And the resistance of these busbars may be measured as an impedance component.

[0091] Therefore, for example, by considering the busbar structure and setting a "predetermined value" for temperature for each secondary battery having the same structure, the positive electrode state estimate can be calculated more accurately using the EIS calculation method. This predetermined value is determined, for example, through experimentation. This predetermined value is stored in the memory unit 13.

[0092] Next, when calculating the positive electrode state estimate using EIS, impedance measurements are performed. It is possible to perform these measurements multiple times, and we will explain how to determine the accuracy when using multiple measured impedance values.

[0093] For example, even if the positive electrode state is estimated using EIS while various conditions such as frequency, SOC, and temperature remain largely unchanged, the output impedance value may differ. In other words, if the impedance value is measured multiple times, the impedance value may fluctuate due to, for example, sensor failure or degradation of secondary batteries other than the positive electrode.

[0094] Furthermore, while impedance values ​​are measured at multiple frequencies, when multiple impedance values ​​are actually measured using EIS, the impedance value used to estimate the state of the positive electrode is generally a value that falls within a certain bandwidth. The reason for using impedance values ​​within this bandwidth is that the impedance values ​​at any frequency within this bandwidth do not change significantly. On the other hand, impedance values ​​measured at frequencies outside this bandwidth may be significantly different.

[0095] In other words, the reliability of the impedance value measured in the EIS can be ensured by determining whether or not it falls within a certain range. This point will be explained below using Figure 6. Figure 6 is an explanatory diagram showing another example of a method for determining the reliability of the positive electrode state estimate obtained by the second positive electrode state estimation method in an embodiment of the present invention.

[0096] Figure 6 shows two explanatory diagrams, Figure 6(A) and Figure 6(B). We will first explain using the explanatory diagram shown in Figure 6(A). In the explanatory diagram in Figure 6(A), the vertical axis represents impedance [mΩ] and the horizontal axis represents frequency [Hz].

[0097] The impedance at multiple frequencies was then measured, and the measured values ​​for each impedance were plotted. These multiple impedance values ​​are connected by a solid line, forming a bell-shaped curve. However, it is not a sharp convex shape, but rather a gentle curve.

[0098] In the explanatory diagram shown in Figure 6(A), there is only one plot representing impedance for each frequency, but this does not mean that the impedance was measured only once at each frequency. For example, with respect to the frequencies in this explanatory diagram, at the frequencies included in the second zone from the left on the horizontal axis, four circles are shown on the vertical axis of the values ​​connected by solid lines to the measured values ​​on both sides.

[0099] In other words, the measured values ​​connected by solid lines are, for example, the average values ​​calculated when the deviation of impedance values ​​measured multiple times at the same frequency is less than or equal to a predetermined value. While an average value is used here, other calculation methods may also be employed.

[0100] Then, the reliability of the positive electrode state estimate obtained by the second positive electrode state estimation method is determined using the impedance values ​​shown in the explanatory diagram in Figure 6(A). Specifically, when measuring impedance, multiple impedances are measured at the same frequency, and if the deviation of the multiple impedances obtained as measurement results is less than or equal to a predetermined value, the reliability is determined to be high.

[0101] This is because if multiple impedance values ​​measured at the same frequency fall within a certain deviation, it indicates that there is no significant variation in impedance values ​​due to disturbances, such as the sensor failure mentioned above. Therefore, in such cases, the reliability of the obtained positive electrode state estimate can be determined to be high, and by performing this process, the positive electrode state can ultimately be estimated with greater accuracy.

[0102] Next, we will describe a method for determining the accuracy of the positive electrode state estimate obtained by the second positive electrode state estimation method using impedance values, which differs from the method explained using Figure 6(A). In the explanatory diagram of Figure 6(B), the vertical and horizontal axes represent the same things as in Figure 6(A). That is, the vertical axis represents impedance [mΩ] and the horizontal axis represents frequency [Hz]. However, unlike Figure 6(A), a solid line frame is shown approximately in the center of the explanatory diagram.

[0103] When the impedance value does not change significantly, it is thought that using the impedance value when determining the likelihood will not have a major impact on the likelihood determination. On the other hand, when the measured impedance value changes significantly, if the frequency bandwidth used in the EIS used to calculate the positive electrode state estimate is not set, it is possible that variability will occur in the likelihood determination, and as a result, the accuracy of the positive electrode state estimate may decrease.

[0104] Therefore, when an estimated positive electrode state is obtained using EIS, the impedance is measured to determine the likelihood of success. If the multiple impedances obtained as measurement results are within a preset frequency band, and the deviation of the impedance values ​​is less than or equal to a preset predetermined value, the degradation state estimation accuracy determination unit 15 determines that the likelihood is high.

