JP2013030394A - Storage battery charging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a storage battery charging apparatus that charges a storage battery comprising a plurality of battery units connected in series and suppresses performance variations among the battery units.SOLUTION: A storage battery charging apparatus includes: a storage battery comprising a plurality of battery units (for example, battery packs 6) connected in series; a temperature difference measuring/recording section for classifying the battery units into at least two groups (for example, a rack-peripheral group G1 and a rack-central group G2), and measuring and recording a temperature difference between the groups of battery units according to a past charging history; a warming section capable of warming the battery units by the group; and a warming control section for deciding a degree of warming by the warming section prior to a start of charging by the group and controlling the warming section on the basis of the temperature difference between the groups of battery units recorded in the temperature difference measuring/recording section such that the temperature difference between the groups of battery units decreases after charging.

Description

  The present invention relates to a storage battery charging device.

  In recent years, the capacity of storage batteries has been increasing, and the introduction of power supply systems that store power consumed in buildings, factories, stores, homes, and the like has been promoted. Such a power supply system is a system capable of discharging a storage battery (supplying power) at an arbitrary timing by charging the storage battery in advance (consuming power), and charging and discharging the storage battery. By controlling this timing, it is possible to control the timing of consuming grid power (power supplied from an electric power company).

  In general, the power charge of the grid power includes a fixed basic charge and a usage-based charge. The electric power company sets the basic charge so that the basic charge becomes cheaper as the maximum value of the amount of grid power consumed per unit time becomes smaller. In addition, the usage fee is set so that the price per unit power of the usage fee is lower at night when the power consumption is lower than during the day when the power consumption is high. Therefore, the user who uses the system power can reduce the power charge of the system power as the consumption of the system power is leveled.

  Therefore, in a power supply system, a user who uses the grid power to charge the storage battery using grid power during a time zone when the power demand of the grid power user is small or a nighttime electricity rate is applied When the demand for power exceeds the predetermined threshold, the amount of power exceeding the predetermined threshold is supplemented by the discharge of the storage battery, whereby the power charge of the grid power can be suppressed.

  In a power supply system, a storage battery that can be repeatedly charged and discharged, such as a lithium ion battery, is usually used in the form of a battery pack having a configuration in which a plurality of battery blocks in which a plurality of battery cells are connected in parallel are connected in series. Further, in a general large-scale power supply system, an assembly in which a plurality of battery pack strings in which a plurality of battery packs are connected in series is connected in parallel is stored in a rack.

JP 2007-330008 A JP 2003-274565 A

  When charging / discharging a storage battery with an electric power supply system, the problem that charging / discharging efficiency worsens in the environment where external temperature is low generate | occur | produces. In particular, in a large power supply system, variations in the amount of power that can be used and the degree of deterioration occur due to a temperature difference between battery packs. At this time, due to the influence of a battery pack that has a small amount of power that can be used, the total amount of power that can be used in the entire system is greatly reduced compared to the total amount of power that can be used by each battery pack, and batteries that have a large degree of deterioration Due to the influence of the pack, there arises a problem that the life of the entire system is remarkably shortened compared to the average life of each battery pack.

  In Patent Document 1, a charging circuit provided with a heater and a fan for heating the secondary battery is proposed. However, the charging circuit heats the entire secondary battery using the heater and the fan. In addition, it is impossible to suppress the occurrence of variations in the performance of the plurality of battery units due to the temperature difference between the plurality of battery units (for example, battery packs) constituting the secondary battery.

  Patent Document 2 proposes a power storage device that heats a secondary battery by internal heat generated by repeated charging and discharging of the secondary battery. However, the power storage device is configured to recharge the entire secondary battery by repeating charging and discharging. Since the heating is performed, it is impossible to suppress the occurrence of variations in the performance of the plurality of battery units due to the temperature difference between the plurality of battery units (for example, battery packs) constituting the secondary battery. Further, in the power storage device, since the secondary battery is repeatedly charged and discharged for heating, there is a problem that the life of the secondary battery is shortened.

  An object of the present invention is to provide a storage battery charging device that can charge a storage battery in which a plurality of battery units are connected in series in view of the above situation, and can suppress performance variations among the plurality of battery units. And

  In order to achieve the above object, a storage battery charging device according to the present invention classifies a storage battery in which a plurality of battery units are connected in series, and a plurality of the battery units into at least two groups. A temperature difference measurement recording unit that measures and records a temperature difference of the battery unit between groups, a heating unit that can heat the battery unit for each group, and the battery between the groups after charging Heating before the start of charging by the heating unit for each group based on the temperature difference of the battery units between the groups recorded in the temperature difference measurement recording unit so as to reduce the temperature difference of the unit. It is set as the structure (1st structure) provided with the heating control part which determines quantity and controls the said heating part.

  In the storage battery charging device according to the first configuration, the heating unit can heat the battery unit for each battery unit and includes a resistor connected in parallel to the battery unit (second Configuration).

