Methods for determining the degradation of secondary batteries

Abstract

To provide a method for determining degradation of a secondary battery which can determine an abnormal degradation of a positive electrode.SOLUTION: The method for determining degradation of a secondary battery includes: (a) acquiring a first internal resistance value of a battery when a current value is at least a first current value, subsequent to continuous discharge for a predetermined time; and (b) evaluating the relation between a first internal resistance value acquired in the step (a) and one of a second internal resistance value and the capacity maintenance rate of the battery when the current value is not larger than a second current value.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to a method for determining the deterioration of a secondary battery.

Background Art

[0002] Patent Document 1 discloses a battery system that acquires the relationship between the rate of change in resistance and current in a secondary battery with a reduced state of charge in a secondary battery in which the battery voltage is affected by the positive electrode potential rather than the negative electrode potential, and determines the deterioration state of the secondary battery by distinguishing between deterioration due to wear of the secondary battery and deterioration due to the salt concentration distribution inside the secondary battery based on the acquired relationship.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In a secondary battery, it is known that the materials constituting the secondary battery deteriorate (deteriorate over time), for example, according to the temperature, SOC (State of Charge), and elapsed time of the secondary battery. In particular, the materials constituting the positive electrode may deteriorate abnormally.

[0005] An object of the present disclosure is to provide a method for determining the deterioration of a secondary battery that can determine abnormal deterioration of the positive electrode.

Means for Solving the Problems

[0006] The present inventors have found that the above problems can be solved by the following means. <Aspect 1> A method for determining the deterioration of a secondary battery, including the following: (a) Obtaining a first internal resistance value of the battery when the current value is equal to or greater than a first current value, following continuous discharge for a predetermined period of time. (b) Evaluate the relationship between the first internal resistance value obtained in step (a) and the second internal resistance value or the battery capacity retention rate when the current value is less than or equal to the second current value. <Aspect 2> The method according to Embodiment 1, wherein in step (b), the relationship between the first internal resistance value obtained in step (a) and the second internal resistance value when the current value is less than or equal to the second current value is evaluated. <Aspect 3> The method according to embodiment 1, wherein in step (b), the relationship between the first internal resistance value obtained in step (a) and the battery capacity retention rate is evaluated. [Effects of the Invention]

[0007] According to this disclosure, a method for determining the degradation of a secondary battery that can identify abnormal degradation of the positive electrode can be provided. [Brief explanation of the drawing]

[0008] [Figure 1] Figure 1 is a graph showing the relationship between discharge time and internal resistance. [Figure 2] Figure 2 is a flowchart showing the process for determining the degradation state of a secondary battery based on the relationship between current and resistance ratio. [Figure 3] Figure 3 is a graph showing the relationship between current and resistance ratio. [Figure 4] Figure 4 is a flowchart showing the process for determining the degradation state of a secondary battery based on the relationship between capacity retention rate and resistance change rate. [Figure 5] Figure 5 is a graph showing the relationship between capacity retention rate and resistance change rate. [Modes for carrying out the invention]

[0009] The embodiments of this disclosure will be described in detail below. However, this disclosure is not limited to the embodiments described below, and can be implemented in various modified forms within the scope of the essence of the disclosure.

[0010] Methods for determining the degradation of secondary batteries The present disclosure method for determining the degradation of a secondary battery includes (a) obtaining a first internal resistance value of the battery when the current value is equal to or greater than a first current value, following continuous discharge for a predetermined period of time, and (b) evaluating the relationship between the first internal resistance value obtained in step (a) and a second internal resistance value or the battery's capacity retention rate when the current value is equal to or less than a second current value.

[0011] The disclosing parties found that secondary batteries with abnormally deteriorated positive electrodes, in addition to deterioration over time, exhibit a higher internal resistance when discharged at a large current for a long period compared to secondary batteries that have only deteriorated over time, leading to this disclosure.

[0012] The method of determining the degradation of a secondary battery according to this disclosure includes (a) obtaining a first internal resistance value Rp when the current value is equal to or greater than a first current value Ip, following a continuous discharge for a predetermined time, in particular obtaining a first internal resistance value Rp when the current value is equal to or greater than a first current value Ip, following a continuous discharge at a predetermined current value for a predetermined time. Here, the “predetermined current value” may be the same value as the first current value Ip, for example, 150A or more, 200A or more, or 250A or more, and 500A or less, 400A or less, or 300A or less.

