JP6566345B2 - Storage battery control device, power storage system - Google Patents

Description

  The present invention relates to a storage battery control device that controls the output of a power storage device, and a power storage system including the storage battery control device.

  Conventionally, a power storage system including a power storage device, a power converter, and a controller has been proposed (see, for example, Patent Document 1). The power converter converts AC power into DC power when charging the power storage device, and converts DC power into AC power when discharging from the power storage device. The controller also controls the operation of the power converter.

  Patent Document 1 aims at discharging a power storage device by a control method suitable for the power situation of a house and extending the life of the power storage device when a power generation device including a solar cell and a power storage device coexist. As described above, a technique for controlling a power converter is described. In Patent Document 1, the controller performs the current control when the voltage of the lead-in line is smaller than the threshold voltage, and operates the power converter so as to execute the voltage control when the voltage of the lead-in line is larger than the threshold value. Is controlling.

Japanese Patent No. 5392357

  In Patent Document 1, current control is performed when discharging from a power storage device, and the discharge current is determined such that the power energy accumulated in the power storage device is used up in a discharge time zone. As a result, in Patent Document 1, the life of the power storage device is extended. However, Patent Document 1 does not specifically describe a technique for controlling the discharge current in order to use up the power energy accumulated in the power storage device.

  An object of the present invention is to provide a storage battery control device that increases the amount of electric power that can be extracted from the power storage device, and further to provide a power storage system that uses this storage battery control device.

The storage battery control device according to the present invention includes an input unit that receives a value of a voltage output from a power storage device, an output unit that gives an operation instruction to a power conversion device that converts DC power output from the power storage device into AC power, A processing unit that determines the instruction to be given to the power conversion device so as to adjust a value of a current output from the power storage device, and the power conversion device is connected so as to be linked to a power system, When the value of the voltage received by the input unit decreases to a predetermined lower limit during a period in which the value of the current output by the power storage device is maintained at a constant current, the processing unit while varying the rate of change of reduction of the current output from said power storage device within a range that can continue the output of the AC power, gives the instruction through the output unit to the power conversion apparatus to reduce the current And wherein the door.

The power storage system according to the present invention receives a value of a power storage device, a power conversion device that converts DC power of the power storage device into AC power, and that is linked to a power system, and a voltage output by the power storage device, A storage battery control device that gives an operation instruction to the power conversion device so as to adjust a value of a current output from the power storage device, and the storage battery control device maintains a current value output from the power storage device at a constant current. When the value of the voltage received during the period when the power is reduced to a predetermined lower limit, thereafter, the rate of change in reduction of the current output from the power storage device within a range in which the power conversion device can continue outputting the AC power The instruction is given to the power converter so as to reduce the current while changing the current .

  According to the configuration of the present invention, the amount of electric power that can be extracted from the power storage device is controlled by controlling the power conversion device so that power can be extracted from the power storage device even after the value of the voltage output from the power storage device has decreased to the lower limit. This has the advantage that it can be achieved.

It is a block diagram which shows embodiment. It is operation | movement explanatory drawing which shows embodiment. It is operation | movement explanatory drawing which shows a comparative example.

  As shown in FIG. 1, the power storage system described below is configured to perform system interconnection with the power system 5, and includes a power storage device 2, a power conversion device 3, a management device 4, and a storage battery control device 10. Hereinafter, the “storage battery control device” is simply referred to as “control device”. As will be described later, the management device 4 functions as a protection device that stops the operations of the power storage device 2 and the power conversion device 3 when an abnormality occurs in the power storage device 2. Since the management device 4 does not function at all times, it is not an essential component in this embodiment. In FIG. 1, a solid line connected to the component represents a power supply system, and a broken line connected to the component represents a signal system (information system).

  The power storage device 2 of the present embodiment has a configuration in which a housing 22 includes a plurality of battery modules 20 including one or a plurality of storage batteries 21, and can be used in a house, a small store, or a small office. It has a certain capacity (for example, 10 kWh). The storage battery 21 is assumed to be a lithium ion battery, but does not prevent other storage batteries such as lead storage batteries from being used. The battery module 20 includes a monitor unit 23 that monitors the terminal voltage (output voltage) and output current of the battery module 20 and the temperature of the battery module 20. The monitor unit 23 has a function of transmitting the measured voltage value, current value, temperature value, and the like as an output value. The output value of the monitor unit 23 is assumed to be a digital value, but an analog value can also be used.

