Battery system

Abstract

The present application relates to a battery system. An ECU performs a process including the steps of: in the case where DC charging is started (YES in S100), acquiring a last stored temperature difference ΔTBpre (S102); acquiring an elapsed time Tp (S104); calculating a correction coefficient K (S106); calculating a temperature difference ΔTBa (S108); performing a DC charging process (S110); in the case where charging is ended (YES in S112), storing a temperature difference ΔTB at the time of charging end as ΔTBpre (S114).

Description

Technical Field

[0001] This invention relates to a battery system. Background Technology

[0002] Japanese Patent Application Publication No. 2021-068637 discloses a technology that uses an electronic control unit (ECU) to infer the state of charge (SOC) by obtaining the current and temperature of the vehicle battery from a monitoring unit. Summary of the Invention

[0003] When charging an energy storage device such as a battery as described above, the temperature of the energy storage device rises, and a cooling device is sometimes used to cool it. If cooling is performed during charging, a temperature difference may sometimes occur between the surface portion and the internal portion of the energy storage device (hereinafter, sometimes referred to as "the inside and outside of the energy storage device"). When a temperature difference occurs between the inside and outside of the energy storage device, the allowable current during charging is calculated taking this temperature difference into account. However, if charging ends or the ECU is in a power-off state, the temperature difference is reset, and therefore, it may be impossible to perform charging that takes into account the temperature difference between the inside and outside of the energy storage device during subsequent charging.

[0004] The present invention was made to solve the above-mentioned problems, and its object is to provide a battery system capable of performing charging that takes into account the temperature difference between the inside and outside of the energy storage device.

[0005] One embodiment of the present invention relates to a battery system comprising: an energy storage device mounted in a vehicle; a cooling device for cooling the energy storage device; a detection device for detecting the surface temperature of the energy storage device; and a control device for controlling the charging of the energy storage device. The control device uses the elapsed time from the end of the last charge to the start of the current charge, and the temperature difference between the surface temperature at the end of the last charge and the internal temperature of the energy storage device, to estimate the temperature difference at the start of the current charge, and uses the estimated temperature difference to control the charging of the energy storage device, storing the temperature difference after the charging of the energy storage device is completed.

[0006] In this way, even if the cooling device operates during the last charge, causing an increase in the temperature difference between the surface and internal temperatures of the energy storage device, the temperature difference at the start of the current charge is inferred using the elapsed time from the end of the last charge to the start of the current charge and the temperature difference at the end of the last charge. This inferred temperature difference is then used to control the charging of the energy storage device, thereby suppressing accelerated capacity degradation. Furthermore, by storing the temperature difference after charging is complete, the temperature difference can be accurately inferred during the next charge, further suppressing accelerated capacity degradation.

[0007] In one embodiment, the control device stores the temperature difference when the vehicle system stops after the charging of the energy storage device is completed.

[0008] In this way, the temperature difference at the start of the current charge can be accurately predicted. Therefore, the predicted temperature difference can be used to charge the energy storage device while suppressing accelerated capacity degradation.

[0009] Furthermore, in one embodiment, the control device acquires a correction coefficient corresponding to the elapsed time and multiplies the acquired correction coefficient by the temperature difference to infer the temperature difference at the start of this charging.

[0010] In this way, by multiplying the correction factor obtained based on the elapsed time with the temperature difference, the temperature difference at the start of this charge can be inferred with high accuracy.

[0011] Furthermore, in one embodiment, the control device sets the correction coefficient to different values ​​when the vehicle is traveling within the elapsed time and when the vehicle is parked within the elapsed time.

[0012] In this way, different correction coefficients are set according to whether the vehicle was driven or parked within the time elapsed, thus enabling a high-precision estimation of the temperature difference at the start of the current charging.

[0013] Furthermore, in one embodiment, the energy storage device includes a lithium-ion secondary battery.

[0014] In this way, the current flowing to the energy storage device can be controlled by accurately predicting the temperature difference at the start of the charge, thereby preventing lithium deposition. Therefore, the accelerated capacity degradation of the energy storage device can be suppressed.

[0015] Furthermore, in one embodiment, a cooling device is disposed against the lower surface of the energy storage device. The internal temperature includes the temperature of the portion inside the energy storage device on the lower surface side.

[0016] In this way, the temperature difference between the lower surface and the surface temperature within the energy storage device, which is easily affected by the operation of the cooling device, can be calculated with high precision. Therefore, the acceleration of capacity degradation can be suppressed.