[0013] The discharge time T can be measured by, for example, a timer or the like. The "predetermined time" can be, for example, a time when the length of the discharge time T is a time equal to or longer than a reference time Tp that can discriminate the deterioration state of the secondary battery verified in advance by tests or the like. For example, as shown in FIG. 1, under predetermined conditions, when the discharge time T is 10 seconds, for the secondary battery in the initial state and the secondary battery deteriorated over time, the internal resistance of the battery with abnormally deteriorated positive electrode becomes significantly large. Therefore, in this case, the reference time Tp can be set to 10 seconds. Here, the initial state is a state used as a reference when evaluating the deterioration of the secondary battery, and can be, for example, the state immediately after manufacturing the secondary battery. Note that in the state immediately after manufacturing the secondary battery, the deterioration of the secondary battery has not occurred.

[0014] The first current value Ip can be any value that can discriminate the deterioration state of the battery. The first current value Ip can be, for example, 150 A or more, 200 A or more, or 250 A or more, and can be 500 A or less, 400 A or less, or 300 A or less.

[0015] In the method of the present disclosure according to the first embodiment, in step (b), it includes evaluating the relationship between the first internal resistance value Rp obtained in step (a) and the second internal resistance value Rd when the current value is not more than the second current value Id.

[0016] That is, the deterioration of the secondary battery can be evaluated using the resistance ratio. The resistance ratio is a value obtained by dividing the first internal resistance value Rp when the current value is not less than the first current value Ip by the second internal resistance value Rd when the current value is not more than the second current value Id.

[0017] Here, the second current value Id can be the current value of the secondary battery in the initial state.

[0018] A system for determining the deterioration of a secondary battery includes an ECU. A typical configuration of the ECU includes at least a ROM (Read Only Memory) storing a program for performing such control, a CPU (Central Processing Unit) capable of executing the program, a RAM (Random Access Memory) for temporarily storing data, and input / output ports.

[0019] Various signals from a voltage sensor, a current sensor, a temperature sensor, etc. are input to the ECU via the input port. Also, drive signals to a load (power consumer and / or power supplier) are output from the ECU via the output port.

[0020] The ECU constitutes a discharge time determination means, an internal resistance acquisition means, and a discrimination means.

[0021] The discharge time determination means is configured to determine whether the length of the discharge time T is equal to or greater than a reference time Tp that can discriminate the deterioration state of the secondary battery. The method for determining the length of the discharge time T is not particularly limited. For example, it may be a method of comparing whether the length of the discharge time T measured by a timer is equal to or greater than a reference time Tp verified and set in advance by tests or the like. The current value during discharge may be a first current value Ip.

[0022] The internal resistance acquisition means is configured to acquire (estimate) the internal resistance of the secondary battery. The method for acquiring the internal resistance is not particularly limited, and a conventionally known method commonly used in a general battery system may be adopted. For example, based on various data detected by a voltage sensor and a current sensor, a method of estimating the internal resistance by dividing the voltage change during charge and discharge by the change in current at that time (for example, a method of linearly approximating a parameter of the current change amount and a parameter based on the voltage change amount and the impedance change amount, and calculating the slope of the approximated straight line as the impedance of the battery) is exemplified.

[0023] The internal resistance acquisition means allows for the acquisition of a first internal resistance value Rp when the current value is equal to or greater than a first current value Ip, and a second internal resistance value Rd when the current value is equal to or less than a second current value Id.

[0024] The discrimination means is configured to determine the degradation state of the secondary battery based on information regarding the first internal resistance value Rp.

[0025] In the first embodiment, the discrimination means identifies the relationship between the current and resistance ratio of the secondary battery. Specifically, the discrimination means calculates the resistance ratio for each current while changing the current of the secondary battery. By plotting the acquired current and resistance ratio on a coordinate system, a map showing the relationship between current and resistance ratio is obtained. Then, by referring to this map, the degradation state of the secondary battery is determined from the relationship between current and resistance ratio. This map can be created in advance by conducting preliminary experiments or the like.

[0026] Next, the process for determining the degradation state of the secondary battery in the first embodiment will be explained using the flowchart shown in Figure 2. The process shown in Figure 2 is executed by the ECU.