[0105] In other words, as long as the impedance value falls within the frame shown in Figure 6(B), there is no significant variation in the multiple measured impedance values. Therefore, the variation in the obtained impedance values ​​is small, and it can be determined that the accuracy of the positive electrode state estimate calculated using the impedance values ​​included in this frame is high. As a result, it can be considered that the accuracy of the positive electrode state estimate calculated using impedance values ​​included in this band is high.

[0106] The deviations between multiple impedance values ​​measured at the same frequency, as well as the deviations between the set frequency band and the multiple impedance values ​​included in that set frequency band, are pre-set and stored in the memory unit 13. Therefore, the degradation state estimation accuracy determination unit 15 uses the deviations stored in the memory unit 13 to determine the likelihood of success.

[0107] As explained above, if the impedance values ​​measured at the same frequency fall within a certain deviation range, and if the deviation of the impedances measured within a predetermined frequency band is below a predetermined value, the positive electrode state estimate obtained by EIS is judged to have high reliability.

[0108] Therefore, the resistance value used in the first positive electrode state estimation method, CCA, can be a value calculated based on the impedance value obtained in EIS that is judged to have a high probability. Specifically, the resistance value used in CCA is a value calculated based on the impedance obtained in EIS performed before or during the execution of CCA.

[0109] In this way, by using highly reliable impedance values ​​obtained in EIS performed at a timing close to when CCA is executed, the accuracy of the positive electrode state estimate calculated using CCA can also be improved.

[0110] [Operation] Next, the calculation of the positive electrode state estimate using the two calculation methods described above, and the process for determining the accuracy of the calculated positive electrode state estimate, will be explained using Figures 7 to 10. First, Figure 7 is a flowchart showing the flow of the secondary battery degradation estimation method in an embodiment of the present invention. Figure 8 is a flowchart showing the flow of the accuracy confirmation in the secondary battery degradation estimation method in an embodiment of the present invention, where the first positive electrode state estimation method is used.

[0111] Regarding CCA, as mentioned above, it is a process that can be executed whether the battery is being charged or discharged. However, here we will explain it using the example of when the secondary battery is being charged while the vehicle V is stationary, rather than when it is being discharged while the vehicle V is running. On the other hand, EIS is a process that is executed while the vehicle is stationary, as mentioned above.

[0112] The determination unit 11 of the degradation estimation device 1 determines whether or not the vehicle V has stopped (ST1). As mentioned above, here we take the case where the secondary battery charging process is performed as an example of a case in which the CCA process may be performed, so first it is determined whether or not the vehicle V has stopped.

[0113] Next, the determination unit 11 determines whether or not normal charging has started for the vehicle V (ST2). As described above, the CCA process cannot accurately calculate the positive electrode state estimate when rapid charging is performed, but it can accurately calculate the positive electrode state estimate when normal charging is performed.

[0114] Therefore, if the determination unit 11 determines that normal charging of the vehicle V has started (YES in ST2), the determination unit 11 selects CCA as the method for estimating the state of the positive electrode (ST3). This determination is transmitted to the positive electrode state calculation unit 12, which calculates an estimated positive electrode state value using CCA (ST4).

[0115] The positive electrode state estimate calculated by the positive electrode state calculation unit 12 is stored in the storage unit 13, and its accuracy is verified by the degradation state estimation accuracy determination unit 15 (ST5). The degradation state estimation accuracy determination unit 15 determines the accuracy while referring to measurement information acquired from the measurement information acquisition unit 14 as appropriate.

[0116] Specifically, the determinations include whether the fluctuation of the State of Charge (SOC) is above a predetermined value (ST6 in Figure 8), whether the C rate is below a predetermined value (ST7), whether the fluctuation of the C rate is below a predetermined value (ST8), and whether the temperature of the secondary battery is above a predetermined value (ST9).

[0117] Furthermore, regarding the conditions for determining these probabilities, if even one of them is met, it can be determined that the probability of the positive electrode state estimate is high. However, in the embodiment of the present invention, the probability is determined to be high only when all of these conditions are met (ST10).

[0118] Then, if the degradation state estimation accuracy determination unit 15 determines that the probability is high, the most recent value, for example, the positive electrode state estimate stored in the memory unit 13, is corrected to the positive electrode state estimate that was determined to have a high probability (ST11).

[0119] On the other hand, if any of the four judgment conditions described above are not met, the degradation state estimation accuracy determination unit 15 determines that the calculated positive electrode state estimate is unlikely to be accurate (ST12). In this case, because the calculated positive electrode state estimate is unlikely to be accurate, the most recently calculated positive electrode state estimate is not modified by the newly calculated positive electrode state estimate (ST13).

