JP6849497B2 - Battery system - Google Patents

Description

The present disclosure relates to a battery system, and more particularly to a battery system including an assembled battery including a plurality of secondary batteries connected in parallel.

Japanese Unexamined Patent Publication No. 2006-138750 (Patent Document 1) discloses a battery monitoring device that monitors the state of a secondary battery block (assembled battery). The secondary battery block is configured by connecting a plurality of parallel cell blocks in series, and each of the plurality of parallel cell blocks is composed of a plurality of cells (secondary batteries) connected in parallel.

In this battery monitoring device, the amount of voltage change of each parallel cell block before and after energization is calculated, and the amount of current change of the secondary battery block before and after energization is calculated. Then, the internal resistance of each parallel cell block is calculated from the calculated voltage change amount and current change amount, and the cell abnormality is determined based on the calculated internal resistance (see Patent Document 1).

Japanese Unexamined Patent Publication No. 2006-138750

In the above battery monitoring device, although it is possible to determine the abnormality of the cell, the current variation due to the temperature variation occurring between the cells of the parallel cell block is not taken into consideration. When the temperature variation occurs between the cells of the parallel cell block, the resistance variation between the cells increases, and as a result, the current variation between the cells increases. Then, for example, even if the battery protection control is performed based on the total current or the average current flowing in the parallel cell block, there is a possibility that the cell in which the relatively large current flows in the parallel cell block cannot be protected.

The present disclosure has been made to solve such a problem, and an object of the present invention is to realize appropriate battery protection control in consideration of current variation due to temperature variation between a plurality of secondary batteries connected in parallel. To provide a battery system.

The battery system of the present disclosure includes an assembled battery, a cooling device, and a control device. The assembled battery includes a plurality of secondary batteries connected in parallel. The cooling device is configured to supply cooling air to the assembled battery. The control device is configured to estimate the current variation among the plurality of secondary batteries due to the temperature variation among the plurality of secondary batteries. The control device uses the calorific value of the highest temperature cell having the highest temperature among the plurality of secondary batteries and the cooling amount of the highest temperature cell by the cooling device to be used as a first temperature index having a correlation with the temperature of the highest temperature cell. Is calculated. Further, the control device uses the calorific value of the lowest temperature cell having the lowest temperature among the plurality of secondary batteries and the cooling amount of the lowest temperature cell by the cooling device, and has a second correlation with the temperature of the lowest temperature cell. Calculate the temperature index. Further, the control device calculates an evaluation value indicating the degree of temperature variation by subtracting the second temperature index from the first temperature index. Then, the control device estimates the degree of current variation using the calculated evaluation value and the temperature of the assembled battery. The calorific value of the maximum temperature cell is calculated using the estimation results of the resistance and current of the maximum temperature cell. The calorific value of the lowest temperature cell is calculated using the estimation results of the resistance and current of the lowest temperature cell. Each cooling amount of the maximum temperature cell and the minimum temperature cell is estimated using the difference between the temperature of the assembled battery and the temperature of the cooling air.

In this battery system, the current variation among the plurality of secondary batteries due to the temperature variation among the plurality of secondary batteries connected in parallel is estimated by the above configuration. Therefore, according to this battery system, for example, it is possible to estimate the maximum current in consideration of the estimated current variation and realize appropriate battery protection control based on the estimated maximum current.

It is a figure which showed schematic the structure of the vehicle which was equipped with the battery system according to the embodiment of this disclosure. It is a figure which showed an example of the detailed structure of the assembled battery shown in FIG. It is a figure which showed an example of the temperature change of the highest temperature cell and the lowest temperature cell among a plurality of cells connected in parallel. It is a figure which showed an example of the relationship between a cell temperature and a resistance. It is a flowchart explaining the procedure of the process concerning the estimation of the current variation which occurs among a plurality of cells connected in parallel. This is a map for determining the coefficient f used in the equations (7) and (8). It is a figure which showed the temperature dependence of the resistance of a cell. It is a figure which showed the residual capacity dependence of the resistance of a cell. It is a figure which showed a plurality of cells connected in parallel. It is a map of the maximum current correction gain G. It is a figure which showed the relationship between the temperature and resistance of a cell.

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The same or corresponding parts in the drawings are designated by the same reference numerals, and the description thereof will not be repeated.

