JP6708957B2 - Battery system - Google Patents

Description

The present invention relates to a technique for controlling charge/discharge of a nickel hydrogen battery.

BACKGROUND ART In recent years, it is known that a phenomenon called a memory effect occurs in secondary batteries that are widely used in various fields, as charging and discharging are repeated. In Japanese Patent Laid-Open No. 8-223812 (Patent Document 1), so-called refresh charging/discharging (discharging that makes the remaining capacity approximately 0% and charging that makes the remaining capacity almost 100%) is appropriately performed when a memory effect occurs. Techniques for eliminating the memory effect by carrying out are disclosed.

JP-A-8-223812

In order to accurately charge and discharge the secondary battery, it is desired to estimate the state of charge (hereinafter also referred to as “SOC”) of the secondary battery with high accuracy. Conventionally, as a method of calculating SOC, a voltage method using a voltage of a secondary battery is known. This voltage system has a characteristic that the open circuit voltage (Open Circuit Voltage, also referred to as “OCV” below) of the secondary battery increases as the SOC increases, and therefore a curve showing the value of OCV with respect to the SOC (hereinafter referred to as “OCV curve”). Is also obtained in advance by experiments or the like and stored in a memory or the like, and the SOC corresponding to the detected value of the voltage of the secondary battery is calculated with reference to this OCV curve.

Conventionally, nickel-hydrogen batteries have been widely used as secondary batteries for hybrid vehicles. The inventors of the present application have newly found through experiments and the like that a phenomenon different from the memory effect occurs in a nickel hydrogen battery. Another phenomenon is that when the SOC stays in a region lower than a predetermined value for a predetermined time or more, the inclination of the OCV curve (the change amount of the OCV when the SOC changes by a unit amount) is new (unused). It is a phenomenon that it becomes flatter than the (time). In this specification, this phenomenon is also referred to as "flattening of OCV curve" for convenience of description. If the OCV curve is flattened, the accuracy of the SOC calculated using the OCV curve at the time of new product will be reduced. Therefore, development of a technique for eliminating the flattening of the OCV curve is desired.

The present invention has been made to solve the above problems, and an object thereof is to eliminate flattening of an OCV curve in a nickel-hydrogen battery when the flattening occurs.

The battery system according to the present invention includes a nickel-hydrogen battery and a control unit that controls charging and discharging of the nickel-hydrogen battery. The control unit determines whether or not the time during which the amount of electricity stored in the nickel-hydrogen battery is staying in the first region, which is lower than a predetermined value, exceeds a predetermined time, and when the time during which the stay is over the predetermined time, The nickel-metal hydride battery is forcibly charged so that the charged amount of the nickel-hydrogen battery becomes larger than a predetermined value, and then the charged amount of the nickel-hydrogen battery falls within the second region in which the lower limit value is larger than the specified value. Control the charging and discharging of.

As described above, in the nickel-hydrogen battery, the inventors of the present application newly found that the OCV curve is flattened when the SOC stays in a region lower than the predetermined value for a predetermined time or longer. Further, as a result of repeated studies, the inventors of the present application have found that the flattening of the OCV curve can be resolved by charging to a SOC that is somewhat high.

In view of this point, the control unit considers that the time during which the amount of electricity stored in the nickel-hydrogen battery stays in the first region lower than the predetermined value exceeds the predetermined time (that is, the OCV curve is flattened). If so, first, forcible charging is performed so that the amount of electricity stored in the nickel-hydrogen battery becomes larger than a predetermined value, and the flattening of the OCV curve is eliminated early. Then, after the forced charging is performed , the charging/discharging is controlled so that the SOC falls within the second region in which the lower limit value is larger than the predetermined value. Therefore, the SOC is maintained in the second region even after the forced charging is performed, and the state in which the flattening of the OCV curve is eliminated can be fixed. As a result, when the OCV curve is flattened in the nickel-hydrogen battery, the flattening can be eliminated.

It is a figure which shows roughly the structure of the vehicle in which a battery system is mounted. It is a figure which shows the voltage curve after the endurance test 1. It is a figure which shows the voltage curve after the endurance test 2. It is a figure which shows the voltage curve after the endurance test 3. It is a figure which shows an OCV curve typically. It is a figure which shows the voltage curve when charging/discharging is repeated twice after the endurance test 2. It is a figure which shows the voltage curve when charging/discharging is repeated twice after the endurance test 3. 3 is a flowchart showing an example of a processing procedure of the ECU.

