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. When the SMR is not connected after the replacement of the battery is detected, the SOC is estimated on the basis of the SOC-OCV characteristics on the basis of the voltage of the single battery when the battery ECU is started. When the SMR is connected, the SOC estimated from the voltage is corrected using the integrated value of the current charged or discharged via the SMR, and the SOC is estimated.

Description

Technical Field

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

[0002] Japanese Patent Publication No. 2018-63115 discloses a technology for appropriately estimating the state of charge (SOC) in a secondary battery with a large stable region. The stable region is 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 estimated using the relationship between the internal resistance and the SOC.

[0003] Generally, 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., there is a situation where the battery is replaced. When a battery is replaced, it is desired to infer the SOC of the replaced battery.

[0004] In Japanese Patent Application Laid-Open No. 2018-63115, the internal resistance of the battery is calculated based on the current and voltage during charge and discharge, and the SOC is estimated. However, in order to calculate the internal resistance of the battery, sufficient time is required after the battery is replaced, and the SOC cannot be estimated at the same time as the battery is replaced. Summary of the invention

[0005] The purpose of the present disclosure is to infer the SOC of a battery when the battery is replaced.

[0006] The battery system disclosed in the present invention comprises: a battery; a voltage sensor for detecting the voltage of the battery; and a control device for estimating the SOC of the battery. When the control device detects that the battery has been replaced, the control device uses the voltage detected by the voltage sensor to estimate the SOC based on the SOC-OCV characteristic of the battery. In addition, when the control device detects that the battery has been replaced, the control device corrects the SOC estimated based on the SOC-OCV characteristic based on the current discharged or charged through the relay when a relay connecting the battery to the load is connected.

[0007] According to this structure, when replacing a battery, the control device uses the voltage detected by the voltage sensor and estimates the SOC based on the SOC-OCV characteristic. When the replacement of the battery is detected, if a relay connecting the battery to the load is connected, there is a possibility that the replaced battery is discharged or the replaced battery is charged. When the replacement of the battery is detected, if a relay connecting the battery to the load is connected, the SOC is corrected based on the current discharged or charged through the relay, thereby being able to estimate the SOC more accurately.

[0008] In addition, the battery system of the present disclosure 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 control device is configured to estimate the SOC based on the SOC-OCV characteristics of the battery using the voltage detected by the voltage sensor when the battery is replaced, and to correct the SOC using the full charge capacity in the SOC-OCV characteristics and the full charge capacity when the battery is degraded, that is, the full charge capacity when degraded, wherein the full charge capacity when degraded is preset.

[0009] When replacing a battery, there is also a case where a used or refurbished battery is used as a replacement battery. According to this structure, when the battery is replaced, the control device uses the voltage detected by the voltage sensor to estimate the SOC based on the SOC-OCV characteristics of the battery. In addition, the SOC is corrected using the full charge capacity in the SOC-OCV characteristics and the pre-set full charge capacity when deteriorated. As a result, even when a battery with a reduced full charge capacity due to deterioration is replaced, the SOC can be estimated more accurately.

[0010] According to the present disclosure, when a battery is replaced, the SOC of the battery can be estimated.

[0011] 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 denote like elements. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 is an overall structural diagram of an electric vehicle equipped with a battery system according to the present embodiment;

[0013] Figure 2 is a diagram for explaining the SOC-OCV characteristics in the single cell (LFP cell) of the present embodiment;

[0014] Figure 3 is a flowchart showing an example of a replacement SOC estimation process executed by the battery ECU in the present embodiment;

[0015] Figure 4is a flowchart showing an example of input and output current integration processing executed by the battery ECU;

[0016] Figure 5 A diagram showing a portion of a flowchart of a replacement SOC estimation process executed by the battery ECU in the second embodiment;

[0017] Figure 6 This is a diagram for explaining the SOC-OCV characteristics after battery replacement. DETAILED DESCRIPTION

[0018] Hereinafter, the embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In addition, in the drawings, the same or corresponding parts are denoted by the same reference numerals, and their description will not be repeated.

[0019] [Implementation Method 1]

[0020] Figure 1 1 is an overall structural 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 includes an electric 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.

[0021] 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.

[0022] When the electric vehicle 1 is braked, the MG 10 is driven by the driving wheels 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.

[0023] 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.