[0120] Therefore, in this case, the most recently calculated positive electrode state estimate is treated as the latest positive electrode state estimate. This positive electrode state estimate is maintained until a positive electrode state estimate calculated using CCA or EIS, which is determined to have a higher probability, is obtained.

[0121] As described above, the degradation state estimation accuracy determination unit 15 determines whether the probability determination conditions are met, and as a result, it is decided whether or not to revise the positive electrode state estimate. However, even if the degradation state estimation accuracy determination unit 15 determines that the probability is low, the positive electrode state estimate calculated by the positive electrode state calculation unit 12 is stored in the storage unit 13 as described above. Therefore, the degradation state estimation accuracy determination unit 15 can combine these positive electrode state estimates, which have been determined to have low probability, and perform a new probability determination.

[0122] Therefore, the flow of this process will be explained below using Figure 9. Figure 9 is a flowchart showing the flow of verifying the accuracy of the positive electrode state estimate obtained by the first positive electrode state estimation method, which is a method for estimating the degradation of a secondary battery in an embodiment of the present invention.

[0123] First, when the positive electrode state calculation unit 12 stores the calculated positive electrode state estimate in the storage unit 13, it also stores in the storage unit 13 the relationship between the positive electrode open-circuit potential and the change in SOC related to the positive electrode state estimate (ST21). When executing the process of generating the positive electrode open-circuit potential characteristics explained using Figure 4, the process of storing this change in the positive electrode open-circuit potential and SOC in the storage unit 13 is a prerequisite.

[0124] When the degradation state estimation accuracy determination unit 15 performs the said process, it grasps the relationship between the open-circuit potential of the positive electrode and the change in SOC related to the latest positive electrode state estimation value calculated by the positive electrode state calculation unit 12 (ST22).

[0125] The degradation state estimation accuracy determination unit 15 then searches the storage unit 13 for information on changes in SOC that can be combined based on the relationship between the open-circuit potential of the positive electrode and the change in SOC related to the latest positive electrode state estimation value obtained (ST23).

[0126] Specifically, the degradation state estimation accuracy determination unit 15 searches for information that matches the above-mentioned conditions from among the information on changes in multiple SOCs stored in the memory unit 13, for example. Alternatively, it may combine each piece of information with the SOC change related to the latest positive electrode state estimate that has been grasped.

[0127] The degradation state estimation accuracy determination unit 15 then determines whether it is possible to combine the latest positive electrode state estimation value with the change in SOC (ST24). If a change in SOC that is determined to be combinable is found (YES in ST24), the degradation state estimation accuracy determination unit 15 combines the multiple pieces of information to generate an open-circuit potential characteristic of a single positive electrode (ST25). This process is explained using Figure 4.

[0128] Then, the degradation state estimation accuracy determination unit 15 determines the reliability of the open-circuit potential characteristics of the generated positive electrode. Specifically, it determines whether the change in the state of affairs (SOC) of the open-circuit potential characteristics of the generated positive electrode is greater than or equal to a predetermined value (ST26).

[0129] As a result, if the change in SOC is determined to be greater than or equal to a predetermined value (YES in ST26), the degradation state estimation accuracy determination unit 15 determines that the reliability of the open-circuit potential characteristics of the generated positive electrode is high (ST27).

[0130] Then, if the degradation state estimation accuracy determination unit 15 determines that the probability is high, the most recent value, for example, the positive electrode state estimate stored in the memory unit 13, is corrected to the positive electrode state estimate that was determined to have high probability (ST28).

[0131] On the other hand, if it is not possible to combine the values ​​to generate an open-circuit potential characteristic of a single positive electrode (ST24 NO), or if the probability determination conditions are not met (ST26 NO), the degradation state estimation accuracy determination unit 15 determines that the probability of the generated open-circuit potential characteristic of the positive electrode is low (ST29). In this case, because the probability of the generated open-circuit potential characteristic of the positive electrode is low, the most recently calculated positive electrode state estimate is not modified by the calculated positive electrode state estimate (ST30).

[0132] Therefore, in this case, the most recently calculated positive electrode state estimate is treated as the latest positive electrode state estimate. This positive electrode state estimate is maintained until a positive electrode state estimate calculated using CCA or EIS, which is determined to have a higher probability, is obtained.

[0133] Next, we will explain the processing flow when EIS is selected for calculating the positive electrode state estimate. Figures 7 and 10 will be used in this explanation. Figure 10 is a flowchart showing the flow of confirmation of accuracy when the second positive electrode state estimation method is used in the secondary battery degradation estimation method according to an embodiment of the present invention.

[0134] It is assumed that the vehicle V, which is equipped with the secondary battery whose degradation state is to be estimated, is stationary (YES in ST1). Then, the determination unit 11 determines whether or not normal charging has started (ST2). As mentioned above, if normal charging has not started (NO in ST2), CCA cannot be performed. In that case, the calculation of the positive electrode state estimate by EIS is started or the determination is made again.