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

<Battery system configuration>
FIG. 1 is a diagram schematically showing a configuration of a vehicle 1 in which a battery system 2 according to the present embodiment is mounted. Note that the vehicle 1 shown in FIG. 1 is a so-called hybrid vehicle, but the battery system 2 according to the present embodiment is not limited to being applied to a hybrid vehicle, and is applicable to all vehicles equipped with a nickel hydrogen battery, and other than vehicles. It is also applicable to.

The vehicle 1 includes a battery system 2, a power control unit (PCU) 30, motor generators (hereinafter referred to as “MG”) 41 and 42, an engine 50, a power split mechanism 60, and a drive. A shaft 70 and a drive wheel 80 are provided. The battery system 2 includes a nickel hydrogen (NiMH) battery 10, a monitoring unit 20, and an electronic control unit (ECU: Electronic Control Unit) 100.

The engine 50 outputs kinetic energy by the combustion energy of fuel. MGs 41 and 42 can function as both a generator and an electric motor.

MG 41 mainly operates as a generator that generates electric power by using a part of the output of engine 50 transmitted through power split device 60. The electric power generated by the MG 41 is supplied to the MG 42 or the nickel hydrogen battery 10 via the PCU 30.

MG 42 is driven by at least one of the power from nickel hydrogen battery 10 and the power generated by MG 41. The drive force of MG 42 is transmitted to drive shaft 70. During regenerative braking of vehicle 1, MG 42 operates as a generator by being driven by the rotational force of the wheels. The nickel-hydrogen battery 10 is charged via the PCU 30 with the electric power generated by the MGs 41 and 42.

Nickel hydrogen battery 10 stores electric power for driving MG 41, 42. The nickel hydrogen battery 10 includes a plurality of nickel hydrogen battery cells connected in series. The monitoring unit 20 includes a voltage sensor 21, a current sensor 22, and a temperature sensor 23. The voltage sensor 21 is configured to be able to detect the terminal voltage of the nickel-hydrogen battery 10 (hereinafter also referred to as “battery voltage VB”). The current sensor 22 is configured to be able to detect the charge/discharge current of the nickel hydrogen battery 10 (hereinafter also referred to as “battery current IB”). The temperature sensor 23 is configured to be able to detect the temperature of the nickel hydrogen battery 10 (hereinafter also referred to as “battery temperature TB”). Each sensor outputs a signal indicating the detection result to ECU 100.

PCU 30 is configured to perform bidirectional power conversion between nickel hydrogen battery 10 and MGs 41, 42 according to a switching command from ECU 100. The PCU 30 is configured to be able to control the states of the MGs 41 and 42 separately. For example, PCU 30 can put MG 41 in a power running state while putting MG 41 in a regenerative (power generating) state.

ECU 100 is configured to include a CPU (Central Processing Unit), a memory, and an input/output interface (none of which is shown). The ECU 100 controls charging/discharging of the nickel hydrogen battery 10 by controlling the engine 50 and the PCU 30 based on the signals from the respective sensors and the information stored in the memory.

When it is necessary to charge the nickel-hydrogen battery 10, the ECU 100 causes the MG 41 to generate power by using a part of the power of the engine 50, and charges the nickel-hydrogen battery 10 with the electric power generated by the MG 41. Control (MG41, MG42). Hereinafter, control of charging nickel-hydrogen battery 10 with electric power generated by MG 41 using a part of the power of engine 50 is also referred to as “P charge”.

The ECU 100 calculates the state of charge (SOC) of the nickel-hydrogen battery 10 in order to precisely control the charge/discharge of the nickel-hydrogen battery 10. Generally, SOC is represented by the ratio of the remaining capacity to the full charge capacity. ECU 100 according to the present embodiment calculates SOC using battery voltage VB. Specifically, the ECU 100 stores an OCV curve obtained by an experiment or the like in a memory in advance, and refers to the OCV curve to calculate an SOC corresponding to the battery voltage VB (a value detected by the voltage sensor 21). ..

ECU 100 controls charging/discharging of nickel-hydrogen battery 10 so that the calculated SOC falls within a predetermined region (a normal SOC region or a high SOC region described later). For example, when the SOC falls below the lower limit value of the predetermined region, ECU 100 increases the SOC by performing the above-mentioned P charge to charge nickel hydrogen battery 10.

