Battery system

Abstract

A battery system according to the present invention includes a battery and a control device that estimates the SOC of the battery. The SOC-OCV characteristic of the battery has a voltage flat region. When the control device is started for the first time, the control device estimates the SOC of the battery on the basis of the SOC-OCV characteristics. In this case, when the voltage VB of the battery is in the voltage flat region a (Va < = VB < = Vb), S1, which is an intermediate value of the voltage flat region a, is estimated as the SOC of the battery.

Description

Technical Field

[0001] The present disclosure relates to battery systems. Background Art

[0002] Japanese Patent Publication No. 2018-63115 discloses a technique for appropriately estimating the state of charge (SOC) in a secondary battery with a wide stable region. The stable region refers to a region where the open circuit voltage (OCV) of the secondary battery hardly changes even if the SOC changes, and is also referred to as a "voltage flat region" in the present disclosure. In Japanese Patent Publication No. 2018-63115, the internal resistance of the secondary battery is calculated based on the current and voltage during charging and discharging, and the SOC is inferred using the relationship between the internal resistance and the SOC.

[0003] Generally speaking, the SOC of a secondary battery is calculated by a control device including an input device, a central processing unit (CPU), a memory, etc. In a battery system composed of a battery (secondary battery), a control device, etc., it is expected that the SOC of the battery can be inferred at the time of shipment of the battery system, when the control device is replaced, or when the control device is first started. In Japanese Patent Application Laid-Open No. 2018-63115, the internal resistance of the battery is calculated based on the current and voltage during charging and discharging, and the SOC is inferred, but when the control device is first started, the internal resistance cannot be calculated based on the current and voltage during charging and discharging. Therefore, it can be considered to infer the SOC based on the SOC-OCV characteristics when the control device is first started.

[0004] However, in a battery whose SOC-OCV characteristics have a voltage flat region, if the battery voltage is in the voltage flat region, the SOC estimated based on the detected voltage deviation varies greatly, so the SOC estimation error becomes larger, and there is a concern that the SOC estimation accuracy will deteriorate. Summary of the invention

[0005] An object of the present disclosure is to suppress deterioration in the estimation accuracy of SOC when a control device is first started.

[0006] The battery system disclosed in the present invention includes a battery, a voltage sensor for detecting the voltage of the battery, and a control device for estimating the SOC of the battery. The SOC-OCV characteristic of the battery has a voltage flat region in which the OCV changes slightly relative to the SOC. The control device is configured to estimate the SOC based on the SOC-OCV characteristic when the control device is first started, and in the case where the voltage detected by the voltage sensor is in the voltage flat region, a predetermined value is estimated as the SOC. The predetermined value is set to a value within the range of the SOC in the voltage flat region.

[0007] According to this configuration, when the control device is first activated, the SOC is estimated based on the SOC-OCV characteristic. When the voltage detected by the voltage sensor is in the voltage flat region of the SOC-OCV characteristic, a predetermined value set in advance is estimated as the SOC.

[0008] If the detected voltage deviates due to detection errors of the voltage sensor, etc., the SOC estimated based on the SOC-OCV characteristic changes greatly in the voltage flat region. When the voltage detected by the voltage sensor exists in the voltage flat region, a predetermined value is estimated as the SOC. As long as the predetermined value is set to a value within the range of the SOC in the voltage flat region, even if the detected voltage deviates, the estimation error of the SOC can be suppressed from becoming larger, thereby suppressing the deterioration of the SOC estimation accuracy.

[0009] Preferably, the predetermined value is a middle value of the SOC in the voltage flat region. If the predetermined value is set to the middle value of the SOC in the voltage flat region, it is possible to reduce an estimation error of the SOC.

[0010] Preferably, the battery is a battery pack having a plurality of cells connected in series, and the voltage sensor detects the voltage of each cell. The control device may also be configured to estimate the SOC of each cell based on the detection value of the voltage sensor, determine a representative SOC from the estimated SOCs of each cell, and use the representative SOC as the SOC of the battery.

[0011] According to this configuration, it is possible to reduce the estimation error of the SOC of the battery pack, thereby suppressing the deterioration of the SOC estimation accuracy.

[0012] Preferably, the control device is configured to determine whether the voltage of each cell is in a voltage flat region or a region outside the voltage flat region in the SOC-OCV characteristic, and determine the representative SOC based on the voltage of the cell in the region where the voltage of each cell is the largest.