FIG. 1 is a diagram schematically showing a configuration of a vehicle 1 equipped with a battery system according to an embodiment of the present disclosure. In the following, a case where the vehicle 1 is a hybrid vehicle will be typically described, but the battery system according to the present disclosure is not limited to the one mounted on the hybrid vehicle, and is equipped with the assembled battery 10 described below. It can be applied to all vehicles that have been used, and also to applications other than vehicles.

With reference to FIG. 1, the vehicle 1 is referred to as a battery system 2, a power control unit (hereinafter referred to as "PCU (Power Control Unit)") 30, and a motor generator (hereinafter referred to as "MG (Motor Generator)". ) 41, 42, an engine 50, a power dividing device 60, a drive shaft 70, and drive wheels 80.

The battery system 2 includes an assembled battery 10, a monitoring unit 20, a cooling device, and an electronic control device (hereinafter referred to as "ECU (Electronic Control Unit)") 100. The cooling device is a device for supplying cooling air to the assembled battery 10, and includes a duct 25, a fan 26, and a temperature sensor 27.

The engine 50 is an internal combustion engine that outputs power by converting the combustion energy generated when the air-fuel mixture is burned into the kinetic energy of movers such as pistons and rotors. The power splitting device 60 includes, for example, a planetary gear mechanism having three rotating shafts of a sun gear, a carrier, and a ring gear. The power dividing device 60 divides the power output from the engine 50 into a power for driving the MG 41 and a power for driving the drive wheels 80.

The MGs 41 and 42 are AC rotating electric machines, for example, three-phase AC synchronous motors in which permanent magnets are embedded in a rotor. The MG 41 is mainly used as a generator driven by the engine 50 via the power dividing device 60. The electric power generated by the MG 41 is supplied to the MG 42 or the assembled battery 10 via the PCU 30.

The MG 42 mainly operates as an electric motor and drives the drive wheels 80. The MG 42 is driven by receiving at least one of the electric power from the assembled battery 10 and the electric power generated by the MG 41, and the driving force of the MG 42 is transmitted to the drive shaft 70. On the other hand, when braking the vehicle or reducing acceleration on a downhill slope, the MG 42 operates as a generator to generate regenerative power. The electric power generated by the MG 42 is supplied to the assembled battery 10 via the PCU 30.

The assembled battery 10 includes a plurality of cells (secondary batteries) connected in parallel (detailed configuration will be described later). Each cell is, for example, a lithium ion battery, a nickel hydrogen battery, or the like. The assembled battery 10 stores electric power for driving the MGs 41 and 42. That is, the assembled battery 10 can supply electric power to the MGs 41 and 42 through the PCU 50. Further, the assembled battery 10 is charged by receiving the generated power through the PCU 30 at the time of power generation of the MGs 41 and 42.

The monitoring unit 20 includes a voltage sensor 21, a current sensor 22, and a temperature sensor 23. The voltage sensor 21 detects the voltage (hereinafter, also referred to as “cell voltage”) VBi of a plurality of cells connected in parallel in the assembled battery 10. The current sensor 22 detects the charge / discharge current IB of the assembled battery 10. A plurality of temperature sensors 23 are provided in the assembled battery 10 and detect the temperature TBj of the assembled battery 10.

The duct 25 is a passage for supplying cooling air to the assembled battery 10. A fan 26 is provided in the duct 25. In this embodiment, the fan 26 is provided in the duct 25 on the intake side, but the fan 26 may be provided in the duct 25 on the exhaust side. The temperature sensor 27 detects the temperature TC of the cooling air supplied to the assembled battery 10 through the duct 25.

The PCU 30 executes bidirectional power conversion between the assembled battery 10 and the MGs 41 and 42 according to the control signal from the ECU 100. The PCU 30 is configured so that the states of the MGs 41 and 42 can be individually controlled. For example, the MG42 can be put into a power running state while the MG41 is in a regenerative (power generation) state. The PCU 30 includes, for example, two inverters provided corresponding to the MGs 41 and 42, and a converter that boosts the DC voltage supplied to each inverter to a voltage higher than the output voltage of the assembled battery 10.

The ECU 100 includes a CPU (Central Processing Unit) 102, a memory (ROM (Read Only Memory) and RAM (Random Access Memory)) 105, and an input / output port (not shown) for inputting / outputting various signals. Consists of. The ECU 100 controls the charging / discharging of the assembled battery 10 by controlling the engine 50 and the PCU 30 based on the signals received from each sensor and the programs and maps stored in the memory 105.