<Flatization of OCV curve>
As described above, the ECU 100 calculates the SOC corresponding to the battery voltage VB (the value detected by the voltage sensor 21) with reference to the OCV curve, and charges the nickel-hydrogen battery 10 so that the calculated SOC falls within a predetermined region. Control the discharge. Therefore, in order to accurately charge and discharge the nickel hydrogen battery 10, it is desired to calculate the SOC with high accuracy.

However, the inventors of the present application have found that, in the nickel-hydrogen battery 10, when the SOC continuously stays in a region where the SOC is lower than a predetermined value (hereinafter referred to as “low SOC region”) for a predetermined time or longer, the OCV curve used for calculating the SOC. It was newly found by various experiments including the following endurance tests 1 to 3 that the slope (change amount of OCV when SOC changes by a unit amount) becomes flatter than that of a new product (when not in use).

<<Experimental content and results>>
The inventors each carry out the following durability tests 1 to 3 on the nickel-hydrogen battery 10, and carry out an experiment to measure the behavior of the battery voltage VB with respect to the SOC (hereinafter also referred to as “voltage curve”) after each durability test. I did.

(Durability Test 1) P1 is left for a predetermined period at 25° C. in a state of almost complete discharge (state of SOC is about several%).

(Durability Test 2) P1 is left for a predetermined period at 65° C. in a state of being almost completely discharged.
(Durability Test 3) The battery is left at 65° C. for a predetermined period of P2 (P2>P1) in a state of being almost completely discharged.

The above durability tests 1 to 3 are tests in which SOC is continuously retained in a low SOC region (region lower than a predetermined value) for a predetermined time T1 or more. The above durability tests 1 to 3 are common in that they are left in a state of being almost completely discharged. The endurance test 2 has the same standing period as the endurance test 1, but is different in that the temperature is high. The endurance test 3 has the same temperature as the endurance test 2, but is different in that the leaving period is long.

The above experimental results are shown in FIGS. FIG. 2 is a diagram showing a voltage curve after the durability test 1. FIG. 3 is a diagram showing a voltage curve after the durability test 2. FIG. 4 is a diagram showing a voltage curve after the durability test 3. In each of FIGS. 2 to 4, the horizontal axis represents SOC, and the vertical axis represents battery voltage VB (CCV: Closed Circuit Voltage) when SOC is changed. Further, in each of FIGS. 2 to 4, the solid line represents the voltage curve after the durability test, and the broken line represents the voltage curve when the product is new. In addition, in each charging voltage curve shown in FIGS. 2 to 4 (and FIGS. 6 and 7 described later), there is a region where the battery voltage VB becomes flat at the maximum level. This is because the voltage at the maximum level is intentionally maintained for a predetermined time in order to bring the battery into a nearly fully charged state.

From all the experimental results, it can be confirmed that after the durability test (solid line), the voltage curve is flatter (the slope of the voltage curve is smaller) than that of the new product (broken line). Further, it can be confirmed that the flattening of the voltage curve depends on the temperature and the standing period. Specifically, comparing FIG. 2 and FIG. 3 in which only the temperature is different, it is confirmed that the voltage curve of FIG. 3 having a higher temperature is flatter than the voltage curve of FIG. 2 having a lower temperature. it can. Further, when comparing FIG. 3 and FIG. 4 which are different only in the leaving period, it is confirmed that the voltage curve of FIG. 4 having a long leaving period is flatter than the voltage curve of FIG. 3 having a short leaving period. it can. The flattening of the voltage curve described above is caused by leaving it in the low SOC region, but even when charging/discharging in the low SOC region, the flattening of the voltage curve occurs as in the case of leaving. Applicant has confirmed in other experiments.

The voltage curves during charging and discharging shown in FIGS. 2 to 4 are the results of measurement with a small current close to no load. That is, it is considered that the OCV curve is also flattened in the nickel hydrogen battery in which the voltage curve is flattened as described above.

FIG. 5 is a diagram schematically showing an OCV curve of the nickel hydrogen battery 10. In FIG. 5, the horizontal axis represents SOC and the vertical axis represents OCV. Further, in FIG. 5, the solid line represents the OCV curve after the durability test, and the alternate long and short dash line represents the OCV curve at the time of new product.