[0135] Specifically, the determination unit 11 determines whether a predetermined time has elapsed since the vehicle V came to a stop (ST41). This is because, when calculating the positive electrode state estimate using EIS, if the polarization state in the secondary battery is not resolved, the positive electrode state estimate cannot be calculated accurately even if the EIS process is executed. The predetermined non-operating time based on the relaxation phenomenon of the secondary battery is determined for each secondary battery through experiments, etc.

[0136] Therefore, if the determination unit 11 determines that a predetermined time has not elapsed since the vehicle V stopped (NO in ST41), the determination unit 11 will again check whether the predetermined time has elapsed.

[0137] On the other hand, if the determination unit 11 determines that a predetermined time has elapsed (YES in ST41), the determination unit 11 selects EIS as the method for estimating the positive electrode state (ST42). This determination is transmitted to the positive electrode state calculation unit 12, which uses EIS to calculate an estimated positive electrode state (ST43).

[0138] The positive electrode state estimate calculated by the positive electrode state calculation unit 12 is stored in the storage unit 13, and its accuracy is verified by the degradation state estimation accuracy determination unit 15 (ST44). The degradation state estimation accuracy determination unit 15 determines the accuracy while referring to measurement information acquired from the measurement information acquisition unit 14 as appropriate.

[0139] Specifically, the determinations include whether the SOC is above a predetermined value (ST45 in Figure 10), whether the temperature of the secondary battery is below a predetermined value (ST46), whether the deviation at the same frequency is above a predetermined value (ST47), and whether the deviation within the bandwidth of the set frequency is above a predetermined value (ST48).

[0140] Furthermore, regarding the conditions for determining these probabilities, if even one of them is met, it can be determined that the probability of the positive electrode state estimate is high. However, in the embodiment of the present invention, the probability is determined to be high only when all of these conditions are met (ST49).

[0141] Then, if the degradation state estimation accuracy determination unit 15 determines that the probability is high, the most recent value, for example, the positive electrode state estimate stored in the memory unit 13, is corrected to the positive electrode state estimate that was determined to have a high probability (ST50).

[0142] On the other hand, if any of the four judgment conditions mentioned above are not met, the degradation state estimation accuracy determination unit 15 determines that the calculated positive electrode state estimate is unlikely to be accurate (ST51). In this case, because the calculated positive electrode state estimate is unlikely to be accurate, the most recently calculated positive electrode state estimate is not modified by the newly calculated positive electrode state estimate (ST52).

[0143] Therefore, in this case, the most recently calculated positive electrode state estimate is treated as the latest positive electrode state estimate. This positive electrode state estimate is maintained until a positive electrode state estimate calculated using CCA or EIS, which is determined to have a higher probability, is obtained.

[0144] Up until now, in order to explain the processing flow of CCA and EIS, we have used examples where one of the positive electrode state estimation methods is selected for each. However, these positive electrode state estimation methods are not necessarily executed alternately; for example, EIS may be executed while CCA is being executed.

[0145] Furthermore, in cases where EIS is performed while CCA is being executed, the resistance value used in CCA may be a value calculated based on the impedance obtained in EIS performed while CCA is being executed.

[0146] In other words, the CCA is executed, and for example, when the SOC value transitions from 0% to 50%, the charging process is temporarily interrupted, and the execution of the CCA is also interrupted. If this interruption time is set to be a sufficient relaxation time, the EIS process can then be executed. After resuming the charging process, it is also possible to calculate a resistance value based on the impedance obtained from the EIS process and then use that resistance value to execute the interrupted CCA process again.

[0147] Furthermore, in the embodiments of the present invention, two methods for determining the accuracy of the positive electrode state estimate obtained by the second positive electrode state estimation method were explained using Figures 6(A) and 6(B). Although these were explained as separate methods, it is also possible to use these two methods in combination.

[0148] In other words, for example, when determining the likelihood, several impedances to be evaluated are selected in advance from a frame as shown in Figure 6(B). Then, each of the selected impedances is measured multiple times as shown in Figure 6(A), and for example, the average value is calculated. If the average value obtained for each of these selected impedances falls within a certain threshold range, the likelihood of the positive electrode state estimate obtained by the second positive electrode state estimation method is determined to be high.

[0149] [Effects of the Example] (1) A method for estimating the degradation of a secondary battery, comprising the steps of: selecting either a first positive electrode state estimation method or a second positive electrode state estimation method for estimating the state of the positive electrode of a secondary battery based on the condition of the vehicle on which the secondary battery to be degraded is installed; confirming the reliability of the positive electrode state estimate of the secondary battery obtained by the first positive electrode state estimation method or the second positive electrode state estimation method, which indicates the accuracy of the degradation state estimation; and determining whether or not to correct the positive electrode state estimate based on the level of the confirmed reliability.