FIG. 2 is a diagram showing an example of a detailed configuration of the assembled battery 10 shown in FIG. With reference to FIG. 2, in the assembled battery 10, a plurality of cells are connected in parallel to form a block (or module), and a plurality of blocks are connected in series to form the assembled battery 10. Specifically, the assembled battery 10 includes blocks 10-1 to 10-M connected in series, and each of blocks 10-1 to 10-M contains N cells connected in parallel.

The voltage sensor 21-1 detects the voltage of the block 10-1. That is, the voltage sensor 21-1 detects the voltage VB1 of the N cells constituting the block 10-1. The voltage sensor 21-2 detects the voltage VB2 of the N cells constituting the block 10-2. The same applies to the voltage sensor 21-M. The current sensor 22 detects the current IB flowing through each block 10-1 to 10-M. That is, the current sensor 22 detects the total current flowing through the N cells of each block.

In the assembled battery 10 including the plurality of cells connected in parallel in this way, temperature variation occurs between the plurality of cells connected in parallel (hereinafter, simply referred to as “cells”). For example, where there are variations in initial resistance (product variations) between cells, a relatively large current flows through cells with relatively low resistance, so the temperature of cells with low initial resistance becomes relatively high. obtain. Further, the temperature variation also occurs depending on the arrangement of the cells (near the center or the end in the case, etc.) and the cooling structure in the case (configuration of the cooling air passage, etc.).

Since the resistance of a hot cell is lower than that of a cold cell, the temperature variation between cells increases the resistance variation between cells. Since a relatively large current flows through a cell having a relatively low resistance, if the resistance variation between cells increases, the current variation between cells increases. That is, the temperature variation between cells increases the current variation between cells. Then, for example, even if the battery protection control is performed using the total current (IB) and the average current (IB / N) flowing in a plurality of cells (blocks) connected in parallel as they are, the cell in which a relatively large current flows It may not be protected.

Therefore, in the battery system 2 according to this embodiment, the current variation between cells due to the temperature variation between cells is estimated. Regarding the temperature variation, it is practically difficult to actually detect the temperature for each cell and measure the temperature variation. Therefore, in this embodiment, each of the highest temperature cell having the highest temperature and the lowest temperature cell having the lowest temperature among the plurality of cells connected in parallel is correlated with the cell temperature in consideration of cell heat generation and cooling. The temperature index having the above temperature is calculated, and the difference between the temperature index (Ftmax) of the highest temperature cell and the temperature index (Ftmin) of the lowest temperature cell is used as an evaluation function (ΔF) indicating the degree of temperature variation between cells. Then, the evaluation function (ΔF) and the temperature of the assembled battery 10 (as described later, in this embodiment, the lowest temperature (TBmin) among the temperatures TBj detected by the temperature sensor 23 is used). The degree of current variation between cells (maximum current correction gain) is estimated using this. The details will be described below.

FIG. 3 is a diagram showing an example of temperature changes between the highest temperature cell and the lowest temperature cell among a plurality of cells connected in parallel. With reference to FIG. 3, lines L1 and L2 show the temperature change of the lowest temperature cell and the temperature change of the highest temperature cell, respectively, and line L3 shows the change of the temperature difference ΔT between the highest temperature cell and the lowest temperature cell, respectively. .. When the current IB starts to flow in the assembled battery 10 at time t0, the temperatures of both cells start to rise. The way the temperature rises differs from cell to cell, and a temperature difference ΔT (temperature variation) occurs between the two cells. This temperature difference ΔT increases the current difference (current variation) between the two cells as described above.

FIG. 4 is a diagram showing an example of the relationship between the cell temperature and the resistance. With reference to FIG. 4, the horizontal axis represents the cell temperature Tce and the vertical axis represents the cell resistance Rce. There is a relationship between the temperature Tce and the resistance Rce of the cell that the resistance Rce decreases as the temperature Tce rises and the magnitude of the resistance change with respect to the temperature change increases as the temperature decreases (the slope of the curve increases).