As shown in FIG. 5, the OCV curve (dashed line) at the time of new product has a certain degree of inclination in the entire SOC range. Therefore, if the OCV is determined, the SOC is also uniquely easy to determine.

On the other hand, the OCV curve (solid line) after the endurance test changes from the OCV curve at the time of a new product, and has a long flat region (a region where the OCV is almost constant regardless of SOC). In this flat region, it is difficult to uniquely determine SOC even if OCV is determined.

In this way, when the time during which the SOC stays in the low SOC region exceeds the predetermined time, the actual OCV curve is flattened like the OCV curve (solid line) after the durability test. The flattening of the OCV curve depends not only on the standing time but also on the temperature. Nevertheless, if the SOC is calculated using the OCV curve (dashed-dotted line) at the time of new product, the calculation accuracy of the SOC decreases. Even if a flattened OCV curve (dashed-dotted line) is used, it is difficult to uniquely determine the SOC even if the OCV value is determined in the flat region, so the SOC cannot be accurately estimated.

Furthermore, as another point of focus, it can be confirmed from the results of all the experiments that the battery voltage VB at the time of charging after the durability test is higher than that at the time of a new product. It is known that in the nickel-hydrogen battery 10, the higher the battery voltage VB, the more the gas generation amount due to the influence of side reactions. As a result, when the SOC stays in the low SOC region for a predetermined time or longer, the amount of gas generated by the side reaction increases as the battery voltage VB increases, which deteriorates the charging efficiency or the battery. There is a concern that the internal pressure of the cell will rise.

<Elimination of flattening of OCV curve>
The inventors conducted various studies to eliminate the flattening of the OCV curve. As a result, the inventors have found that the flattening of the OCV curve can be resolved by charging to a SOC that is somewhat high.

FIG. 6 is a diagram showing a voltage curve when charging and discharging are repeated twice after the durability test 2. FIG. 7 is a diagram showing a voltage curve when charging and discharging are repeated twice after the durability test 3. 6 and 7, the alternate long and short dash line represents the voltage curve during the first charging/discharging, and the alternate long and two short dashes line represents the voltage curve during the second charging/discharging.

From the examination results of FIGS. 6 and 7, it is confirmed that by repeating charging and discharging, the state where the voltage curve is flattened gradually recovers to the state of a new product having a slope. In particular, it can be confirmed that the inclination recovery after the first charging is remarkable. From the results of this examination, it is considered that the flattening of the OCV curve can be resolved by charging to a high SOC to some extent.

In view of the above points, the ECU 100 according to the present embodiment, when the nickel-hydrogen battery 10 is used (charged/discharged or left) in a state where the SOC continues to stay in the low SOC region for a predetermined time or longer, first, The SOC is increased to a value larger than a predetermined value (upper limit value in the low SOC range) by performing the forced charge to forcibly charge the nickel hydrogen battery 10. This eliminates the flattening of the OCV curve at an early stage.

Further, ECU 100 repeats charging and discharging in a relatively high SOC region for a while after the forced charging. As a result, the state in which the flattening of the OCV curve is eliminated is fixed. That is, if the SOC is returned to the low SOC region immediately after the forced charging, there is a concern that the OCV curve will be immediately returned to the flattened state, so that charging/discharging should be repeated in the high SOC region for a while. Thus, it is possible to prevent the OCV curve from immediately returning to a flattened state.

FIG. 8 is a flowchart showing an example of a processing procedure performed when the ECU 100 determines the battery control mode. In the present embodiment, the battery control mode includes a normal SOC mode and a high SOC mode.

The normal SOC mode is a mode in which the charge/discharge of the nickel-hydrogen battery 10 is controlled so that the SOC falls within a region from the control lower limit value SN1 to the control upper limit value SN2 (hereinafter also referred to as “normal SOC region”). For example, when the control lower limit value SN1 and the control upper limit value SN2 in the normal SOC mode are 30% and 70%, respectively, the SOC control center value in the normal SOC mode is 50%. The above values are merely examples, and the present invention is not limited to these.