[0017] According to the present invention, a battery system capable of performing charging that takes into account the temperature difference between the inside and outside of the energy storage device can be provided. Attached Figure Description

[0018] Hereinafter, with reference to the accompanying drawings, the features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will be described, in which the same reference numerals denote the same elements, and wherein:

[0019] Figure 1This is a diagram illustrating an example of the structure of a vehicle equipped with the battery system according to this embodiment.

[0020] Figure 2 This is a graph illustrating an example of the change in the battery temperature difference ΔTB after cooling begins.

[0021] Figure 3 This is a flowchart illustrating an example of a process performed by the ECU.

[0022] Figure 4 This is a timing diagram used to illustrate an example of ECU operation. Detailed Implementation

[0023] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Furthermore, the same or corresponding parts in the drawings will be labeled with the same symbols, and their descriptions will not be repeated.

[0024] Figure 1 This is a diagram illustrating an example of the structure of a vehicle 200 equipped with the battery system 100 according to this embodiment.

[0025] Vehicle 200 is electrically connected to power station 300 via cable 310, thereby enabling the transmission and reception (charging and discharging) of power between them. DC power is transmitted and received between connector 311 at the end of cable 310 and vehicle 200. Additionally, power station 300 transmits and receives power with power system PG. For example, upon request from vehicle 200, power station 300 converts AC power from power system PG into DC power and supplies it to vehicle 200. Vehicle 200 uses the supplied DC power to fast charge battery 40 (DC charging).

[0026] The vehicle 200 is equipped with a battery system 100, a socket 210, and an electric motor generator (MG) 220.

[0027] Vehicle 200 is configured to operate using electricity stored in battery 40 of battery system 100. Vehicle 200 is, for example, a battery electric vehicle (BEV) without an engine (internal combustion engine). However, it is not limited to this; vehicle 200 may be a plug-in hybrid electric vehicle (PHEV) with an internal combustion engine, or other electric vehicles (xEV).

[0028] The socket 210 is disposed on an external part of the vehicle 200 and has a shape that can be engaged with the connector 311. The external part includes a cover that covers the socket 210 when it is not in use, thereby preventing it from being exposed to the outside.

[0029] MG220 is, for example, a three-phase AC rotary motor. MG220 functions as the driving motor for vehicle 200. MG220 is driven by AC power from battery system 100, causing the drive wheels of vehicle 200 to rotate. Furthermore, MG220 regenerates electricity and outputs the generated AC power to battery system 100. Additionally, the number of driving motors in vehicle 200 is not particularly limited to one; it can be two or more.

[0030] The battery system 100 includes an electronic control unit (ECU) 10, a power control unit (PCU) 20, a battery 40, a system main relay (SMR) 50, and a relay 60.

[0031] like Figure 1 As shown in section (A), ECU 10 is a control device including a processor 11, a random access memory (RAM) 12, and a storage device 13. The storage device 13 is configured to store stored information. In addition to storing programs, the storage device 13 also stores information used in the programs (e.g., graphs, mathematical formulas, and various parameters). In this embodiment, various processes of ECU 10 can be executed by the processor 11 executing the programs stored in the storage device 13. However, these processes can also be executed using only hardware (electronic circuitry) without using software. ECU 10, for example, controls the operation of PCU 20, SMR 50, and relay 60.

[0032] A voltage sensor 43, a current sensor 44, and a battery temperature sensor 45 are connected to the ECU 10. The voltage sensor 43 detects the voltage VB of the battery 40 and outputs a signal indicating the detection result to the ECU 10. The current sensor 44 detects the current IB flowing to the battery 40 and outputs a signal indicating the detection result to the ECU 10. Furthermore, the battery temperature sensor 45 detects the temperature TB of the battery 40 and outputs a signal indicating the detection result to the ECU 10. For example, as... Figure 1 As shown in part (B), the battery temperature sensor 45 detects the temperature of the upper surface of the battery 40 as temperature TB.

[0033] The storage battery 40 includes multiple battery cells. The number of battery cells is predetermined, for example, arranged along the thickness direction of the battery cells. The predetermined number is not particularly limited. The battery cells are secondary batteries, typically lithium-ion secondary batteries. Lithium-ion secondary batteries are batteries that use lithium as a charge carrier; besides lithium-ion secondary batteries with a liquid electrolyte, they can also include all-solid-state batteries using a solid electrolyte. Furthermore, the storage battery 40 can be any energy storage device; for example, a large-capacity capacitor can be used instead of the storage battery 40.