  The power conversion device 3 has a function of performing power conversion bidirectionally between the power storage device 2 and the power system 5. That is, the power conversion device 3 converts the DC power of the power storage device 2 into AC power, and supplies this AC power to the load 51 together with the AC power supplied from the power system 5. Further, the power conversion device 3 converts AC power supplied from the power system 5 into DC power, and charges the power storage device 2 with this DC power. As described above, the power storage device 2 performs charging and discharging. Hereinafter, the operation of discharging the power storage device 2 (that is, supplying power to the load 51) will be described.

  The power conversion device 3 includes a circuit that adjusts the DC voltage output from the power storage device 2 and a circuit that converts DC power to AC power. These circuits are a kind of switching power supply, and the power conversion device 3 limits the input current from the power storage device 2 by controlling the ON / OFF timing of the switching element, Adjust the output current. Since the power system 5 is connected to the AC-side terminal of the power conversion device 3, the frequency and phase of the AC power output from the power conversion device 3 are equal to those of the power system 5 when the power storage device 2 is discharged. Adjusted. Further, the voltage value of the terminal on the AC side of the power converter 3 is equal to that of the power system 5. That is, even if the terminal voltage of the battery module 20 decreases during the discharge of the power storage device 2, the value of the output voltage of the power conversion device 3 is equal to the voltage value of the power system 5 if the power system 5 is normal without a power failure. Become.

  The control device 10 instructs the operation of the power conversion device 3. Control device 10 receives a voltage value output from power storage device 2 through management device 4, and gives an instruction regarding the operation of power conversion device 3 based on the voltage value. When the management device 4 detects an abnormality in the battery module 20 based on the output value of the monitor unit 23, the management device 4 stops the operation of the power storage device 2 and the power conversion device 3. In addition, although the control apparatus 10 assumes the structure accommodated in the housing | casing different from the power converter device 3, you may share a power converter device 3 and a housing | casing, and the power converter device 3 and a circuit board. Can be shared. Alternatively, the control device 10 can share the housing with the management device 4 and can share the circuit board with the management device 4.

  The control device 10 includes an input unit 11, an output unit 12, and a processing unit 13. The input unit 11 receives the voltage value measured by the monitor unit 23. The output unit 12 gives an operation instruction to the power storage device 2 and the power conversion device 3. Processing unit 13 uses the voltage value received by input unit 11 from monitor unit 23 to determine the content of the instruction given to power conversion device 3 so as to adjust the value of the current output from power storage device 2. The contents of the instruction by the processing unit 13 are determined as follows.

  By the way, when the power storage device 2 is discharged, as indicated by a solid line (characteristic vx) in FIG. 2, the value of the voltage output from the power storage device 2 decreases as the remaining capacity of the battery module 20 decreases. The value of the voltage output from the power storage device 2 may be considered to coincide with the value of the terminal voltage of the battery module 20 at normal times. In other words, the output value of the monitor unit 23 received by the input unit 11 can be used as the value of the voltage output by the power storage device 2.

  Control device 10 instructs power conversion device 3 to keep the value of the current output from power storage device 2 at a constant value during normal operation. That is, when the power storage device 2 is discharged, the processing unit 13 measures the voltage value measured by the monitor unit 23 so as to keep the current output from the power storage device 2 at a constant current as shown by a one-dot chain line (characteristic ix) in FIG. The content of the instruction given to the power conversion device 3 is changed in accordance with the fluctuations in. That is, the processing unit 13 changes the operation of the power conversion device 3 so that the input impedance of the power conversion device 3 that is the external impedance of the power storage device 2 decreases as the voltage value measured by the monitor unit 23 decreases. .

  When the voltage value monitored by monitor unit 23 reaches lower limit value Vi, processing unit 13 instructs power conversion device 3 to reduce the output current of power storage device 2. That is, when the voltage value monitored by the monitor unit 23 reaches the lower limit value Vi, the processing unit 13 instructs the power conversion device 3 to increase the input impedance.