[0024] The converter boosts the voltage supplied from the battery 100 and supplies the voltage to the inverter when the battery 100 is discharged. The inverter converts the DC power supplied from the converter into AC power to drive the MG 10.

[0025] The inverter converts AC power generated by MG 10 into DC power and supplies it to the converter when charging battery 100. The converter steps down the voltage supplied from the inverter to a voltage suitable for charging battery 100 and supplies it to battery 100.

[0026] The SMR 50 is electrically connected to a power line connecting the battery 100 and the PCU 40. When the SMR 50 is turned on (i.e., in a connected state) by 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 turned off (i.e., in a disconnected state) by a control signal from the control ECU 500, the electrical connection between the battery 100 and the PCU 40 is disconnected.

[0027] 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.

[0028] 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 and output to the battery 100 (cell 100a). In addition, when the battery 100 is discharged, the current IB is a negative (-) value, and when the battery 100 is charged, the current IB is a positive (+) value. The temperature sensor 230 detects the temperature TB of each cell 100a. Each sensor outputs its detection result to the battery ECU 300.

[0029] The electric vehicle 1 has a socket 60, and the battery 100 can be externally charged using an electric vehicle supply equipment (EVSE, Electric Vehicle Supply Equipment) 400. The socket 60 is configured to be connected to a connector 420 provided at the front end of a charging cable 410 of the EVSE 400. The socket 60 is electrically connected to a power line connected to the battery 100 via a charging circuit 70. In the present embodiment, when the SMR 50 is closed, the socket 60 is connected to the battery 100, so that external charging can be performed. In addition, the charging circuit 70 may also include a charging relay. In addition, it may also be configured such that the power line between the battery 100 and the SMR 50 is connected to the socket 60 (charging circuit 70) via a charging relay, and the battery 100 can be externally charged by closing the charging relay. The SMR 50 or the charging relay is equivalent to the "relay" of the present disclosure.

[0030] 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 may also be composed of the battery 100 (single cell 100a), the monitoring unit 200, the battery ECU 300, and the like.

[0031] 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 and program stored in the memory 502.

[0032] 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 , for the relationship between SOC and OCV (hereinafter, this relationship is also referred to as "SOC-OCV characteristic" or "OCV curve"), there is a wide range of regions (voltage flat regions) where the change in OCV is small relative to the change in SOC, and the change in the OCV curve is small. Therefore, if the SOC of the battery 100 (single cell 100a) is estimated based on the SOC-OCV characteristic using the voltage VB detected by the voltage sensor 210, the estimation accuracy of the SOC deteriorates in the voltage flat region. Therefore, it is desirable to use the current integration method (coulomb counting method) to estimate the SOC.

[0033] In the battery system S, there is a situation where the battery 100 is replaced. When the battery 100 is replaced, it is desired to infer the SOC of the replaced battery 100. When the battery 100 is replaced, if the current integration method is used to infer the SOC, a lot of time is required until the SOC inference is completed. Therefore, it is possible to consider detecting the voltage VB when replacing the battery 100, using the detected voltage VB, and inferring the SOC based on the SOC-OCV characteristic. In this case, it is preferred to use the voltage VB detected before connecting the battery 100 to the load to infer the SOC after the battery 100 is replaced. This is because the voltage VB before connecting the battery 100 to the load is equivalent to the OCV.

[0034] After the battery ECU 300 is activated, it is determined (detected) that the battery 100 has been replaced, for example, based on the presence or absence of a battery replacement signal input from a service tool. Therefore, when the replacement of the battery 100 is detected, there is a case where the SMR 50 is already in a connected state and the battery 100 is connected to the load. In this case, when the replacement of the battery 100 is detected, there is a concern that the estimation error of the SOC will become large even if the voltage VB detected by the voltage sensor 210 is used to estimate the SOC.

[0035] In this embodiment, when replacement of the battery 100 is detected, when an SMR 50 (or a charging relay) that connects the battery 100 to a load is connected, the SOC estimated based on the SOC-OCV characteristic is corrected based on the current discharged or charged through the SMR 50 (or the charging relay), thereby reducing the SOC estimation error.

[0036] Figure 3 1 is a flowchart showing an example of the replacement SOC estimation process performed by the battery ECU 300 in the present embodiment. The flowchart is executed when the IG switch (power switch) 250 is turned on and the battery ECU 300 is started. In step (hereinafter, the step is abbreviated as "S") 10, the voltage VB detected by the voltage sensor 210 is stored in the memory 302 as the starting voltage VBs.