[0027] In step S101, the ECU starts discharging the secondary battery.

[0028] In step S102, the ECU determines whether the discharge time T is greater than or equal to the reference time Tp. If the discharge time T is greater than or equal to the reference time Tp, the ECU executes the process in step S103. If the discharge time T is less than or equal to the reference time Tp, the ECU repeats the process in step S101. The current value during discharge may be the first current value Ip.

[0029] In step S103, the ECU obtains a first internal resistance value Rp for the secondary battery. The first internal resistance value Rp can be calculated from the voltage of the secondary battery and a current value greater than or equal to a first current value Ip. The ECU can obtain the voltage of the secondary battery based on the output of the voltage sensor. The ECU can also obtain the current of the secondary battery based on the output of the current sensor.

[0030] In step S104, the ECU obtains the resistance ratio using the first internal resistance value Rp calculated in step S103. Here, the second internal resistance value Rd can be determined in advance through experiments or other means and stored in the ROM. In this case, the ECU can calculate the resistance ratio by dividing the first internal resistance value Rp calculated in step S103 by the second internal resistance value Rd read from the ROM.

[0031] The internal resistance of a secondary battery can change depending on its temperature. Therefore, if information showing the relationship between internal resistance and temperature is obtained in advance, the internal resistance corresponding to a given temperature can be identified by obtaining the battery's temperature. The temperature of a secondary battery can be obtained using a temperature sensor. The information showing the relationship between internal resistance and temperature can be stored in ROM. This information showing the relationship between internal resistance and temperature can be represented as a map or a function.

[0032] In step S105, the ECU obtains the relationship between the current and resistance ratio of the secondary battery. Specifically, the ECU calculates the resistance ratio for each current while changing the current of the secondary battery. That is, the ECU calculates a first internal resistance value Rp for each current, and calculates the resistance ratio based on this first internal resistance value Rp and a second internal resistance value Rd. By plotting the obtained current and resistance ratio on the coordinate system shown in Figure 3, the relationship between current and resistance ratio is obtained.

[0033] In step S106, the ECU determines whether the resistance ratio increases when a large current flows. For example, the ECU determines whether the resistance ratio when a large current flows is greater than a threshold. If the resistance ratio when a large current flows is greater than the threshold, it can be determined that the positive electrode of the secondary battery is abnormally degraded (S107). Conversely, if the resistance ratio when a large current flows is less than the threshold, it can be determined that the positive electrode of the secondary battery is not abnormally degraded.

[0034] Here, the threshold can be any resistance ratio value at a current value where the degradation state of the secondary battery cannot be determined. For example, as shown in Figure 3, the resistance ratio at a current of 60A can be set to 1, and this value can be used as the threshold. In this case, if the resistance ratio significantly exceeds the threshold, it can be determined that the positive electrode of the secondary battery is abnormally degraded. In Figure 3, this corresponds to the case when a current of 250A flows. In contrast, in a battery where the positive electrode is not abnormally degraded, such an increase in the resistance ratio does not occur.

[0035] Another method for determining the degradation state of a secondary battery from the relationship between current and resistance ratio is to compare it with a map of a secondary battery that has degraded over time. That is, a map showing the relationship between current and resistance ratio of a secondary battery that has degraded over time can be created in advance, and by comparing this map with a map plotting the calculated values, the degradation state of the secondary battery can be determined. As shown in Figure 3, in a secondary battery where the positive electrode has degraded abnormally, the resistance ratio when a large current is flowing is greater than the resistance ratio when a small current is flowing. In contrast, in a battery where the positive electrode has not degraded abnormally, such an increase in resistance ratio does not occur.

[0036] Thus, when the resistance ratio increases when a large current is flowing, the ECU determines that the positive electrode of the secondary battery is abnormally degraded (S107). On the other hand, when the resistance ratio does not increase when a large current is flowing, the ECU determines that the positive electrode of the secondary battery is not abnormally degraded and terminates the process shown in Figure 2.

[0037] In the method of the present disclosure according to the second embodiment, in step (b), the relationship between the first internal resistance value obtained in step (a) and the capacity retention rate of the battery is evaluated.

[0038] In the second embodiment, the ECU further includes means for acquiring battery capacity.