[0013] According to this structure, it is determined whether the voltage of each single cell is in the voltage flat region or in the region outside the voltage flat region in the SOC-OCV characteristic. Furthermore, the representative SOC is determined based on the voltage of the single cell in the region where the voltage of each single cell is the largest, so even if the SOC of each single cell varies, it is possible to suppress the deterioration of the estimation accuracy of the SOC of the battery pack.

[0014] The battery system may be mounted on a vehicle, and the control device may include a nonvolatile memory.

[0015] According to this configuration, when the control device is activated for the first time, such as when the vehicle is shipped or when the control device is replaced, the SOC of the battery can be estimated, and the estimated SOC can be stored in the nonvolatile memory.

[0016] According to the present disclosure, it is possible to suppress deterioration in the estimation accuracy of the SOC when the control device is first started.

[0017] Features, advantages, and technical and industrial significance of exemplary embodiments of the present invention are described below with reference to the accompanying drawings, in which like reference numerals represent like elements. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is an overall configuration diagram of an electric vehicle equipped with a battery system according to the present embodiment;

[0019] Figure 2 is a diagram for explaining the SOC-OCV characteristics in the single cell (LFP cell) of the present embodiment;

[0020] Figure 3 is a flowchart showing an example of the first SOC estimation process executed by the battery ECU in the present embodiment;

[0021] Figure 4 This is a flowchart showing an example of the first SOC estimation process executed by the battery ECU in the modification. DETAILED DESCRIPTION

[0022] Hereinafter, the embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In addition, in the accompanying drawings, the same or corresponding parts are denoted by the same reference numerals, and their description will not be repeated.

[0023] Figure 1 1 is an overall configuration diagram of an electric vehicle (electrified vehicle) 1 equipped with a battery system S according to the present embodiment. In the present embodiment, the electric vehicle 1 is, for example, a battery electric vehicle. The electric vehicle 1 includes a motor generator (MG) 10 as a rotating electric machine, a power transmission gear 20, a drive wheel 30, a power control unit (PCU) 40, a system main relay (SMR) 50, a battery 100, a monitoring unit 200, a battery electronic control unit (ECU) 300 as an example of a control device, and a control ECU 500.

[0024] MG 10 is, for example, an embedded permanent magnet synchronous motor (IPM motor) and has a function as an electric motor and a generator. Output torque of MG 10 is transmitted to drive wheels 30 via a power transmission gear 20 including a speed reducer and a differential device.

[0025] When the electric vehicle 1 is braked, the MG 10 is driven by the driving wheel 30 and operates as a generator. Thus, the MG 10 also functions as a braking device for performing regenerative braking that converts the kinetic energy of the electric vehicle 1 into electric power. The regenerative electric power generated by the regenerative braking force in the MG 10 is stored in the battery 100.

[0026] The PCU 40 is a power conversion device that converts electric power bidirectionally between the MG 10 and the battery 100. The PCU 40 includes an inverter and a converter that operate based on a control signal from the control ECU 500, for example.

[0027] When the battery 100 is discharged, the converter boosts the voltage supplied from the battery 100 and supplies the voltage to the inverter. The inverter converts the DC power supplied from the converter into AC power to drive the MG 10.

[0028] On the other hand, when charging battery 100 , the inverter converts AC power generated by MG 10 into DC power and supplies it to the converter. The converter steps down the voltage supplied from the inverter to a voltage suitable for charging battery 100 and supplies it to battery 100 .

[0029] The SMR 50 is electrically connected to a power line connecting the battery 100 and the PCU 40. When the SMR 50 is closed (ON) (i.e., in a conductive state) according to a control signal from the control ECU 500, electric power can be exchanged between the battery 100 and the PCU 40. On the other hand, when the SMR 50 is disconnected (OFF) (i.e., in a disconnected state) according to a control signal from the control ECU 500, the electrical connection between the battery 100 and the PCU 40 is disconnected.

[0030] The battery 100 stores electric power for driving the MG 10. The battery 100 is a rechargeable DC power source (secondary battery), and is a battery pack in which a plurality of single batteries (battery cells) 100a are electrically connected in series. The battery 100 and the single battery 100a are equivalent to the "battery" of the present disclosure. The single battery 100a can be composed of a lithium ion battery, for example. In the present embodiment, as the single battery 100a, a lithium iron phosphate battery (LFP battery) using phosphorus (P), iron (Fe), and lithium (Li) as a positive electrode material is used.