[0037] In the next S11, it is determined whether the battery has been replaced. For example, when the control ECU 500 receives a replacement signal from the service tool ST used by the operator who replaces the battery 100, it can be determined that the battery has been replaced. In addition, when the identification number (ID) of the battery 100 stored in the non-volatile memory of the memory 302 or the memory 502 is different from the identification number of the battery 100, it can be determined that the battery has been replaced. If there is no battery replacement, the process proceeds to S12, and if there is a battery replacement, the process proceeds to S13.

[0038] In S12, it is determined whether the IG switch 250 has been operated from on to off. When the IG switch 250 has been operated to off, the current routine ends. When the IG switch 250 is on, the routine returns to S11 to determine whether the battery has been replaced.

[0039] In S13, it is determined whether the SMR 50 (or the charging relay) is connected (whether it is in the on state). When the SMR 50 is not connected (in the off state), the process proceeds to S14. When the SMR 50 is connected, the process proceeds to S15.

[0040] In S14, the starting voltage VBs is used, based on the SOC-OCV characteristic ( Figure 2 ), the SOC of the cell 100a is calculated. The SOC of all the cells 100a included in the battery 100 is calculated based on the initial voltage VBs of each cell 100a, and this program ends.

[0041] In S15, the starting voltage VBs is used, based on the SOC-OCV characteristic ( Figure 2 ), the SOC of the battery cell 100a is calculated, and the value after the SOC is corrected by the current integrated value ΣQ is calculated as the SOC. When the SOC of all the battery cells 100a is corrected by the current integrated value ΣQ to calculate the SOC, this routine ends.

[0042] Figure 4 1 is a flowchart showing an example of input / output current integration processing performed by the battery ECU 300. When the battery ECU 300 is activated, the flowchart is repeatedly processed every predetermined period. In S20, it is determined whether the SMR 50 is in the on state. When the SMR 50 is connected and in the on state, the process proceeds to S21, and when the SMR 50 is disconnected and in the off state, the process proceeds to S22.

[0043] In S21, after calculating the current integrated value ΣQ [Ah] by integrating the current IB detected by the current sensor 220, this program ends. When the battery 100 is discharged, the current IB is a negative (-) value, and when the battery 100 is charged, the current IB is a positive (+) value. Therefore, the current integrated value ΣQ decreases during discharge and increases during charge. In S22, after setting the current integrated value ΣQ to 0 (zero), this program ends.

[0044] Reference Figure 3 In S15, the SOC calculated based on the SOC-OCV characteristic is corrected using the current integrated value ΣQ. When the SOC calculated based on the SOC-OCV characteristic is set to "SOC1", it is calculated as "SOC = SOC1-(ΣQ / X)". Figure 2 In the SOC-OCV characteristic, X[Ah] is the capacity (full charge capacity) of the battery 100a when the SOC is 100[%], which is a preset value. In addition, after the SOC is estimated in S14 and S15, the SOC can be calculated (updated at any time) by the current integration method using the current IB detected by the current sensor 220.

[0045] According to the present embodiment, when the replacement of the battery 100 is detected, when the SMR 50 (or charging relay) connecting the battery 100 to the load is connected, the SOC (SOC1) estimated from the SOC-OCV characteristic using the starting voltage VBs is corrected based on the current (current integrated value ΣQ) discharged or charged via the SMR 50 (or charging relay). Thus, after the replacement of the battery 100, the SOC is estimated from the SOC-OCV characteristic using the voltage VB (starting voltage VBs) detected before the battery 100 is connected to the load, and the SOC is corrected based on the current integrated value ΣQ charged and discharged via the SMR 50 (or charging relay), so that the SOC estimation error can be reduced.

[0046] [Implementation Method 2]

[0047] There are also cases where used or refurbished batteries are used as replacement batteries. In this case, if the SOC is inferred based on the OCV curve (SOC-OCV characteristics) when the battery is new, the inferred SOC is lower than the actual SOC, and there is a concern that the replaced battery will be overcharged. Therefore, when a battery replacement is detected, there is a case where the OCV curve used to calculate the SOC is switched to the OCV curve after battery deterioration to calculate the SOC. The OCV curve after deterioration can be set according to a pre-assumed degradation state. For example, when a battery replacement is detected, an OCV curve with a capacity retention rate equivalent to 75[%] is used to calculate the SOC.