[0034] SMR50 switches the connection / disconnection of the power line from battery 40 to PCU20 according to the control signal received by ECU10. SMR50 is in a closed state (connected state) when vehicle 200 is in motion. SMR50 is also in a closed state when power is exchanged between battery 40 and socket 210 (and thus, power station 300).

[0035] Relay 60 is located between socket 210 and the branch point of the power line connecting PCU20 and SMR50. Relay 60 switches the connection / disconnection of the power line (charging and discharging line) from socket 210 to the branch point according to the control signal received by ECU10.

[0036] In this embodiment, the charging and discharging lines via the socket 210 and the repeater 60 are connected to the power lines connecting the SMR50 and the PCU20.

[0037] In the plugged-in vehicle 200, external charging (i.e., charging of the battery 40 from the power station 300 based on electricity) and external discharging (i.e., discharging the battery 40 to the power station 300) are both possible. Alternatively, the vehicle 200 may only be capable of performing external charging. When external charging or external discharging is performed, the relay 60 is controlled to be in a closed state (connected state); when neither external charging nor external discharging is performed, the relay 60 is controlled to be in an open state (disconnected state).

[0038] PCU20 includes circuitry (e.g., an inverter and converter) that uses power supplied from battery 40 to drive MG220. The inverter and converter are, for example, composed of a combination of multiple switching elements, which perform switching operations to convert power or boost voltage according to control signals received from ECU10.

[0039] The vehicle 200 is also equipped with a cooling device 240, which is disposed against the lower surface of the battery 40 and cools the battery 40. The cooling device 240 may be configured as a medium passage (not shown) that uses a pump or the like to circulate refrigerant to exchange heat between the outside air and the battery 40, or it may be configured to exchange heat between the battery 40 and a heat exchanger (not shown) whose temperature is adjusted by a refrigeration cycle.

[0040] In the vehicle 200 with the structure described above, when the battery 40 is charged using the power station 300, the battery 40 heats up due to Joule heat generated in its internal resistance. Therefore, the cooling device 240 is activated to cool the battery 40. If cooling is performed during charging, the difference between the surface temperature and the internal temperature of the battery 40 (hereinafter referred to as temperature difference) may increase. Here, the internal temperature includes the temperature of the portion of the battery 40 on its lower surface side (the side closest to the cooling device 240). The surface temperature refers to the temperature of the upper surface of the battery 40 (battery cell) where the battery temperature sensor 45 is installed.

[0041] Figure 2 This is a graph illustrating an example of the change in temperature difference ΔTB of the battery 40 after cooling begins. Figure 2 The vertical axis represents the temperature difference ΔTB. Figure 2 The horizontal axis represents time. Figure 2 LN1 represents the time variation of the temperature difference ΔTB. The temperature difference ΔTB is represented by the value obtained by subtracting the surface temperature of the battery 40 (sensor detection value) from the internal temperature of the battery 40.

[0042] If cooling begins during charging, the portion of the lower surface of battery 40 (refer to...) Figure 1 The heat at location B of part (B) is removed by the cooling device 240. Therefore, the internal temperature of the lower surface side of the battery 40 decreases. On the other hand, the heat at the upper surface part (B) of the battery 40... Figure 1 The temperature (surface temperature) of part (B) at location A) will not be affected by the operation of the cooling device 240 during the initial cooling phase, and will therefore remain constant. As a result, as... Figure 2As shown in LN1, the temperature difference ΔTB increases over time (increasing in the negative direction), reaching a peak at time T(0) (temperature difference ΔTB(0)). If the cooling device 240 continues to operate and draws heat from the battery 40, the surface temperature of the battery 40 will also decrease. Therefore, after time T(0), the temperature difference ΔTB decreases (decreasing in the negative direction), and then the heat generated and dissipated in the battery 40 reach equilibrium and converge under certain conditions. Thus, if the cooling device 240 operates during charging, a temperature difference ΔTB is generated, and the ECU 10 considers the temperature difference ΔTB during charging to calculate the allowable charging current. However, if the vehicle 200 system stops and the ECU 10 becomes disconnected at the end of charging, the temperature difference ΔTB temporarily stored in the memory is reset to its initial value (e.g., zero), and sometimes it is not possible to implement charging that takes the temperature difference ΔTB into account during the next charging. Therefore, sometimes capacity degradation is accelerated, shortening battery life.