The high SOC mode is a mode in which charge/discharge of the nickel-hydrogen battery 10 is controlled so that the SOC falls within a relatively high SOC range (hereinafter, also referred to as “high SOC range”). For example, when the control lower limit value SH1 and the control upper limit value SH2 in the high SOC mode are 60% and 80%, respectively, the SOC control center value in the high SOC mode is 70%. The above values are merely examples, and the present invention is not limited to these. However, the control lower limit value SH1 in the high SOC mode is at least larger than the control lower limit value SN1 in the normal SOC mode, and is larger than the upper limit value (predetermined value) in the low SOC region where the flattening of the OCV curve is concerned. Must be set to.

In step (hereinafter, step is abbreviated as “S”) 10, ECU 100 determines whether or not the battery control mode is the high SOC mode.

When the battery control mode is not the high SOC mode (NO in S10), that is, when the battery control mode is the normal SOC mode, ECU 100 determines in S11 that the SOC continues to stay in the low SOC region ( Hereinafter referred to as "low SOC retention time"). The low SOC residence time is calculated from the SOC history up to the present. The low SOC residence time is reset when the SOC exceeds a predetermined value. The SOC is calculated by the voltage method using the OCV value calculated from the battery voltage VB (value detected by the voltage sensor 21) as described above.

In S12, the ECU 100 determines whether or not the low SOC residence time exceeds the predetermined time T1. This determination is a process for determining whether or not the OCV curve is flattened. As described above, the flattening of the OCV curve depends not only on time but also on temperature. Therefore, the ECU 100 sets the predetermined time T1 to an appropriate value according to the history of the battery temperature TB during the low SOC retention time. For example, it is considered that the higher the battery temperature TB, the earlier the flattening of the OCV curve occurs, so the ECU 100 shortens the predetermined time T1. This makes it possible to accurately determine whether or not the OCV curve is flattened.

When the low SOC residence time exceeds the predetermined time T1 (YES in S12), it is considered that the OCV curve is flattened, and therefore the ECU 100 increases the SOC to the control center value of the high SOC in S13. Until that time, the above-described P charge is performed to perform the forced charge (first charge) to forcibly charge the nickel hydrogen battery 10. By this forced charging (first charging), the SOC increases to a value larger than the predetermined value, so that the flattening of the OCV curve can be eliminated early.

After that, the ECU 100 shifts the processing to S14 and changes the battery control mode from the current normal SOC mode to the high SOC mode. This suppresses the SOC from immediately returning to the low SOC region (that is, the OCV curve returns to the flattened state) after the forced charging.

On the other hand, when the battery control mode is the high SOC mode (YES in S10), ECU 100 determines in S15 the time during which the high SOC mode is continued (hereinafter, also referred to as “high SOC continuation time”) for a predetermined time T2. It is determined whether or not

If the high SOC continuation time does not exceed the predetermined time T2 (NO in S15), there is a concern that the OCV curve may return to a flattened state when the normal SOC mode is restored at this time, so the ECU 100 , The process proceeds to S14, and the battery control mode is maintained in the high SOC mode.

If the high SOC duration time exceeds the predetermined time T2 (YES in S15), it is considered that the OCV curve does not return to the flattened state even if the normal SOC mode is restored at the present time. Is moved to S16, and the battery control mode is returned from the high SOC mode to the normal SOC mode.

As described above, when the low SOC residence time exceeds the predetermined time T1 during the normal SOC mode, ECU 100 according to the present embodiment first performs forced charging, and switches the battery control mode to the high SOC mode after forced charging. As a result, the nickel-hydrogen battery is charged until the SOC increases to a value higher than the predetermined value, and thereafter the SOC is maintained in the high SOC range. As a result, when the OCV curve is flattened in the nickel-hydrogen battery 10, the flattening can be eliminated.

The above-described embodiment can be modified as follows, for example.
[Modification 1]
In the above embodiment, the case where the voltage method using the OCV curve is adopted as the SOC calculation method has been described.

However, the SOC calculation method is not necessarily limited to the voltage method. For example, in addition to calculating the SOC by the voltage method described above, the SOC is calculated by the current integration method that is another known SOC calculation method, and the two SOCs are weighted to calculate the final SOC. You can

When the final SOC is calculated by weighting the SOC according to the voltage method and the SOC according to the current integration method, at the time of the first charge when the low SOC residence time exceeds the predetermined time T1 (at the time of forced charge in S13 of FIG. 8). In (), when determining whether the SOC has reached the high SOC region, the weighting of the SOC by the current integration method may be increased. When the low SOC residence time exceeds the predetermined time T1, the battery voltage VB is generally higher than that of a new product (see FIG. 2-4). Therefore, when the SOC by the voltage method reaches the high SOC region. When the forced charging is finished at, there is a concern that the actual SOC has not reached the high SOC region and the flattening of the OCV curve cannot be resolved. In view of this point, at the time of the first charge, the actual SOC can be more reliably reached to the high SOC region by increasing the weighting of the SOC by the current integration.