[0048] Even when switching to the OCV curve after degradation, the degradation state of the replaced battery is sometimes different from the pre-assumed degradation state. Therefore, it is required to accurately calculate the degradation degree (capacity retention rate) of the replaced battery. When calculating the degradation degree of the battery, it is preferred to temporarily discharge the battery completely (SOC = 0 [%]), and then charge it to full charge to calculate the full charge capacity.

[0049] There is a case where the degradation state of the replaced battery is better than the assumed degradation state, and the full charge capacity of the replaced battery is greater than the full charge capacity of the OCV curve after degradation. In this case, the SOC calculated based on the OCV curve after degradation is calculated as a value smaller than the actual SCO. Therefore, if the calculated SOC is used to control the charge and discharge of the battery, there is a case where the actual SOC does not reach 0[%] even if the SOC is discharged to 0[%] in order to fully discharge the battery, and there is a case where the full charge capacity of the replaced battery cannot be calculated with high accuracy.

[0050] In the second embodiment, the SOC at the time of battery replacement can be estimated so that the battery after replacement can be discharged until the SOC becomes 0[%]. Figure 5 FIG. 1 is a diagram showing a part of a flowchart of a replacement SOC estimation process executed by the battery ECU 300 in the second embodiment. Figure 3 S14 of the flowchart shown is replaced by Figure 5 S141 and S142 shown, and will be in S15 ( Figure 3 ) is replaced by the SOC calculated in S142.

[0051] In S141, the same as S14( Figure 3 ) is the same, using the starting voltage VBs, based on the SOC-OCV characteristics ( Figure 2 ) to calculate the SOC of the cell 100a. When the SOC of all the cells 100a included in the battery 100 is calculated based on the initial voltage VBs of each cell 100a, the process proceeds to S142.

[0052] In S142, use Figure 2 The SOC calculated in S141 is corrected based on the full charge capacity X [Ah] in the SOC-OCV characteristic (OCV curve) and the full charge capacity Y [Ah] of the SOC-OCV characteristic (OCV curve) used in the charge and discharge control after the battery is replaced. Figure 6 This is a diagram illustrating the SOC-OCV characteristics after battery replacement. Figure 6 In FIG. 1 , the vertical axis is the OCV [V] of the cell 100 a and the horizontal axis is the SOC [%] of the cell 100 a . Figure 6 The solid OCV curve is Figure 2 The OCV curve of the battery 100 is the same as that of the battery 100 when it is new. Figure 6 In the figure, the dashed OCV curve is the SOC-OCV characteristic for controlling the charge and discharge of the battery 100 after the battery is replaced. In an LPF battery having a voltage flat region, even if degradation occurs, the OCV curve in the low SOC region hardly changes, and the solid SOC curve and the dashed OCV curve overlap in the low SOC region.

[0053] exist Figure 6 In the solid OCV curve, X[Ah] is the capacity when SOC=100[%], which is the full charge capacity of the solid OCV curve. The full charge capacity X is equivalent to the "full charge capacity in the SOC-OCV characteristics" in the present disclosure. In the dotted OCV curve, Y[Ah] is the capacity when SOC=100[%], which is the full charge capacity of the dotted OCV curve. The full charge capacity Y is equivalent to the "full charge capacity at deterioration" in the present disclosure. The dotted OCV curve is pre-set in order to perform charge and discharge control of the battery 100 after the battery is replaced. For example, it can be a curve with a capacity maintenance rate equivalent to 75[%] relative to the solid OCV curve. In this case, it becomes "X / Y=1.33".

[0054] In S142 , the SOC calculated in S141 is multiplied by “X / Y” to correct the SOC (SOC←SOC×(X / Y)). When the SOC of all the cells 100 a included in the battery 100 is corrected, this routine ends.

[0055] In addition, in this second embodiment, in S15 ( Figure 3 ) is replaced with the SOC calculated in S142, and the value after the SOC is corrected by the current integrated value ΣQ is calculated as the SOC. When the SOC of all the single cells 100a is corrected by the current integrated value ΣQ, this program ends.