[0031] The monitoring unit 200 includes a voltage sensor 210, a current sensor 220, and a temperature sensor 230. The voltage sensor 210 detects the voltage VB of the cell 100a (the voltage VB between the terminals of the cell 100a). The current sensor 220 detects the current IB input to and output from the battery 100 (cell 100a). The temperature sensor 230 detects the temperature TB of each cell 100a. Each sensor outputs its detection result to the battery ECU 300.

[0032] The electric vehicle 1 has an inlet 60, and the battery 100 can be externally charged using an electric vehicle supply equipment (EVSE, Electric Vehicle Supply Equipment) 400. The inlet 60 is configured to be connected to a connector 420 at the front end of a charging cable 410 provided on the EVSE 400. The inlet 60 is electrically connected to a power line connected to the battery 100 via a charging circuit 70. In the present embodiment, after the SMR 50 is closed, the inlet 60 is connected to the battery 100, so that external charging can be performed. In addition, it can also be configured such that the power line between the battery 100 and the SMR 50 is connected to the inlet 60 (charging circuit 70) via a charging relay, and the charging relay is closed, thereby enabling external charging of the battery 100.

[0033] The battery ECU 300 includes a CPU 301 and a memory 302. The memory 302 includes a nonvolatile memory (e.g., a flash memory). The battery ECU 300 uses the signal received from the monitoring unit 200 to infer the SOC of the battery 100 and outputs it to the control ECU 500. The battery system S is composed of the battery 100 (single cell 100a), the monitoring unit 200, the battery ECU 300, and the like.

[0034] The control ECU 500 includes a CPU 501 and a memory 502. The memory 502 includes a nonvolatile memory (e.g., a flash memory). The control ECU 500 controls each device so that the electric vehicle 1 is in a desired state based on information such as a signal received from the battery ECU 300, signals from various sensors (e.g., an accelerator opening signal, a vehicle speed signal, etc.) not shown, and a map stored in the memory 502 and a program.

[0035] Figure 2 This is a diagram for explaining the SOC-OCV characteristics of the single cell (LFP cell) 100a of this embodiment. Figure 2 In FIG. 1 , the vertical axis is the OCV [V] of the single cell 100a, and the horizontal axis is the SOC [%] of the single cell 100a. Figure 2As shown in FIG. 1 , in the relationship between SOC and OCV (hereinafter, this relationship is also referred to as an OCV curve), there is a wide range of regions (voltage flat regions) where the change in OCV is small with respect to the change in SOC and the change in the OCV curve is small. Therefore, if the SOC of the battery 100 (cell 100a) is estimated based on the SOC-OCV characteristics using the voltage VB detected by the voltage sensor 210, the estimation error of the SOC in the voltage flat region becomes large due to deviations caused by the detection error of the voltage VB and the like.

[0036] In order to reduce the estimation error of SOC, for example, when the battery 100 is externally charged, the SOC is set to 100 [%] when the battery 100 is fully charged and ends. Thereafter, the SOC is estimated by a current integration method using the integrated value of the input and output currents IB. By estimating the SOC in this way, the estimation error of the SOC in the voltage flat region can be reduced. In addition, when the IG switch (power switch) 250 is turned off, the SOC at that moment is stored in the non-volatile memory of the battery ECU 300. Thus, the SOC can be obtained by reading the SOC from the non-volatile memory when the IG switch 250 is turned on next time.

[0037] When the electric vehicle 1 is shipped from the factory or the battery ECU 300 is replaced, the SOC of the battery 100 is not stored in the nonvolatile memory when the battery ECU 300 is started for the first time. Therefore, it is conceivable to use the voltage VB and estimate the SOC based on the SOC-OCV characteristic when the battery ECU 300 is started for the first time. However, in the voltage flat region of the SOC-OCV characteristic, the estimation error of the SOC becomes large due to the deviation of the voltage VB caused by the detection accuracy of the voltage sensor 210, etc.