  Even if the voltage value monitored by the monitor unit 23 reaches the lower limit value Vi, the battery module 20 still has a charge. Therefore, when the output current of the power storage device 2 is reduced, the value of the terminal voltage of the battery module 20 is It rises and becomes larger than the lower limit Vi. When the voltage value monitored by the monitor unit 23 decreases to the lower limit value Vi by such an operation, the output power of the power conversion device 3 is reduced, but the power conversion device 3 continues the power without stopping the operation. It becomes possible to output. In other words, the processing unit 13 determines the content of the instruction so as to reduce the current output from the power storage device 2 within a range in which the power conversion device 3 can continue outputting AC power.

  The lower limit value Vi is appropriately set according to the type of the battery module 20, the conditions for using the power storage device 2, and the like. For example, when charging and discharging are repeated so that the remaining capacity is kept in the range of about 40 to 70% in order to suppress deterioration of the battery module 20, the lower limit value Vi is set to be relatively high. For the purpose of reducing the amount of power received from the power system in order to respond to a power saving request or the like, the lower limit value Vi is set to be relatively low.

  By the way, even after the voltage value monitored by the monitor unit 23 reaches the lower limit value Vi, it is desirable to gradually decrease the output power of the power conversion device 3. By gradually lowering the output power of the power conversion device 3, the possibility of voltage or current oscillation in the power system 5 is reduced. Therefore, after the voltage value monitored by the monitor unit 23 reaches the lower limit value Vi, the processing unit 13 gives an instruction to the power conversion device 3 so that the value of the output current of the power storage device 2 decreases with time. .

  In the present embodiment, as indicated by the alternate long and short dash line (characteristic ix) in FIG. 2, after time t1 when the voltage value (characteristic vx) monitored by the monitor unit 23 reaches the lower limit value Vi, the processing unit 13 is relatively The current value is reduced with a large slope α1. Thereafter, when the voltage value reaches the reference value Vb, the processing unit 13 reduces the current value with a relatively small gradient α2. That is, when the current value up to time t1 is ia and the elapsed time from time t1 is represented by t, the current value is α1 · t + ia during the period of voltage value Vi to Vb. If the time when the voltage value reaches Vb is t2, the current value after time t2 is expressed as α2 · (t−t2) + {α1 · (t2−t1) + ia}. Here, the inclination α1 is set, for example, in the range of about −2 to −5, and the inclination α2 is set, for example, in the range of about 0 to −0.5.

  When the voltage value monitored by the monitor unit 23 again reaches the lower limit value Vi, the processing unit 13 repeats the same operation. However, a limiting condition is set for repetition. The limiting conditions will be described later.

  For example, as shown in FIG. 3, the reference value Vb is obtained when the output current (characteristic ix) of the power storage device 2 is stopped when the voltage value (characteristic vx) monitored by the monitor unit 23 reaches the lower limit value Vi. It is set based on the maximum voltage value Vmax (> Vi) monitored by the monitor unit 23. Here, the reference value Vb is set to n × (Vmax−Vi) + Vi. n is determined in the range of about 0.2 to 0.4. The reference value Vb can be determined as appropriate, and the above-described formula is an example. For example, the reference value Vb may be matched with the lower limit value Vi. In short, the value of the output current of the power storage device 2 is set so that the voltage value monitored by the monitor unit 23 is maintained in a predetermined range (for example, a range between the lower limit value Vi and the reference value Vb) equal to or higher than the lower limit value Vi. You just have to decide.

  If the output current of the power storage device 2 is changed as described above after the voltage value monitored by the monitor unit 23 reaches the lower limit value Vi, the value of the terminal voltage of the battery module 20 is shown by a solid line (characteristic vx) in FIG. It changes as shown in. That is, the value of the terminal voltage of the battery module 20 decreases to the lower limit value Vi, then increases to the reference value Vb, and again decreases to the lower limit value Vi. When this operation is repeated, the value of the output current of the power storage device 2 gradually decreases. Therefore, the processing unit 13 has a minimum value Imin for the output current. The minimum value Imin is set to 0.5 A, for example. After the output current is reduced to the minimum value Imin, the processing unit 13 is configured such that the power storage device 2 and the power conversion device at the time when the voltage value monitored by the monitor unit 23 is reduced to the lower limit value Vi (for example, time t3 in FIG. 2). The operation with 3 is stopped. That is, the minimum value Imin relating to the output current is the above-described repeated limiting condition.