[0039] The battery capacity acquisition means is configured to acquire (estimate) the chargeable and dischargeable battery capacity of a secondary battery. The method for acquiring the battery capacity is not particularly limited, and conventionally known methods commonly used in general battery systems can be employed. For example, one example is a method for estimating the battery capacity of a secondary battery according to a battery model equation based on various data detected by a voltage sensor, current sensor, and temperature sensor, etc. (For example, a method in which the open-circuit voltage characteristics of the positive and negative electrodes of a secondary battery, which have been determined in advance, are stored, and the amount of active material, capacity density, and resistance value of the positive and negative electrodes are extracted by referring to this stored data and the data detected by the voltage sensor, current sensor, and temperature sensor, and the battery capacity of the secondary battery is estimated using these extracted parameters).

[0040] In the second embodiment, the discrimination means calculates the capacity retention rate by subtracting the acquired battery capacity from the initial capacity. The discrimination means also calculates the resistance change rate by subtracting the initial internal resistance value from the acquired internal resistance value. Then, the degradation state of the secondary battery is determined by referring to a map showing the relationship between the capacity retention rate and the resistance change rate of a secondary battery that has deteriorated over time, as shown in Figure 5. This map can be created in advance by conducting preliminary experiments. The initial capacity and initial internal resistance value can also be determined in advance by conducting preliminary experiments.

[0041] Next, the process for determining the degradation of the secondary battery in the second embodiment will be explained using the flowchart shown in Figure 4. The process shown in Figure 4 is executed by the ECU.

[0042] In step S201, the ECU obtains the battery capacity of the secondary battery. For example, the battery capacity can be estimated according to a battery model equation based on data detected by a voltage sensor, a current sensor, and a temperature sensor.

[0043] In step 202, the ECU starts discharging the secondary battery.

[0044] In step 203, the ECU determines whether the discharge time T is greater than or equal to the reference time Tp. If the discharge time T is greater than or equal to the reference time Tp, the ECU executes the process in step S204. If the discharge time T is less than or equal to the reference time Tp, the ECU executes the process in step S202 again. The current value during discharge may be the first current value Ip.

[0045] In step 204, the ECU obtains a first internal resistance value Rp when the current value is a first current value Ip. For example, the ECU can estimate the internal resistance value by dividing the voltage change during charging and discharging by the current change at that time, based on various data detected by the voltage sensor and the current sensor.

[0046] In step S205, the ECU calculates the capacity retention rate (ΔAh) by subtracting the acquired battery capacity from the initial capacity. The ECU also calculates the resistance change rate (ΔR) by subtracting the initial internal resistance value from the acquired internal resistance value. Based on the calculated values, the ECU obtains the relationship between the capacity retention rate and the resistance change rate.

[0047] In step S206, the ECU refers to a map stored in ROM that shows the relationship between the capacity retention rate and the resistance change rate of a secondary battery that has deteriorated over time, and determines the deterioration state of the secondary battery. Specifically, as shown in Figure 5, if the resistance change rate at low capacity in the relationship map obtained in step S205 is larger than the resistance change rate at low capacity in the map for a secondary battery that has deteriorated over time, the ECU determines that abnormal deterioration of the positive electrode has occurred (S207). Conversely, if the ECU compares each map and there is no difference in the resistance change rate at low capacity, it determines that abnormal deterioration of the positive electrode has not occurred and terminates the process shown in Figure 4.

[0048] For example, nickel-metal hydride batteries or lithium-ion batteries can be used as secondary batteries. The secondary battery can be installed in a vehicle, for example, and the vehicle can be driven using the output of the secondary battery.

Claims

1. A method for determining the degradation of a secondary battery, including both (a) and (b) below: (a) Obtaining a first internal resistance value of the battery when the current value is equal to or greater than a first current value, following continuous discharge for a predetermined period of time. (b) Evaluate the relationship between the first internal resistance value obtained in step (a) and the second internal resistance value or the battery capacity retention rate when the current value is the current value of the secondary battery in its initial state.

2. The method according to claim 1, wherein in step (b), the relationship between the first internal resistance value obtained in step (a) and the second internal resistance value when the current value is less than or equal to the second current value is evaluated.

3. The method according to claim 1, wherein in step (b), the relationship between the first internal resistance value obtained in step (a) and the battery capacity retention rate is evaluated.

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