Point P1 indicates the lowest temperature cell among the plurality of cells connected in parallel, and point P2 indicates the highest temperature cell among the plurality of cells connected in parallel. The temperature difference ΔT (temperature variation) between the highest temperature cell and the lowest temperature cell causes a resistance difference ΔR (resistance variation) between the two cells. Then, due to this resistance difference ΔR (resistance variation), a current difference (current variation) occurs between the maximum temperature cell and the minimum temperature cell.

Since the magnitude of the resistance change with respect to the temperature change is larger at lower temperatures, the resistance difference ΔR (resistance variation) between cells and the current difference (current variation) caused by the resistance difference ΔR between cells can be larger at lower temperatures. Therefore, in the battery system 2 according to this embodiment, the degree of current variation is estimated by further using the temperature of the assembled battery 10 together with the evaluation function (ΔF) having a correlation with the temperature difference ΔT.

FIG. 5 is a flowchart illustrating a processing procedure related to estimation of current variation between cells. The process shown in this flowchart is repeatedly executed by the ECU 100 shown in FIG. 1 at a predetermined calculation cycle dt for each block constituting the assembled battery 10.

With reference to FIG. 5, the ECU 100 calculates the temperature index Ftmax (first temperature index) of the highest temperature cell among the plurality of cells connected in parallel by the following equation (step S10).

In each equation, "t" indicates the operation value in the current operation cycle, and "t-1" indicates the operation value in the previous operation cycle. Qtmax indicates the calorific value of the maximum temperature cell (heat generation term associated with energization), and Ctmax indicates the cooling amount of the maximum temperature cell (cooling term by the cooling device). Fk is a predetermined correction coefficient. In the formula (2), Rtmax and Itmax indicate the resistance and current of the maximum temperature cell, respectively, and Qktmax is a predetermined constant (smoothing constant). Rtmax and Itmax are calculated by the formulas (7) and (9) described later, respectively.

Further, in the formula (3), TBmin is the lowest temperature among the temperatures TBj detected by the temperature sensor 23, and TBoffset1 makes the cooling term of the highest temperature cell larger than the cooling term of the lowest temperature cell described later. It is an offset value to be calculated. TC is the temperature of the cooling air detected by the temperature sensor 27 (FIG. 1). h (t) is a cooling term correction coefficient, which is a coefficient determined by the air volume of the cooling air produced by the cooling device. Instead of TBmin, another temperature TBj indicating the temperature of the assembled battery 10 may be adopted, but it is preferable to adopt the temperature TBmin closest to the temperature of the lowest temperature cell as in the formula (3). ..

Next, the ECU 100 calculates the temperature index Ftmin (second temperature index) of the lowest temperature cell among the plurality of cells connected in parallel by the following equation (step S20).

Qtmin indicates the calorific value of the lowest temperature cell (heat generation term associated with energization), and Ctmin indicates the cooling amount of the lowest temperature cell (cooling term by the cooling device). In the formula (5), Rtmin and Itmin indicate the resistance and current of the lowest temperature cell, respectively, and Qktmin is a predetermined constant (smoothing constant). Rtmin and Itmin are calculated by the formulas (8) and (10) described later, respectively.

Rtmax (resistance of the highest temperature cell) in the formula (2) and Rtmin (resistance of the lowest temperature cell) in the formula (5) are estimated by the following formulas.

Rivmin and Rivmax represent the variation (product variation) of the initial resistance existing between the cells, and indicate the minimum value and the maximum value of the initial resistance, respectively. Since the highest temperature cell has a higher cell temperature and a lower resistance than the lowest temperature cell, Rivmin is used to calculate the resistance Rtmax of the highest temperature cell, and Rivmax is used to calculate the resistance Rtmin of the lowest temperature cell. It is used. Rivmin and Rivmax are determined in advance by experiments and the like.

f is a coefficient indicating a decrease in resistance from the initial resistance value (Rivmin, Rivmax), and is a function (map) having the cell temperature and the remaining capacity (RAHR) as arguments.

FIG. 6 is a map for determining the coefficient f used in the equations (7) and (8). With reference to FIG. 6, the coefficient f is determined based on the cell temperature and residual capacity (RAHR). Basically, the lower the temperature and the lower the RAHR, the larger the coefficient f, and the higher the temperature and the higher the RAHR, the smaller the coefficient f. The specific value of the map is determined in advance through experiments and the like.