[0150] By adopting this method for estimating the degradation of secondary batteries, when estimating the degradation state of the positive electrode of secondary batteries installed in vehicles, it is possible to appropriately select a method for estimating the degradation state according to the usage conditions of the vehicle, and by ensuring the accuracy of the obtained positive electrode state estimate, it is possible to estimate the degradation state with higher accuracy at all stages from the time of manufacture to the end of use of the secondary battery.

[0151] Furthermore, by continuously monitoring the degradation status of the secondary battery installed in vehicle V using the first and second positive electrode state estimation methods, when vehicle V's use ends, it is possible to immediately proceed to recycling without having to reassess the degradation status of the secondary battery. This reduces the costs associated with collecting the decommissioned vehicle V and inspecting the secondary battery.

[0152] (2) In the secondary battery degradation estimation method described in (1) above, the first positive electrode state estimation method is a method that uses the voltage value measured during charging and discharging of the secondary battery, and estimates the state of the positive electrode based on the difference between the initial open-circuit potential of the positive electrode of the secondary battery and the open-circuit potential of the positive electrode of the secondary battery at the time of measurement.

[0153] The first positive electrode state estimation method described above is a so-called CCA estimation method, and is a positive electrode state estimation method that can be used when charging and discharging are taking place. By combining it with EIS, which will be described later, it is possible to select a positive electrode state estimation method according to the vehicle's usage conditions.

[0154] (3) In the secondary battery degradation estimation method described in (2) above, in the step of confirming the certainty of the estimated positive electrode state of the secondary battery, the certainty is determined to be high if the change in SOC during charging and discharging of the secondary battery is equal to or greater than a predetermined value set in advance.

[0155] When the change in SOC is large, the difference between the initial open-circuit potential of the positive electrode of the secondary battery and the open-circuit potential of the positive electrode at the time of measurement is easier to grasp, allowing for a more accurate understanding of the state of the positive electrode.

[0156] (4) In the method for estimating the degradation of a secondary battery described in (2) or (3) above, in the step of confirming the certainty of the estimated positive electrode state of the secondary battery, the certainty is determined to be high if the C rate during charging and discharging of the secondary battery is less than or equal to a predetermined value set in advance.

[0157] By adopting the positive electrode state estimate for low C rate, it is possible to calculate a more accurate positive electrode state estimate using the open-circuit potential of the positive electrode. Therefore, the state of the positive electrode can be understood with greater precision.

[0158] (5) In the method for estimating the degradation of a secondary battery in any of (2) to (4) above, in the step of confirming the certainty of the estimated positive electrode state of the secondary battery, the certainty is determined to be high if the fluctuation of the C rate during charging and discharging of the secondary battery is less than or equal to a predetermined value set in advance.

[0159] By adopting the positive electrode state estimate when the C rate fluctuation is below a predetermined value, it is possible to calculate a more accurate positive electrode state estimate using the open-circuit potential of the positive electrode. Therefore, the state of the positive electrode can be understood with greater accuracy.

[0160] (6) In the method for estimating the degradation of a secondary battery in any of (1) to (5) above, in the step of confirming the certainty of the estimated positive electrode state of the secondary battery, the certainty is determined to be high if the temperature of the secondary battery is above a predetermined value set in advance.

[0161] As the temperature of the secondary battery decreases, its resistance increases, requiring a larger resistance correction when estimating the open-circuit voltage from the closed-circuit voltage. Conversely, as the temperature of the secondary battery increases, its resistance decreases, making it easier to calculate the open-circuit potential of the positive electrode. Therefore, it becomes easier to calculate an estimated positive electrode state using the open-circuit potential of the positive electrode, allowing for a more accurate understanding of the positive electrode state.

[0162] (7) In the method for estimating the degradation of a secondary battery in any of (2) to (6) above, the first positive electrode state estimation method is performed when multiple charge and discharge operations are performed on the secondary battery, and comprises the steps of acquiring and storing the relationship between the open-circuit potential of the positive electrode and the change in SOC obtained by the execution of the first positive electrode state estimation method each time, and generating an open-circuit potential characteristic of one positive electrode by combining the acquired multiple changes in SOC, and determining that the probability is high if the generated open-circuit potential characteristic of the positive electrode is greater than or equal to a preset change in SOC.

[0163] By combining multiple SOC changes to generate open-circuit potential characteristics of the positive electrode, taking into account the characteristic that the positive electrode does not degrade easily, it is possible to estimate the state of the positive electrode with greater accuracy. Furthermore, by adopting this method, it is not necessary to obtain results that directly lead to the estimation of the state of the positive electrode, which can provide an opportunity to increase the timing of performing CCA.