[0043] Therefore, in this embodiment, the ECU 10 performs the following operation: The ECU 10 uses the elapsed time from the end of the last charge to the start of the current charge, and the temperature difference ΔTB from the end of the last charge, to estimate the temperature difference ΔTB at the start of the current charge. The ECU 10 uses the estimated temperature difference to control the charging of the battery 40. Then, the ECU 10 stores the temperature difference ΔTB after the charging of the battery 40 is completed.

[0044] In this way, even if the cooling device 240 operates during the last charge and the temperature difference ΔTB in the battery 40 increases, the temperature difference at the start of the current charge can be inferred using the elapsed time from the end of the last charge to the start of the current charge, and the temperature difference ΔTB at the end of the last charge. This inferred temperature difference can then be used to control the charging of the battery 40, thereby suppressing accelerated capacity degradation. Furthermore, by storing the temperature difference ΔTB after charging is complete, the temperature difference ΔTB can be inferred with high accuracy during the next charge, further suppressing accelerated capacity degradation.

[0045] The following is for reference Figure 3 An example of the processing performed by ECU10 will be explained. Figure 3 This is a flowchart illustrating an example of a process performed by ECU10.

[0046] In step (hereinafter referred to as S) 100, ECU 10 determines whether to start DC charging. ECU 10 determines, for example, that DC charging will begin when connector 311 is installed in socket 210 and the state of charge (SOC) of battery 40 is below a threshold. Alternatively, ECU 10 may determine that DC charging will begin when power is requested from power station 300. If DC charging is determined to begin ("Yes" in S100), the process proceeds to S102.

[0047] In S102, ECU10 acquires the previously stored temperature difference ΔTBpre. ECU10, for example, acquires the temperature difference ΔTBpre stored in storage device 13 at the end of the previous DC charge. This post-processing then proceeds to S104.

[0048] In S104, ECU10 acquires the elapsed time Tp from the end point of the last DC charge to the current time. ECU10 starts time measurement, for example, by starting a timer at the end point of the last DC charge. ECU10 acquires the measured value based on the timer's duration as the elapsed time Tp. This post-processing then proceeds to S106.

[0049] In S106, ECU10 calculates the correction factor K. A graph showing the relationship between the correction factor K and the elapsed time Tp is pre-stored in the storage device 13 of ECU10. This graph is adapted, for example, through experiments. ECU10 uses the elapsed time Tp and the graph obtained in S104 to calculate the correction factor K. This post-processing then proceeds to S108.

[0050] In S108, ECU10 calculates the temperature difference ΔTBa at the start of this charge. ECU10 uses the formula ΔTBa = ΔTBpre × K to calculate the temperature difference ΔTBa. This post-processing is then transferred to S110.

[0051] In S110, ECU10 performs DC charging processing. ECU10 requests charging power from power station 300. Power station 300 begins supplying charging power according to the request from ECU10. After starting DC charging processing, ECU10 also acquires the temperature difference ΔTB at each preset time interval and uses the acquired temperature difference ΔTB to set the charging current during DC charging. ECU10 requests charging power from power station 300 to allow the set charging current to flow. For example, if battery 40 is composed of a lithium-ion secondary battery, ECU10 uses the acquired temperature difference ΔTB to estimate the range of charging current (allowable charging current) that prevents lithium deposition within battery 40, and sets the charging current within the estimated range. Furthermore, as a method for obtaining the temperature difference ΔTB, it can be obtained, for example, based on the surface temperature detected by the battery temperature sensor 45 and the internal temperature inferred from the temperature of the refrigerant inside the cooling device 240 detected using a sensor not shown. Alternatively, it can be obtained by calculating a correction factor K based on the elapsed time since the most recent temperature difference ΔTB was obtained, using different graphs for the operating and stopped states of the cooling device 240, and multiplying it by the most recently obtained temperature difference ΔTB to obtain the current temperature difference ΔTB. This post-processing proceeds to S110.

[0052] In S112, ECU10 determines whether charging has ended. After the DC charging process begins, ECU10 monitors the SOC of battery 40. When the SOC exceeds a threshold, it requests power station 300 to stop supplying charging power. Power station 300 stops supplying charging power according to the request from ECU10. ECU10 determines that charging has ended when it stops supplying charging power from power station 300. If charging is determined to be ended ("Yes" in S112), the process proceeds to S114. Otherwise, if charging is determined not to be ended ("No" in S112), the process returns to S112.