FIG. 7 is a diagram showing the temperature dependence of the resistance of the cell. With reference to FIG. 7, the horizontal axis represents the cell temperature Tce and the vertical axis represents the cell resistance Rce. When the temperature Tce decreases, the resistance Rce increases, and as the temperature Tce increases, the resistance Rce decreases.

Point P3 indicates the lowest temperature cell among the plurality of cells connected in parallel. In this embodiment, as shown in the formula (8), the resistance Rtmin of the lowest temperature cell is estimated using TBmin indicating the lowest temperature in the assembled battery 10.

Point P4 indicates the highest temperature cell among the plurality of cells connected in parallel. In this embodiment, as shown by the formula (7), the value obtained by adding a predetermined offset amount TBoffset2 to TBmin indicating the minimum temperature in the assembled battery 10 is defined as the temperature of the maximum temperature cell, and Rtmax of the maximum temperature cell is set. Presumed.

FIG. 8 is a diagram showing the residual capacitance dependence of the resistance of the cell. With reference to FIG. 8, the horizontal axis represents the remaining capacity (RAHR (%)) in block units, and the vertical axis represents the resistance Rce of the cell. As RAHR decreases, resistance Rce increases.

Point P5 indicates the lowest temperature cell among the plurality of cells connected in parallel. As can be seen from the figure, in the region where RAHR is low, the sensitivity of the change in RAHR to the change in resistance becomes high. Therefore, in this embodiment, as shown by the equation (8), the RAHR of each block is increased. The resistance Rtmin of the lowest temperature cell is estimated using RAHRmin, which shows the lowest RAHR.

Point P6 indicates the highest temperature cell among the plurality of cells connected in parallel. In this embodiment, as shown by the formula (7), the resistance Rtmax of the maximum temperature cell is estimated by setting the value obtained by adding a predetermined offset amount RAHRoffset to RAHRmin as the RAHR of the maximum temperature cell.

Regarding Itmax (current of the highest temperature cell) in the formula (2) and Itmin (current of the lowest temperature cell) in the formula (5), as shown in FIG. 9, a plurality of cells connected in parallel are the highest. It is estimated by the following equation, assuming that it is either the temperature cell or the lowest temperature cell, and also considering the disconnection of one cell (the disconnection may increase the current of other cells and increase the current variation). ..

N is the number of cells in parallel in each block (Fig. 2). N1 is the number of the highest temperature cells among the N cells connected in parallel, and N2 is the number of cells that are disconnected. The equations (9) and (10) are Rtmax (resistance of the highest temperature cell) and Rtmin (resistance of the lowest temperature cell) calculated by the above equations (7) and (8), and the model shown in FIG. Can be easily derived using.

In this embodiment, N1 = 1 is set as the state in which the current variation is the largest in the situation where the assembled battery 10 can be used (the current concentration of the maximum temperature cell is the highest), and N2 is set to N2. The worst value in the usable state of the assembled battery 10 (for example, N2 = 2 with respect to N = 15) is set.

In this way, using Rtmax and Itmax calculated by the formulas (7) and (9), the calorific value Qtmax of the maximum temperature cell is calculated by the formula (2). Then, using the calculated calorific value Qtmax and the cooling amount Ctmax calculated by the formula (3), the temperature index Ftmax (first temperature index) of the maximum temperature cell is calculated by the formula (1) (1st temperature index). Step S10).

Further, using Rtmin and Itmin calculated by the formulas (8) and (10), the calorific value Qtmin of the lowest temperature cell is calculated by the formula (5). Then, using the calculated calorific value Qtmin and the cooling amount Ctmin calculated by the formula (6), the temperature index Ftmin (second temperature index) of the lowest temperature cell is calculated by the formula (4) ( Step S20).

With reference to FIG. 5 again, when the temperature index Ftmax of the highest temperature cell and the temperature index Ftmin of the lowest temperature cell are calculated, the ECU 100 subtracts the temperature index Ftmin of the lowest temperature cell from the temperature index Ftmax of the highest temperature cell. As a result, the evaluation function ΔF indicating the degree of temperature variation between cells is calculated (step S30).

Next, the ECU 100 uses the calculated evaluation function ΔF and the temperature TBmin indicating the minimum temperature in the assembled battery 10 to calculate the maximum current correction gain G indicating the degree of current variation between cells (step S40). ..