[0038] Reference Figure 2 , in the voltage flat region a where SOC is A to B [%], the OCV when SOC = A is Va [V], and the OCV when SOC = B is Vb [V] (for example, A = 29 [%], B = 61 [%], Va = 3.28 [V], Vb = 3.31 [V].). If based on Figure 2The SOC is estimated based on the SOC-OCV characteristic shown in the figure. When the voltage VB deviates between Va and Vb in the voltage flat region a, the estimated SOC changes from A to B (for example, if the magnitude of the deviation of the voltage VB is 0.03 [V], the SOC changes within the range of 29 to 60 [%].). Therefore, if the SOC is estimated based on the SOC-OCV characteristic, the estimation error of the SOC becomes larger in the voltage flat region a, and the estimation accuracy of the SOC deteriorates. In the present embodiment, when the SOC is estimated based on the SOC-OCV characteristic at the first start-up of the battery ECU 300, when the voltage VB is in the voltage flat region, the intermediate value of the SOC in the voltage flat region is regarded as the estimated value of the SOC. Thus, the estimation error of the SOC is reduced to suppress the deterioration of the estimation accuracy of the SOC.

[0039] Figure 3 1 is a flowchart showing an example of the first SOC estimation process performed by the battery ECU 300 in the present embodiment. This flowchart is executed when the IG switch 250 is turned from off to on. In step (hereinafter, step is abbreviated as "S") 10, it is determined whether it is the first start of the battery ECU 300. For example, when the value of the SOC stored in the non-volatile memory included in the memory 302 is set to the specification of "0" when the battery ECU 300 is shipped, when the SOC read from the non-volatile memory is "0", it can be determined that it is the first start (first start) of the battery ECU 300. In addition, when the cumulative driving distance of the electric vehicle 1 is stored (saved) in the non-volatile memory of the memory 302, the cumulative driving distance stored in the non-volatile memory of the control ECU 500 is compared with the cumulative driving distance read from the non-volatile memory of the memory 302. When the cumulative driving distances are different, it can also be determined that it is the first start of the battery ECU 300. If it is determined in S10 that the battery ECU 300 is started for the first time, the process proceeds to S11 , and if it is not the first time, the routine ends.

[0040] In S11, it is determined whether the voltage VB detected by the voltage sensor 210 is in the voltage flat region a. If "Va≤VB≤Vb" is satisfied and the voltage VB is in the voltage flat region a, an affirmative determination is made and the process proceeds to S12. If "VB<Va" or "VB>Vb" is satisfied and the voltage VB is not in the voltage flat region a, a negative determination is made and the process proceeds to S13.

[0041] In S12, the SOC of the battery cell 100a where the voltage VB is detected is calculated as S1 [%], and then the process proceeds to S16. S1 is the middle value of the SOC in the voltage flat region a. Figure 2In the example, it is "S1=(A+B) / 2".

[0042] In S13, it is determined whether the voltage VB detected this time is in the voltage flat region b. If "Vc≤VB≤Vd" is satisfied and the voltage VB is in the voltage flat region b, an affirmative determination is made and the process proceeds to S14. If "VB<Vc" or "VB>Vd" is satisfied and the voltage VB does not exist in the voltage flat region b, a negative determination is made and the process proceeds to S15.

[0043] In S14, the SOC of the current battery cell 100a is calculated as S2 [%], and then the process proceeds to S16. S2 is the middle value of the SOC in the voltage flat region b. Figure 2 In the example, it is "S2=(C+D) / 2".

[0044] In S15, using voltage VB, according to Figure 2 The SOC-OCV characteristic of the battery 100a is calculated, and then the process proceeds to S16. By applying the voltage VB to Figure 2 The SOC is calculated based on the value of the OCV, and the SOC is calculated to be a value in the range of B to C. The SOC calculated in S15 is calculated in increments of 1 [%], and the decimal points are rounded off or discarded.

[0045] In S16, it is determined whether the SOC of all the cells 100a included in the battery 100 has been calculated. If the SOC of all the cells 100a has not been calculated, the process returns to S11 to calculate the SOC of the next cell 100a. If the SOC of all the cells 100a has been calculated, an affirmative determination is made in S16, and the process proceeds to S17.