[Modification 2]
In the above-described embodiment, the battery control mode is returned from the high SOC mode to the normal SOC mode when the time during which the high SOC mode is continued exceeds the “predetermined time T2” (S15 in FIG. 8, (See S16).

However, it is assumed that the recovery speed of the OCV curve after switching to the high SOC mode (the speed until the flattening is resolved and the fixed state is fixed) depends on the fluctuation range of the battery temperature TB and the SOC. It In view of this point, the “predetermined time T2” used in the process of S15 of FIG. 8 may be changed according to the fluctuation range of the battery temperature TB and the SOC. This makes it possible to return to the normal SOC mode at an appropriate timing without unnecessarily continuing the high SOC mode.

[Modification 3]
In the above-described embodiment, in S13 of FIG. 8, the case of performing the P charge is described as a method of increasing the SOC to the high SOC region. However, instead of or in addition to the P charge, the output of the nickel-hydrogen battery 10 may be changed. It is also possible to temporarily reduce the possible power WOUT (unit: watt) to suppress the decrease in SOC due to the discharge of the nickel-hydrogen battery 10, thereby promoting the increase in SOC. Alternatively, the receivable electric power WIN (unit: watt) of the nickel-hydrogen battery 10 may be temporarily increased to promote an increase in SOC due to the charging of the nickel-hydrogen battery 10.

[Modification 4]
In the above-described embodiment, in S12 of FIG. 8, it is determined whether or not the OCV curve is flattened by using the time (low SOC retention time) itself as an index.

However, the index used for determining whether the OCV curve is flattened is not limited to the time itself. For example, a standardized evaluation value may be newly provided and whether or not the OCV curve is flattened may be determined using this evaluation value as an index.

As a new evaluation value, for example, an evaluation value E defined by the following equation is calculated and accumulated for each predetermined period Δt, and when the accumulated evaluation value E exceeds a predetermined amount, the OCV curve is flattened. May be determined to have occurred.

E(t+Δt)=E(t)+K·Δt
In the above equation, “t” represents time (time), “E(t+Δt)” represents the current value of the evaluation value E, and “E(t)” represents the previous value of the evaluation value E. “K” is a flattening point at each temperature.

Further, “K” may have a fluctuation range dependency of SOC. By introducing such an evaluation value E, it is possible to more effectively carry out the present control while the vehicle is traveling in which the temperature greatly changes.

Further, as described above, the elimination speed of the flattening of the OCV curve depends on the battery temperature TB and the SOC fluctuation range. In view of this point, in S15 of FIG. 8, an evaluation index similar to the above-described evaluation value E is newly provided, and it is determined whether or not to return from the high SOC mode to the normal SOC mode by using this evaluation index. May be. By doing so, it is possible to more efficiently determine whether or not to return to the normal SOC mode.

The embodiments disclosed this time are to be considered as illustrative in all points and not restrictive. The scope of the present invention is shown not by the above description but by the claims, and is intended to include meanings equivalent to the claims and all modifications within the scope.

1 vehicle, 2 battery system, 10 nickel hydrogen battery, 20 monitoring unit, 21 voltage sensor, 22 current sensor, 23 temperature sensor, 30 PCU, 41, 42 MG, 50 engine, 60 power split mechanism, 70 drive shaft, 80 drive Wheel, 100 ECU.

Claims (1)

Ni-MH battery,
A control unit for controlling charge and discharge of the nickel-hydrogen battery,
The control unit is
It is determined whether or not the time during which the amount of electricity stored in the nickel-hydrogen battery stays in the first region lower than a predetermined value exceeds a predetermined time,
When the staying time exceeds the predetermined time, forcible charging is performed so that the charged amount of the nickel-hydrogen battery becomes a value larger than the predetermined value, and then the lower limit value is larger than the predetermined value. A battery system for controlling charging/discharging of the nickel-hydrogen battery such that the amount of electricity stored in the nickel-hydrogen battery falls within a second region.

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