[0164] (8) In the secondary battery degradation estimation method described in (1) above, the second positive electrode state estimation method measures the impedance when an alternating current is applied to the secondary battery, obtains the positive electrode parameters in the equivalent circuit from the impedance and the equivalent circuit of the secondary battery, and estimates the state of the positive electrode from the positive electrode parameters.

[0165] The second positive electrode state estimation method described above is a so-called EIS estimation method, and is a positive electrode state estimation method that can be used while the vehicle is stationary. By combining it with the CCA described above, it is possible to select a positive electrode state estimation method according to the vehicle's usage conditions.

[0166] (9) In the secondary battery degradation estimation method described in (8) above, the second positive electrode state estimation method is performed when a predetermined non-operating time based on the relaxation phenomenon of the secondary battery has elapsed, which can be determined to have eliminated the polarization characteristics.

[0167] As the secondary battery's state stabilizes after a period of non-operation, the positive electrode state estimate can be calculated with high accuracy by using the second positive electrode state estimation method in this state.

[0168] (10) In the secondary battery degradation estimation method described in (8) or (9) above, in the step of confirming the accuracy of the estimated positive electrode state of the secondary battery, the accuracy is determined to be high if the SOC during execution of the second positive electrode state estimation method is equal to or greater than a predetermined value set in advance. A higher SOC allows for more accurate estimation of the positive electrode state.

[0169] (11) In the method for estimating the degradation of a secondary battery in any of (8) to (10) above, in the step of confirming the certainty of the estimated positive electrode state of the secondary battery, the certainty is determined to be high if the temperature of the secondary battery at the time of execution of the second positive electrode state estimation method is below a predetermined value set in advance.

[0170] The higher the temperature of the secondary battery, the lower the impedance value. However, when measuring impedance, there is a distance between the secondary battery being measured and the measuring device, so the impedance may be measured from the wires connecting the two. Therefore, if the impedance value is small, it becomes more difficult to separate the impedance value in the secondary battery from other impedance values, making it difficult to calculate an accurate estimate of the positive electrode state. For this reason, it is required that the temperature of the secondary battery be below a predetermined value.

[0171] (12) In the method for estimating the degradation of a secondary battery in any of (8) to (11) above, in the second method for estimating the positive electrode state, when measuring impedance, multiple impedances are measured at the same frequency, and the probability is determined to be high if the deviation of the multiple impedances obtained as measurement results is less than or equal to a predetermined value set in advance.

[0172] If the deviation of multiple measured impedance values ​​falls below a predetermined value, the variability of the measured values ​​decreases. Therefore, it becomes possible to calculate the estimated positive electrode state with greater accuracy.

[0173] (13) In the method for estimating the degradation of a secondary battery in any of (8) to (11) above, in the second method for estimating the positive electrode state, when measuring impedance, multiple impedances are measured within a preset frequency band, and the probability is determined to be high if the deviation of the multiple impedances obtained as measurement results is less than or equal to a preset predetermined value.

[0174] If the deviation of multiple impedance values ​​measured within the set frequency band is below a predetermined value, the variation in the measured values ​​will be reduced. Therefore, the estimated positive electrode state can be calculated with greater accuracy.

[0175] (14) In the secondary battery degradation estimation method in any of (1) to (13) above, the resistance value used when performing the first positive electrode state estimation method is the value calculated based on the impedance obtained in the second positive electrode state estimation method.

[0176] According to the calculation method of CCA, the first method for estimating the positive electrode state, the more accurate the resistance value (impedance value), the more accurately the positive electrode state estimate can be calculated. Therefore, by using a resistance value based on the impedance value measured by EIS, the second method for estimating the positive electrode state, the calculation of the positive electrode state estimate using CCA can also be performed with high accuracy.

[0177] (15) In the secondary battery degradation estimation method described in (14) above, the resistance value used in the first positive electrode state estimation method is a value calculated based on the impedance obtained in the second positive electrode state estimation method, which is performed before or while the first positive electrode state estimation method is being performed.

[0178] By using the resistance values ​​calculated in EIS before or during the execution of CCA, it is possible to calculate the positive electrode state estimate with greater accuracy.

[0179] (16) A secondary battery degradation estimation device comprises: a determination unit that determines whether to use a first positive electrode state estimation method or a second positive electrode state estimation method for a secondary battery to be mounted on a vehicle and subject to degradation estimation; a positive electrode state calculation unit that calculates a positive electrode state estimate value indicating the state of the positive electrode of the secondary battery using the selected first positive electrode state estimation method or second positive electrode state estimation method; and a degradation state estimation accuracy determination unit that checks the reliability of the positive electrode state estimate value calculated by the positive electrode state calculation unit and corrects the positive electrode state estimate value if it is determined that the reliability is high.