  In the above operation example, after the voltage value monitored by the monitor unit 23 reaches the lower limit value Vi, the processing unit 13 decreases the output current of the power storage device 2, and when the value of the output current decreases to the minimum value Imin, Next, when the voltage value reaches the lower limit Vi, discharging of the power storage device 2 is stopped. As a condition for stopping the discharge of the power storage device 2, in addition to the condition that the voltage value monitored by the monitor unit 23 reaches the lower limit value Vi, the elapsed time after the output current value reaches the minimum value Imin can be used. It is. For example, the voltage value has not recovered to the reference value Vb when a predetermined holding time T1 (for example, 30 seconds) has elapsed since the output current value has reached the minimum value Imin (for example, time t4 in FIG. 2). In such a case, a configuration in which discharging of the power storage device 2 is stopped may be employed. Note that the voltage value included in the condition for stopping the discharge of the power storage device 2 is not limited to the reference value Vb as long as the voltage value is larger than the lower limit value Vi.

  In the configuration example described above, the condition for stopping the discharge of the power storage device 2 is that the terminal voltage or the output current of the battery module 20 reaches the minimum value Imin after the value of the output current of the power storage device 2 reaches the minimum value Imin. The time elapsed since then. However, other conditions may be sufficient as the conditions which stop discharge of the electrical storage apparatus 2. FIG. For example, in a state in which the value of the output current of the power storage device 2 is kept constant, the power is stored in the elapsed time from the time (time t1) when the terminal voltage of the battery module 20 monitored by the monitor unit 23 decreases to the lower limit value Vi. It is possible to determine the timing for stopping the discharge of the device 2. Further, discharging of power storage device 2 may be stopped when the value of the output current of power storage device 2 reaches minimum value Imin.

  In the operation example described above, when the terminal voltage of the battery module 20 monitored by the monitor unit 23 is lowered to the lower limit value Vi in a state where the value of the current output from the power storage device 2 is kept constant, the current is set to a predetermined slope. The terminal voltage of the battery module 20 is increased. Two kinds of slopes for reducing the current are alternately selected, and the larger slope is adopted during the period when the terminal voltage of the battery module 20 rises to the reference value Vb, and the terminal voltage reaches the reference value Vb. The smaller slope is then adopted.

  However, instead of alternately selecting two types of inclinations, it is possible to employ three or more types of inclinations. For example, the rule that a relatively large slope is selected during a period when the terminal voltage of the battery module 20 rises and a relatively small slope is selected during a period when the terminal voltage falls follows the rise and fall of the terminal voltage. You may employ | adopt the structure which changes an inclination whenever performing.

  Note that when the control device 10 stops the operation of the power conversion device 3 as in the above-described operation example, the remaining capacity of the battery module 20 in the power storage device 2 is reduced, so the power from the power storage device 2 is reduced. It cannot be used continuously. Therefore, the control device 10 is configured to be able to operate the power conversion device 3 after confirming that the remaining capacity of the battery module 20 has been recovered. For example, the control device 10 is configured to allow the power storage device 2 to discharge if the terminal voltage of the battery module 20 monitored by the monitor unit 23 is equal to or higher than a predetermined determination value.

  As described above, by controlling the operation of the power conversion device 3 so as to reduce the output current of the power storage device 2 after the value of the terminal voltage of the battery module 20 reaches the lower limit value Vi, The stored charge can be used effectively. Assume that the amount of power received from the power system 5 is reduced by using the power stored in the power storage device 2 when power saving is requested. In this case, if the discharging of the power storage device 2 is stopped when the terminal voltage of the battery module 20 is reduced to the lower limit value Vi, the operation time of the load 51 may be shortened. The period during which 2 can be discharged can be extended. For example, when power saving is requested, the period during which power can be supplied from the power storage device 2 to the load 51 is extended. Therefore, for the user of the load 51, the amount of power received from the power system 5 can be reduced while saving power. The disadvantage can be reduced.

  In the configuration example described above, the terminal voltage of the battery module 20 monitored by the monitor unit 23 is used, but it is possible to use SOC (State Of Charge). In addition, since the discharge characteristics of the battery module 20 change depending on the temperature of the battery module 20, when the control device 10 instructs the operation of the power conversion device 3, the content of the instruction is taken into account the temperature monitored by the monitor unit 23. It may be changed.