[0053] In S114, ECU10 stores the temperature difference ΔTB at the end of charging as ΔTBpre. This post-processing ends. Additionally, if it is determined that DC charging has not started ("No" in S100), this process ends.

[0054] Reference Figure 4 The operation of ECU10 based on the structure and flowchart described above will be explained. Figure 4 This is a timing diagram used to illustrate an example of the operation of ECU10. Figure 4 The vertical axis represents the charging mode, the operating status of the cooling device 240, the temperature difference ΔTB, and the surface temperature of the battery 40. Figure 4 The horizontal axis represents time. Figure 4 LN2 represents the time variation of the charging mode. Figure 4LN3 indicates the time-varying operating status of the cooling device 240. Figure 4 LN4 (dashed line) represents the time variation of the temperature difference ΔTB when the charge is reset to its initial value at the end of the charge. Figure 4 LN5 represents the time variation of the temperature difference ΔTB. Figure 4 LN6 indicates the time-varying surface temperature of battery 40.

[0055] For example, such as Figure 4 As shown in LN2, during the period from time T(1) to time T(2) which is not a charging period and as Figure 4 When the cooling device 240 shown in LN3 is in a stopped state, the temperature of the battery 40 becomes a uniform temperature. Therefore, as Figure 4 As shown in LN6, when the surface temperature of the battery 40 is maintained at temperature TB(2), as Figure 4 As shown in LN5, the temperature difference ΔTB becomes zero.

[0056] On the other hand, if connector 311 is installed in socket 210 at time T(2), then as Figure 4 As shown in LN2, the charging mode is switched to the DC charging mode, and DC charging begins ("Yes" in S100). The previously stored temperature difference ΔTBpre is obtained (S102), the elapsed time Tp is obtained (S104), and the correction coefficient K is calculated (S106). Then, the temperature difference ΔTBa is calculated using the above formula. If the temperature difference ΔTpre is zero, the temperature difference ΔTBa is also calculated as zero. Then DC charging begins. At this time, if the surface temperature of the battery 40 is higher than the threshold, as... Figure 4 As shown in LN3, the cooling device 240 operates. As a result, a temperature difference ΔTB (1) is created due to heat loss from the lower surface of the battery 40. The cooling device 240 operates as follows... Figure 4 As shown in LN6, the surface temperature of the battery 40 also decreases over time, decreasing to temperature TB (3).

[0057] If, at time T(3), it is determined that the connector 311 has been disassembled and charging has ended (S112 is "Yes"), the value of ΔTB at the end of charging, ΔTB(1), is stored as ΔTBpre in the storage device 13 of the ECU10. At this time, the cooling device 240 stops working at the same time as DC charging stops.

[0058] After time T(3), battery 40 is placed. At this time, as Figure 4As shown in LN6, the surface temperature of the battery 40 is maintained at TB (3), and the cooling device 240 becomes a non-operating state. Therefore, the internal temperature of the battery 40 gradually rises, and the temperature difference ΔTB changes in a manner close to zero.

[0059] If connector 311 is reinstalled at time T(4), ECU10 will again request charging power to power station 300, and power station 300 will resume supplying charging power upon request. Therefore, as Figure 4 As shown in LN2, the charging mode switches back to the mode during DC charging, indicating that DC charging has started (S100 indicates "Yes").

[0060] At this time, when the surface temperature of battery 40 is below the threshold, such as Figure 4 As shown in LN3, the cooling device 240 is kept in a stopped state. Therefore, the temperature difference ΔTBpre (=ΔTB (1)) stored last time is obtained (S102), the elapsed time Tp from the end point of the last DC charge (=time T (3)) to the current time is obtained (S104), and the correction coefficient K is calculated based on the elapsed time Tp and the graph (S106).

[0061] The calculated correction coefficient K is multiplied by the previously stored temperature difference ΔTBpre to infer the temperature difference ΔTBa in time T (4) (S108). DC charging is performed using the temperature difference ΔTBa (S110). At this time, for example, if the battery 40 is composed of a lithium-ion secondary battery, the ECU 10 uses the temperature obtained by adding the temperature difference ΔTBa to the battery temperature TB to set the allowable charging current that does not deposit lithium, sets the charging power to not exceed the set allowable charging current, and requests the set charging power from the power station 300. Therefore, with... Figure 4 Compared to LN4, the charging current is set considering the temperature difference ΔTBa, thus suppressing the accelerated capacity degradation of the battery 40. Furthermore, the ECU 10 can, for example, use a graph showing the relationship between temperature (obtained by adding the battery temperature TB and the temperature difference ΔTBa) and the allowable charging current to set the allowable charging current.