[0046] In S17, the SOC of the battery 100 is estimated based on the majority rule. The largest SOC is extracted from the SOCs of all the single cells 100a calculated in S12, S14, and S15 as the representative SOC. Then, the representative SOC is adopted as the SOC of each single cell 100a and the battery 100, and this procedure ends. In addition, if the SOC estimation is completed by the first SOC estimation process, the SOC estimation based on the current integration method using the integrated value of the input and output current IB is started thereafter (the SOC is updated at any time by the current integration method).

[0047] According to the present embodiment, when the battery ECU 300 is first started, the SOC is estimated based on the SOC-OCV characteristic. When the voltage VB detected by the voltage sensor 210 is in the voltage flat region a of the SOC-OCV characteristic, S1 is estimated as the SOC, and when it is in the voltage flat region b, S2 is estimated as the SOC.

[0048] If the voltage VB deviates due to a detection error of the voltage VB, the SOC estimated based on the SOC-OCV characteristic greatly changes in the voltage flat regions a and b, and the estimation error of the SOC increases. However, since the intermediate values ​​of the SOC in the voltage flat regions a and b, that is, S1 and S2, are estimated as the SOC, the estimation error of the SOC can be suppressed from increasing, thereby suppressing the deterioration of the SOC estimation accuracy.

[0049] According to the present embodiment, the SOC of each cell 100a is estimated, and a representative SOC is determined from the estimated SOC of the cell 100a, and the representative SOC is adopted as the SOC of the cell 100a and the battery 100. Even if the SOC estimation error of any cell 100a becomes large due to the variation of the voltage VB, the SOC with a small estimation error can be adopted as the SOC of the cell 100a and the battery 100.

[0050] In the above embodiment, when the IG switch 250 is turned from off to on, the Figure 3 . When the electric vehicle 1 is shipped from the factory, when the battery ECU 300 is started for the first time, S10 may be omitted and the process may be started from S11. In this case, for example, the process may be started from S11 after the battery ECU 300 is connected to the power supply (+B). In addition, the control ECU 500 may start the process from S11 after the IG switch 250 is first detected to be turned on from off.

[0051] In the above embodiment, S1 is set to the intermediate value of SOC in the voltage flat region a, that is, "S1 = (A + B) / 2". However, S1 may not be an intermediate value, as long as it is a value that does not exceed the range of the voltage flat region a. For example, S1 may be set to a value between "(A + B) × 0.2" and "(A + B) × 0.8". Similarly, in the voltage flat region b, S2 may be set to a value between "(C + D) × 0.2" and "(C + D) × 0.8".

[0052] Modifications

[0053] Figure 4 1 is a flowchart showing an example of the first SOC estimation process executed by the battery ECU 300 in the modified example. This flowchart is also executed when the IG switch 250 is operated from off to on. Figure 3 In the flowchart shown, S12 is changed to S12a, S14 is changed to S14a, S15 is changed to S15a, S16 is changed to S16a, and S17 is changed to S17a. The other steps are the same as Figure 3The processing is the same as that of , so its description is omitted.

[0054] In S12a, the counter C1 is incremented by 1. In S14a, the counter C2 is incremented by 1. In S15a, the counter C3 is incremented by 1. The initial values ​​of the counters C1, C2, and C3 are 0.

[0055] In S16a, it is determined whether the region where the voltage VB exists has been determined in all the cells 100a included in the battery 100. If the region where the voltage VB exists has not been determined in all the cells 100a, the process returns to S11 to determine the region where the voltage VB exists in the next cell 100a. If the region where the voltage VB exists in all the cells 100a has been determined, an affirmative determination is made in S16a, and the process proceeds to S17a.

[0056] In S17a, the SOC of the battery 100 is estimated by the majority rule. When the value of the counter C1 is greater than the counter C2 and the counter C3, S1 (= (A + B) / 2) is set as the representative SOC. When the value of the counter C2 is greater than the counter C1 and the counter C3, S2 (= (C + D) / 2) is set as the representative SOC.

[0057] When the value of the counter C3 is greater than the counter C1 and the counter C2, the average value of the SOC calculated based on the SOC-OCV characteristic using the voltage VB of each battery cell 100a counted in S15a is set as the representative SOC. Then, the representative SOC is adopted as the SOC of each battery cell 100a and the battery 100, and the program ends. In addition, when the SOC estimation is completed by the first SOC estimation process, the SOC estimation based on the current integration method using the integrated value of the input and output current IB is started thereafter (the SOC is updated at any time by the current integration method).