  In the configuration example described above, the control device 10 gives an instruction to the power conversion device 3 based on the terminal voltage of the battery module 20 monitored by the monitor unit 23. On the other hand, the power converter device 3 may be configured to monitor the output voltage of the power storage device 2 and to give an instruction to the power converter device 3 based on the output voltage. For example, when the electric wire connecting the power storage device 2 and the power conversion device 3 is relatively long, the voltage drop due to the wire becomes relatively large. Therefore, this voltage drop is combined with the voltage drop due to the internal resistance of the battery module 20. It is necessary to consider. If the output voltage of the power storage device 2 is monitored in the power conversion device 3, a voltage value obtained by subtracting the voltage drop due to the electric wire can be obtained. Therefore, rather than using the voltage value monitored by the monitor unit 23 provided in the battery module 20, it is possible to accurately instruct the power conversion device 3.

  The control apparatus 10 described above can be realized using an electronic circuit in which individual electronic components are connected, but can also be configured using a device including a processor that operates according to a program. This type of device may be a micro processing unit (MPU) that requires a semiconductor memory as a separate component, or a microcomputer (micro controller) that is a device that integrally includes a processor and a semiconductor memory. The program may be written in advance in a ROM (Read Only Memory) built in the control device 10, or may be provided through an electric communication line such as the Internet or a mobile communication network. The program may be provided using a computer-readable recording medium (optical disk, semiconductor memory) or the like.

  The storage battery control device 10 of the present embodiment described above includes the input unit 11, the output unit 12, and the processing unit 13. The input unit 11 receives the value of the voltage output from the power storage device 2, and the output unit 12 gives an operation instruction to the power conversion device 3 that converts DC power output from the power storage device 2 into AC power. Processing unit 13 determines an instruction to be given to power conversion device 3 so as to adjust the value of the current output from power storage device 2. The power converter 3 is connected so as to be linked to the power system 5. When the value of the voltage received by the input unit 11 decreases to a predetermined lower limit value Vi while the current value output from the power storage device 2 is maintained at a constant current, the processing unit 13 thereafter An instruction is given to the power conversion device 3 through the output unit 12 so as to reduce the output current. However, the current output from the power storage device 2 is reduced within a range in which the power conversion device 3 can continue outputting AC power.

  Further, the power storage system described above includes a power storage device 2, a power conversion device 3, and a storage battery control device 10. The power conversion device 3 converts the DC power of the power storage device 2 into AC power and is connected to the power system 5. The storage battery control device 10 receives the value of the voltage output from the power storage device 2 and gives an operation instruction to the power conversion device 3 so as to adjust the value of the current output from the power storage device 2. When the value of the voltage received during the period in which the value of the current output from the power storage device 2 is maintained at a constant current decreases to the predetermined lower limit value Vi, the storage battery control device 10 thereafter processes the current output from the power storage device 2 An instruction is given to the power conversion device 3 so as to reduce the power consumption. However, the current output from the power storage device 2 is reduced within a range in which the power conversion device 3 can continue outputting AC power.

  According to the configuration of the storage battery control device 10 and the power storage system described above, the power storage device 2 that could not be used due to a voltage drop due to the internal impedance of the power storage device 2, the impedance of the electric wire connecting the power storage device 2 and the power conversion device 3, or the like. Power is available. In general, when a constant current is output from the power storage device 2, if the voltage output from the power storage device 2 decreases to the lower limit value Vi, then the power of the power storage device 2 cannot be extracted unless the power storage device 2 is charged. On the other hand, in the configuration of the present embodiment, since the current extracted from the power storage device 2 is reduced after the voltage output from the power storage device 2 has decreased to the lower limit value Vi, power is continuously extracted from the power storage device 2. It becomes possible. As a result, the amount of power that can be supplied from power storage device 2 is increased as compared to the case where a constant current is output from power storage device 2.

  Moreover, in the storage battery control apparatus 10, the process part 13 reduces the value of an electric current with progress of time in the period which reduces the electric current output from the electrical storage apparatus 2 in the range in which the power converter device 3 can continue the output of alternating current power. It is desirable to give an instruction to the power converter 3 so that

  According to this configuration, as the remaining capacity of the power storage device 2 decreases and the voltage output from the power storage device 2 decreases, the current taken out from the power storage device 2 is reduced, so that the amount of power that can be supplied from the power storage device 2 is increased. It becomes possible to plan.