[0180] By using such a secondary battery degradation estimation device, when estimating the degradation state of the positive electrode of a secondary battery installed in a vehicle, it is possible to appropriately select a method for estimating the degradation state according to the vehicle's usage conditions, and by ensuring the accuracy of the obtained positive electrode state estimate, more accurate degradation state estimation can be performed at all stages from the time of manufacture to the end of use of the secondary battery.

[0181] Furthermore, by using the secondary battery degradation estimation device, the degradation status of the secondary battery installed in vehicle V can be monitored in real time. Therefore, when vehicle V is no longer in use, it can be immediately moved to the recycling process without having to reassess the degradation status of the secondary battery. This reduces the costs of collecting the vehicle V after its use, transporting the secondary battery to the inspection facility, and conducting the secondary battery inspection.

[0182] (17) In the secondary battery degradation estimation device described in (16) above, the degradation state estimation accuracy determination unit determines the accuracy of the positive electrode state estimation value by comparing the change in SOC during charging and discharging, the C rate during charging and discharging, the fluctuation of the C rate during charging and discharging, the temperature of the secondary battery, and the result of comparing the open-circuit potential characteristics of one positive electrode, which are generated by combining multiple changes in SOC obtained by performing the first positive electrode state estimation method, with a preset change in SOC, when the determination unit has adopted the first positive electrode state estimation method.

[0183] When the first positive electrode state estimation method is adopted, the reliability of the calculated positive electrode state estimate can be determined and reflected in the positive electrode state estimate as appropriate, thereby enabling a more accurate estimation of the secondary battery's degradation state.

[0184] (18) In the secondary battery degradation estimation device described in (16) above, if the determination unit adopts the second positive electrode state estimation method, the degradation state estimation accuracy determination unit determines the accuracy of the positive electrode state estimation value using at least one of the following: the deviation of multiple impedances obtained as a result of measuring multiple impedances at the same frequency when measuring the SOC, the temperature of the secondary battery, and the impedance when measuring the impedance during the execution of the second positive electrode state estimation method, or the deviation of multiple impedances obtained as a result of measuring multiple impedances within a preset frequency band when measuring the impedance.

[0185] When the second positive electrode state estimation method is adopted, the reliability of the calculated positive electrode state estimate can be determined and reflected in the positive electrode state estimate as appropriate, thereby enabling a more accurate estimation of the secondary battery's degradation state. [Explanation of symbols]

[0186] 1...Degradation estimation device, 11...Determination unit, 12...Positive electrode state calculation unit, 13...Storage unit, 14...Measurement information acquisition unit, 15...Degradation state estimation accuracy determination unit, 2...Battery monitoring system, 3...Vehicle control unit, 4...In-vehicle communication unit, A...Antenna, AC...AC power supply, BP...Battery pack, C...Charger, CP...Charging port, CS...Current sensor, I...Inverter, M...Drive motor, V...Vehicle

Claims

1. The steps include selecting either a first positive electrode state estimation method or a second positive electrode state estimation method for estimating the state of the positive electrode of a secondary battery based on the condition of the vehicle equipped with the secondary battery to be degraded, The steps include: confirming the reliability of the estimated positive electrode state of the secondary battery obtained by the first positive electrode state estimation method or the second positive electrode state estimation method, which indicates the accuracy of the estimation of the degradation state; The steps include determining whether or not to modify the estimated positive electrode state value based on the confirmed level of certainty, A method for estimating the degradation of a secondary battery, characterized by comprising the following:

2. The first method for estimating the positive electrode state is a method that uses a voltage value measured during charging and discharging of the secondary battery, and is characterized in that it estimates the state of the positive electrode based on the difference between the initial open-circuit potential of the positive electrode of the secondary battery and the open-circuit potential of the positive electrode of the secondary battery at the time of measurement.

3. The method for estimating the degradation of a secondary battery according to claim 2, characterized in that, in the step of confirming the certainty of the estimated positive electrode state of the secondary battery, the certainty is determined to be high if the change in SOC during charging and discharging of the secondary battery is greater than or equal to a predetermined value set in advance.

4. The method for estimating the degradation of a secondary battery according to claim 2, characterized in that, in the step of confirming the certainty of the estimated positive electrode state of the secondary battery, the certainty is determined to be high when the C rate in charging and discharging the secondary battery is less than or equal to a predetermined value set in advance.

5. The method for estimating the degradation of a secondary battery according to claim 2, characterized in that, in the step of confirming the certainty of the estimated positive electrode state of the secondary battery, the certainty is determined to be high when the fluctuation of the C rate during charging and discharging of the secondary battery is less than or equal to a predetermined value set in advance.