[0058] In this modification, when the battery ECU 300 is started for the first time, the Figure 2 The SOC is estimated by the SOC-OCV characteristic shown in FIG. It is determined whether the voltage VB detected by the voltage sensor 210 is in the voltage flat region a, the voltage flat region b, or a region other than the voltage flat region, and the representative SOC is determined based on the voltage VB in the region where the voltage VB is the largest. In the case of the voltage flat region a of the SOC-OCV characteristic, S1 is set as the representative SOC, and in the case of the voltage flat region b, S2 is set as the representative SOC. Furthermore, the representative SOC is adopted as the SOC of each single cell 100a and the battery 100.

[0059] If the voltage VB deviates due to a detection error of the voltage VB, the SOC estimated based on the SOC-OCV characteristic greatly changes in the voltage flat regions a and b, and the estimation error of the SOC increases. However, since the intermediate values ​​of the SOC in the voltage flat regions a and b, that is, S1 and S2, are estimated as the SOC, the estimation error of the SOC can be suppressed from increasing, thereby suppressing the deterioration of the SOC estimation accuracy.

[0060] In the above embodiment, a lithium iron phosphate battery (LFP battery) is used as the cell 100a. However, the cell 100a may be another type of battery as long as it has a region (voltage flat region) where the OCV curve has a small change.

[0061] Vehicles to which the battery system S of the present disclosure can be applied are not limited to Figure 1 The electric vehicle 1 shown. For example, the present disclosure can also be applied to a plug-in hybrid electric vehicle having an engine and an electric generator, and can also be applied to a fuel cell electric vehicle having a battery and capable of external charging. In addition, it can also be an industrial vehicle such as a forklift. In addition, the battery system S can also be a battery for fixed installation.

[0062] Furthermore, in the case of adopting a configuration in which the SOC at the time when the IG switch 250 is turned off is stored in the nonvolatile memory of the memory 302 and the nonvolatile memory of the memory 502, when the battery ECU 300 is replaced, the SOC stored in the nonvolatile memory of the memory 502 can be used to estimate the SOC at the time of the first start of the battery ECU 300. For example, it can also be configured so that: when the IG switch 250 is turned off, the SOC at the time of the first start of the battery ECU 300 is estimated. Figure 3 or Figure 4 When a positive determination is made in S10 of the control ECU 500, it is determined whether the SOC is stored in the non-volatile memory (memory 502) of the control ECU 500. If the SOC is stored, the value of the SOC is determined as the representative SOC, the SOC is estimated, and the program ends. If the SOC is not stored in the non-volatile memory of the control ECU 500, the processing after S11 is performed.

[0063] All points of the embodiments disclosed this time should be considered as illustrative and not limiting of the present invention. The scope of the present invention is not limited by the description of the above embodiments, but is indicated by the claims, and is intended to include the meaning equivalent to the claims and all changes within the scope thereof.

Claims

1. A battery system, wherein: The battery system comprises: Battery; a voltage sensor to detect the voltage of the battery; and a control device, inferring the SOC of the battery, The SOC-OCV characteristics of the battery have a voltage flat region where the OCV changes slightly with respect to the SOC. The control device is configured such that when the control device is started for the first time, inferring the SOC based on the SOC-OCV characteristic, and when the voltage detected by the voltage sensor is in the voltage flat region, a predetermined value set in advance is estimated as the SOC, The predetermined value is set to a value within the range of the SOC in the voltage flat region.

2. The battery system according to claim 1, wherein: The prescribed value is a middle value of the SOC in the voltage flat region.

3. The battery system according to claim 1 or 2, wherein: The battery is a battery pack having a plurality of single cells connected in series. The voltage sensor detects the voltage of each of the plurality of battery cells. The control device is composed of: estimating the SOC of each of the plurality of battery cells based on the detection value of the voltage sensor, And determine the representative SOC from the estimated SOC of each battery, The representative SOC is adopted as the SOC of the battery.

4. The battery system according to claim 3, wherein: The control device is composed of: determining whether the voltage of each of the battery cells is in the voltage flat region or in a region outside the voltage flat region in the SOC-OCV characteristic; The representative SOC is determined based on the voltage of the cell in the region where the voltage of each cell is the highest.

5. The battery system according to claim 3, wherein: The battery system is mounted on a vehicle. The control device includes a nonvolatile memory.

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