  In the storage battery control device 10, the processing unit 13 receives the value of this current from the input unit 11 during a period in which the current output from the power storage device 2 is reduced within a range in which the power conversion device 3 can continue outputting AC power. It is desirable to determine the voltage value so as to be maintained within a predetermined range equal to or higher than the lower limit value Vi.

  According to this configuration, the current extracted from the power storage device 2 is adjusted so that the voltage value received by the input unit 11 can be maintained near the lower limit Vi, and the period during which power can be extracted from the power storage device 2 is extended relatively long. Is possible.

  In the storage battery control device 10, the processing unit 13 is configured to stop discharging the power storage device 2 when a predetermined holding time has elapsed since the value of the current output from the power storage device 2 has reached a predetermined minimum value Imin. May be.

  When this configuration is adopted, when the value of the current output from the power storage device 2 is reduced to the minimum value Imin, the power storage device 2 is discharged after the holding time has elapsed, so that overdischarge of the power storage device 2 is prevented. be able to.

  Further, in the storage battery control device 10, the processing unit 13 determines that the voltage value received by the input unit 11 decreases to the lower limit value Vi after the value of the current output from the power storage device 2 reaches a predetermined minimum value Imin. There is a case where the storage device 2 is configured to stop discharging.

  When this configuration is adopted, when the voltage output from the power storage device 2 decreases to the lower limit value Vi, the discharge of the power storage device 2 is stopped, so the remaining capacity of the power storage device 2 when the discharge of the power storage device 2 is stopped is reduced. It is possible to manage with relatively high accuracy. For example, when the remaining capacity affects the degree of deterioration of the power storage device 2, it is easy to avoid a situation where the power storage device 2 deteriorates more than expected.

  The above-described embodiment is an example of the present invention. For this reason, the present invention is not limited to the above-described embodiment, and various modifications can be made according to design and the like as long as the technical idea according to the present invention is not deviated from this embodiment. Of course, it can be changed.

2 power storage device 3 power conversion device 5 power system 10 storage battery control device 11 input unit 12 output unit 13 processing unit Imin minimum value Vi lower limit value

Claims (6)

An input unit for receiving a voltage value output from the power storage device;
An output unit that gives an operation instruction to a power converter that converts the DC power output by the power storage device into AC power;
A processing unit for determining the instruction to be given to the power converter so as to adjust a value of a current output from the power storage device;
The power conversion device is connected so as to be linked to a power system,
The processor is
When the value of the voltage received by the input unit decreases to a predetermined lower limit during a period in which the value of the current output by the power storage device is maintained at a constant current, the power conversion device thereafter supplies the AC power. Storage battery control characterized in that the instruction is given to the power conversion device through the output unit so as to reduce the current while changing the rate of change in reduction of the current output from the power storage device within a range in which output can be continued. apparatus. The processor is
Instructing the power converter to reduce the value of the current with time in a period in which the current output from the power storage device is reduced within a range in which the power converter can continue to output the AC power. The storage battery control device according to claim 1. The processor is
The current value is maintained within a predetermined range in which the voltage value is equal to or greater than the lower limit value in a period in which the current output from the power storage device is reduced within a range in which the power conversion device can continue to output the AC power. The storage battery control device according to claim 1 or 2, wherein the storage battery control device is defined as follows. The processor is
The structure of stopping discharge of the said electrical storage apparatus, if predetermined | prescribed holding | maintenance time passes from the time of the value of the electric current which the said electrical storage apparatus outputs reaches | attains the predetermined minimum value. The storage battery control device according to item. The processor is
After the value of the current output from the power storage device reaches a predetermined minimum value, the discharge of the power storage device is stopped when the value of the voltage received by the input unit decreases to the lower limit value. The storage battery control device according to any one of claims 1 to 3. A power storage device;
A power converter that converts direct current power of the power storage device into alternating current power and is linked to a power system;
A storage battery control device that receives a value of a voltage output from the power storage device and gives an operation instruction to the power converter so as to adjust a value of a current output from the power storage device;
The storage battery control device
When the value of the voltage received during the period in which the value of the current output from the power storage device is maintained at a constant current decreases to a predetermined lower limit value, the power conversion device can continue to output the AC power thereafter. A power storage system, wherein the instruction is given to the power conversion device so as to decrease the current while changing a rate of change in reduction of a current output from the power storage device within a range.

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