[0062] After time T(4), ECU10 acquires the temperature difference ΔTB at each preset time interval during DC charging, and uses the acquired temperature difference ΔTB to set the allowable charging current. For example... Figure 4 As shown in LN6, the surface temperature of battery 40 increases with time T (4) after time.

[0063] In time T(5), when the SOC of the battery 40 reaches a fully charged state above the threshold, charging is determined to end (in S112, this is "Yes"). Figure 4As shown in LN2, the charging mode is turned off, and the temperature difference ΔTB (=0) at the end of charging is stored in the storage device 13 as ΔTBpre (S114).

[0064] As described above, according to the battery system 100 of this embodiment, even when the cooling device 240 operates during the last charge and the temperature difference ΔTB in the battery 40 increases, the temperature difference ΔTBa at the start of the current charge can be estimated using the elapsed time Tp from the end of the last charge to the start of the current charge and the temperature difference ΔTBpre at the end of the last charge. The estimated temperature difference ΔTBa is then used to control the charging of the battery 40, thereby suppressing accelerated capacity degradation. Furthermore, by storing the temperature difference ΔTB after charging is completed as ΔTBpre, the temperature difference ΔTBa can be estimated with high accuracy during the next charge, further suppressing accelerated capacity degradation. Therefore, a battery system capable of performing charging that takes into account the internal and external temperature differences of the storage device can be provided.

[0065] The following describes some variations.

[0066] In the above embodiment, the method described is to calculate the correction factor K based on the elapsed time Tp, independent of the vehicle's state, during the period from the end of the last charge to the start of the current charge. However, for example, the correction factor K can be set to different values ​​depending on whether the vehicle 200 is parked during the period from the end of the last charge to the start of the current charge or whether the vehicle 200 is in motion during the same period. In this way, the temperature difference ΔTB at the start of the current charge can be estimated with high accuracy.

[0067] Furthermore, in the above embodiment, the case of inferring the temperature difference ΔTB at the start of DC charging was described as an example. However, if the vehicle 200 is configured to perform AC charging, the temperature difference ΔTB at the start of AC charging can also be inferred in the same way.

[0068] Furthermore, in the above embodiment, the method of storing the temperature difference ΔTB at the time of charging completion as ΔTBpre has been described. However, for example, the temperature difference ΔTB can also be stored as ΔTBpre when the vehicle 200 system stops after charging is completed.

[0069] In addition, the above-mentioned variations can be implemented by combining all or part of them appropriately.

[0070] It should be considered that the embodiments disclosed herein are illustrative in all respects and not restrictive. The scope of the invention is not shown in the foregoing description, but is indicated by the technical solutions, and is intended to include all modifications within the meaning and scope equivalent to the technical solutions.

Claims

1. A battery system, characterized in that, have: An energy storage device mounted on a vehicle; A cooling device for cooling the energy storage device; A detection device that detects the surface temperature of the energy storage device; and A control device that controls the charging of the energy storage device. The control device performs the following processing: The temperature difference at the start of the current charge is inferred by using the elapsed time from the end of the last charge to the start of the current charge, and the temperature difference between the surface temperature at the end of the last charge and the internal temperature of the energy storage device. The inferred temperature difference is used to control the charging of the energy storage device; and The temperature difference is stored after the energy storage device has finished charging.

2. The battery system according to claim 1, characterized in that, The control device stores the temperature difference when the vehicle system stops after the charging of the energy storage device is completed.

3. The battery system according to claim 1, characterized in that, The control device acquires a correction coefficient corresponding to the elapsed time, and multiplies the acquired correction coefficient by the temperature difference to infer the temperature difference at the start of the current charging.

4. The battery system according to claim 3, characterized in that, The control device sets the correction coefficient to different values ​​depending on whether the vehicle is traveling during the elapsed time or when the vehicle is parked during the elapsed time.

5. The battery system according to claim 1, characterized in that, The energy storage device includes a lithium-ion secondary battery.

6. The battery system according to claim 1, characterized in that, The cooling device is disposed against the lower surface of the energy storage device. The internal temperature includes the temperature of the portion of the lower surface side within the energy storage device.

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