[0056] In the second embodiment, when the battery is replaced, the SOC is estimated based on the SOC-OCV characteristic using the initial voltage VBs detected by the voltage sensor 210. Furthermore, the SOC is corrected using the full charge capacity X and the full charge capacity Y. For example, Figure 6As shown, when the voltage VBs is Va at the beginning, the SOC estimated based on the SOC-OCV characteristic (solid line) is 45[%]. When the capacity retention rate of the OCV curve (dashed line) pre-set for charge and discharge control of the battery 100 after the battery is replaced is 75[%], it is "X / Y=1.33", so the SOC becomes 60[%] (=45×1.33). Therefore, from SOC=60[%], using the OCV curve (dashed line), discharge is performed to full discharge (SOC=0[%]), thereby enabling the actual SOC to be discharged to 0[%]. Thus, by charging to full charge after full discharge, the full charge capacity of the replaced battery 100 can be accurately calculated. In addition, after calculating the full charge capacity of the replaced battery 100, the SOC is estimated by using a new OCV curve using the full charge capacity. In addition, the current integration method can also be used to estimate the SOC.

[0057] In order to suppress degradation, etc., replacement batteries are generally circulated in a state where the SOC is in the low SOC area. When the SOC is inferred using a pre-set OCV curve after degradation, in a battery whose OCV curve in the low SOC area changes according to the degradation state of the battery, if the degradation state of the replaced battery is different from that of the pre-set OCV curve, the inferred SOC will have a large error. Therefore, the actual SOC cannot be discharged to 0[%]. In an LPF battery with a voltage flat area, even if degradation occurs, the OCV curve in the low SOC area hardly changes. Therefore, even if the full charge capacity of the replaced battery is different from the pre-set full charge capacity Y (full charge capacity after degradation), the actual SOC can be discharged to 0[%] by using the full charge capacity X and the full charge capacity Y to correct the SOC.

[0058] In this second embodiment, it is also possible to omit Figure 3 In S13 and S15 of the flowchart shown, when an affirmative determination is made in S11, the process proceeds to S141 and S142.

[0059] As embodiments of the present disclosure, the following structures can be cited.

[0060] 1) A battery system comprising: a battery; a voltage sensor for detecting a voltage of the battery; and a control device for estimating the SOC of the battery, wherein the SOC-OCV characteristic of the battery has a voltage flat region in which the change of the OCV relative to the SOC is small, and the control device is configured to use the voltage detected by the voltage sensor and estimate the SOC based on the SOC-OCV characteristic when the battery is replaced, and to correct the SOC using a full charge capacity X in the SOC-OCV characteristic and a full charge capacity Y in the SOC-OCV characteristic after battery deterioration, and to discharge the battery in a manner of fully discharging the battery using the corrected SOC and the SOC-OCV characteristic after battery deterioration.

[0061] 2) Based on 1, the control device is configured to charge the battery to full charge when the battery is completely discharged, thereby calculating the full charge capacity of the replaced battery.

[0062] 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) in which the OCV curve has a small change.

[0063] 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 be applied to a plug-in hybrid electric vehicle having an engine and an electric motor, 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.

[0064] 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 control device is configured to: upon detecting that the battery has been replaced, using the voltage detected by the voltage sensor and inferring the SOC based on the SOC-OCV characteristic of the battery, and When a relay connecting the battery and a load is connected, the SOC estimated based on the SOC-OCV characteristic is corrected based on the current discharged or charged via the relay.

2. The battery system according to claim 1, wherein: The full charge capacity of the battery when the battery is deteriorated, i.e., the full charge capacity when deteriorated, is preset. The control device is composed of: The SOC estimated based on the SOC-OCV characteristic is corrected using the full charge capacity and the degradation-time full charge capacity in the SOC-OCV characteristic.

3. 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 control device is configured such that when the battery is replaced, using the voltage detected by the voltage sensor and inferring the SOC based on the SOC-OCV characteristic of the battery, and The SOC is corrected using the full charge capacity in the SOC-OCV characteristic and the full charge capacity when the battery is degraded, that is, the full charge capacity when the battery is degraded. The full charge capacity at the time of degradation is preset.

4. The battery system according to claim 1 or 3, wherein: The SOC-OCV characteristic of the battery has a voltage flat region in which the change of OCV with respect to SOC is small.

5. The battery system according to claim 1 or 3, wherein: A signal indicating that the battery has been replaced is input from a service tool.

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