FIG. 10 is a diagram showing a map of the maximum current correction gain G. With reference to FIG. 10, the maximum current correction gain G is determined by the evaluation function ΔF indicating the degree of temperature variation between cells and the temperature TBmin. The larger the value of the maximum current correction gain G, the larger the current variation. Generally, the larger the value of the evaluation function ΔF (the larger the temperature variation), and the lower the temperature TBmin, the larger the current variation. The maximum current correction gain G is a large value.

The reason why the temperature TBmin indicating the lowest temperature in the assembled battery 10 is used is that the lower the temperature, the larger the current variation between the cells.

FIG. 11 is a diagram showing the relationship between the cell temperature and the resistance. With reference to FIG. 11, the horizontal axis represents the cell temperature Tce and the vertical axis represents the cell resistance Rce. As shown in FIG. 11, even if the temperature difference ΔT (evaluation function ΔF) between cells is the same, the lower the cell temperature, the larger the resistance difference ΔR (resistance variation) between cells, and as a result, The current variation between cells also increases. Therefore, in this embodiment, as shown in the map shown in FIG. 10, the maximum current correction gain G is calculated in consideration of not only the evaluation function ΔF indicating the degree of temperature variation between cells but also the temperature of the assembled battery 10. Will be done. Further, in this embodiment, as the strictest temperature condition, the temperature TBmin indicating the lowest temperature in the assembled battery 10 is used.

With reference to FIG. 5 again, when the maximum current correction gain G indicating the degree of current variation between cells is calculated, the ECU 100 adds the maximum current correction gain G to the current IB of the assembled battery 10 detected by the current sensor 22. Is multiplied to calculate the maximum current value Imax (step S50). Then, although not particularly shown, the ECU 100 limits the current IB so that the maximum current value Imax does not exceed the upper limit based on the maximum current value Imax, or the electric power calculated based on the maximum current value Imax is generated. Various restriction controls such as restricting the input / output of the assembled battery 10 so as not to exceed the upper limit are executed.

As described above, in this embodiment, the evaluation function ΔF indicating the degree of temperature variation between cells is calculated, and the evaluation function ΔF and the temperature TBmin of the assembled battery 10 are used to determine the current variation between cells. The degree is estimated (calculation of maximum current correction gain G). Therefore, according to this embodiment, it is possible to realize appropriate battery protection control by using the maximum current value Imax obtained by multiplying the maximum current correction gain G by the actual current IB.

The embodiments disclosed this time should be considered to be exemplary in all respects and not restrictive. The scope of the present disclosure is shown by the scope of claims rather than the description of the embodiment described above, and is intended to include all modifications within the meaning and scope equivalent to the scope of claims.

1 vehicle, 2 battery system, 10 sets of batteries, 10-1 to 10-M blocks, 20 monitoring units, 21,21 to 21-M voltage sensors, 22 current sensors, 23, 27 temperature sensors, 25 ducts, 26 Fan, 30 PCU, 41, 42 MG, 50 engine, 60 power divider, 70 drive shaft, 80 drive wheels, 100 ECU, 102 CPU, 105 memory.

Claims (1)

An assembled battery that includes multiple secondary batteries connected in parallel,
A cooling device configured to supply cooling air to the assembled battery,
A control device configured to estimate the current variation between the plurality of secondary batteries due to the temperature variation among the plurality of secondary batteries is provided.
The control device is
A first temperature index having a correlation with the temperature of the maximum temperature cell by using the calorific value of the maximum temperature cell having the highest temperature among the plurality of secondary batteries and the cooling amount of the maximum temperature cell by the cooling device. Is calculated and
A second temperature index that correlates with the temperature of the lowest temperature cell using the calorific value of the lowest temperature cell among the plurality of secondary batteries and the cooling amount of the lowest temperature cell by the cooling device. Is calculated and
By subtracting the second temperature index from the first temperature index, an evaluation value indicating the degree of the temperature variation is calculated.
Using the calculated evaluation value and the temperature of the assembled battery, the degree of the current variation is estimated.
The calorific value of the maximum temperature cell is calculated using the estimation results of the resistance and current of the maximum temperature cell.
The calorific value of the minimum temperature cell is calculated using the estimation results of the resistance and current of the minimum temperature cell.
A battery system in which the cooling amounts of the maximum temperature cell and the minimum temperature cell are estimated using the difference between the temperature of the assembled battery and the temperature of the cooling air.

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