6. The method for estimating the degradation of a secondary battery according to claim 2, characterized in that, in the step of confirming the certainty of the estimated positive electrode state of the secondary battery, the certainty is determined to be high when the temperature of the secondary battery is above a predetermined value set in advance.

7. The first positive electrode state estimation method is performed when multiple charge-discharge operations are performed on the secondary battery, and each time, it includes the steps of acquiring and storing the relationship between the open-circuit potential of the positive electrode and the change in SOC obtained by the execution of the first positive electrode state estimation method, The method comprises the step of generating an open-circuit potential characteristic of a single positive electrode by combining the acquired changes in multiple states of equilibrium, The method for estimating the degradation of a secondary battery according to claim 2, characterized in that the probability is determined to be high when the generated open-circuit potential characteristics are greater than or equal to a preset change in SOC.

8. The second method for estimating the positive electrode state is characterized by measuring the impedance when an alternating current is applied to the secondary battery, determining the positive electrode parameter in the equivalent circuit from the impedance and the equivalent circuit of the secondary battery, and estimating the state of the positive electrode from the positive electrode parameter, as described in claim 1.

9. The method for estimating the degradation of a secondary battery according to claim 8, characterized in that the second positive electrode state estimation method is performed when a non-operating time based on a predetermined relaxation phenomenon of the secondary battery has elapsed, which can be determined to have eliminated the polarization characteristics.

10. The method for estimating the degradation of a secondary battery according to claim 8 or 9, characterized in that, in the step of confirming the certainty of the estimated positive electrode state of the secondary battery, the certainty is determined to be high when the SOC during execution of the second positive electrode state estimation method is greater than or equal to a predetermined value set in advance.

11. The method for estimating the degradation of a secondary battery according to claim 8, characterized in that, in the step of confirming the certainty of the estimated positive electrode state of the secondary battery, the certainty is determined to be high when the temperature of the secondary battery at the time of execution of the second positive electrode state estimation method is below a predetermined value set in advance.

12. The method for estimating the degradation of a secondary battery according to claim 8, characterized in that, in the second positive electrode state estimation method, when measuring the impedance, multiple impedances are measured at the same frequency, and the probability is determined to be high when the deviation of the multiple impedances obtained as measurement results is less than or equal to a predetermined value set in advance.

13. The method for estimating the degradation of a secondary battery according to claim 8, characterized in that, in the second positive electrode state estimation method, when measuring the impedance, a plurality of the impedances are measured within a preset frequency band, and the probability is determined to be high when the deviation of the plurality of impedances obtained as measurement results is less than or equal to a preset predetermined value.

14. The method for estimating the degradation of a secondary battery according to claim 1, characterized in that the resistance value used when performing the first positive electrode state estimation method is a value calculated based on the impedance obtained in the second positive electrode state estimation method.

15. The method for estimating the degradation of a secondary battery according to claim 14, characterized in that the resistance value used in the first positive electrode state estimation method is a value calculated based on the impedance obtained in the second positive electrode state estimation method performed before or during the execution of the first positive electrode state estimation method.

16. A determination unit that determines whether to use the first positive electrode state estimation method or the second positive electrode state estimation method for the secondary battery that is the subject of degradation estimation installed in the vehicle, A positive electrode state calculation unit calculates a positive electrode state estimate value indicating the state of the positive electrode of the secondary battery using the selected first positive electrode state estimation method or the second positive electrode state estimation method, A degradation state estimation accuracy determination unit checks the reliability of the positive electrode state estimate calculated by the positive electrode state calculation unit, and corrects the positive electrode state estimate if it is determined that the reliability is high. A device for estimating the degradation of a secondary battery, characterized by comprising the following:

17. The degradation state estimation accuracy determination unit, when the determination unit employs the first positive electrode state estimation method, determines the reliability of the positive electrode state estimation value using at least one of the following: the change in SOC during charging and discharging of the secondary battery, the C rate during charging and discharging, the fluctuation of the C rate during charging and discharging, the temperature of the secondary battery, and the result of comparing the open-circuit potential characteristics of one positive electrode generated by combining a plurality of changes in SOC obtained by executing the first positive electrode state estimation method with a preset change in SOC, as described in 16, the degradation estimation device for a secondary battery.

18. The degradation state estimation accuracy determination unit, when the determination unit adopts the second positive electrode state estimation method, determines the reliability of the positive electrode state estimation value using at least one of the following: the deviation of multiple impedances obtained as a result of measuring multiple impedances at the same frequency when measuring the SOC, the temperature of the secondary battery, and the impedance when measuring the impedance during execution of the second positive electrode state estimation method, or the deviation of multiple impedances obtained as a result of measuring multiple impedances within a preset frequency band when measuring the impedance, as described in 16, the secondary battery degradation estimation device.

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