  The storage battery charging device of the second configuration may be configured (third configuration) including a bypass circuit that is installed for each battery unit and can bypass the battery unit.

  In the storage battery charging device according to any one of the first to third configurations, the heating control unit is based on the temperature difference of the battery unit between the groups recorded in the temperature difference measurement recording unit. It is good also as a structure (4th structure) which determines the start time of the heating by the said heating part for every group.

  The storage battery charging device having any one of the first to fourth configurations includes an outside air temperature detecting unit that measures or estimates the outside air temperature of the charging device, and the charging device measured or estimated by the outside air temperature detecting unit. When the outside air temperature is equal to or higher than a predetermined value, a configuration (fifth configuration) may be employed in which control of the heating unit by the heating control unit is stopped.

  In the storage battery charging device according to any one of the first to fifth configurations, the resistance of the heating unit also serves as a resistance of a balance discharge unit that performs balance discharge to reduce a charge amount difference between the battery units. Good.

  A storage battery charging device according to the present invention includes a storage battery in which a plurality of battery units are connected in series, and a plurality of the battery units are classified into at least two groups. A temperature difference measurement recording unit that measures and records a temperature difference, a heating unit that can heat the battery unit for each group, and a temperature difference of the battery unit between the groups after charging is reduced. Thus, based on the temperature difference of the battery unit between the groups recorded in the temperature difference measurement recording unit, a heating amount before the start of charging by the heating unit is determined for each group, and the heating is performed. It is the structure provided with the heating control part which controls a warm part. According to such a configuration, since the temperature difference of the battery units between the groups after charging is reduced, it is possible to suppress variations in performance (for example, the amount of power that can be used and the degree of deterioration) of the plurality of battery units. it can.

It is a figure which shows schematic structure of an electric power supply system provided with the charging device of the storage battery which concerns on this invention. It is a figure which shows the example of 1 structure of a storage battery. It is a figure which shows the other structural example of a storage battery. It is a figure which shows one example of arrangement | positioning of the principal part of an electric power supply system provided with the charging device of the storage battery which concerns on this invention. It is a figure which shows the other example of arrangement | positioning of the principal part of an electric power supply system provided with the charging device of the storage battery which concerns on this invention. It is a figure which shows the principal part structural example of BMU. It is a time chart which shows the temperature and charge amount of a battery unit in the charging device of the storage battery which concerns on this invention. It is a figure which shows transition of the operation state in the charging device of the storage battery which concerns on this invention. It is a time chart which shows the temperature and charge amount of a battery unit in the conventional charging device of a storage battery.

  Embodiments of the present invention will be described below with reference to the drawings. In the following description, the storage battery charging device according to the present invention is provided in the power supply system, but the present invention is not limited to this. The storage battery charging device according to the present invention may be used alone, for example, as a so-called charger, or may be provided in a moving body such as an electric vehicle using the storage battery as a power source. The storage battery charging device according to the present invention is preferably a charging device that periodically charges the storage battery.

  FIG. 1 is a block diagram showing a schematic configuration of a power supply system including a storage battery charging device according to the present invention. In FIG. 1, thick lines connecting the blocks indicate power lines, and thin lines connecting the blocks indicate communication lines. In the present embodiment, each communication line is realized by wired communication from the viewpoint of emphasizing reliability, but can also be realized by wireless communication. The communication may be performed by TCP (Transmission Control Protocol), for example.

  The power supply system shown in FIG. 1 includes a PCS (Power Conditioning System) management control unit 1, a PCS 2, a storage battery 3, a BMU (Battery Management Unit) 4, and a master BMU 5. In the power supply system shown in FIG. 1, the PCSs 2 that charge and discharge the storage batteries 3 are connected via the PCS management control unit 1 so that the plurality of storage batteries 3 are connected in parallel. In this embodiment, the “temperature difference measurement recording unit” and the “heating control unit” described in the claims are provided in the BMU 4, and the “heating unit” and “resistance” described in the claims are provided. And the “bypass circuit” is built in the storage battery 3, and the “outside air temperature detection unit” described in the claims is realized by using a temperature sensor provided in the storage battery 3.

  The PCS management control unit 1 is connected to an external load 100 and a power system 200. The load 100 is a load having an AC power input terminal, and the power system 200 is a power system that supplies AC power. The PCS management control unit 1 controls the operation of each PCS 2 based on the charge / discharge command from the master BMU 5 and monitors the state of each PCS 2.

  Each PCS 2 is a bi-directional AC / DC power converter, which converts AC power supplied from the power system 200 via the PCS management controller 1 during charging into DC power and is connected in series during discharging. DC power supplied from the storage battery 3 is converted into AC power. Unlike this embodiment, when the PCS management control unit 1 is connected to an external DC load (a load having a DC power input terminal) and a DC power source (for example, a solar cell), each PCS 2 is connected to a bidirectional DC. / DC power converter may be changed.

  Each BMU 4 monitors the state of the corresponding storage battery 3 and transmits log information regarding the state of the corresponding storage battery 3 and the state of itself (BMU 4) to the master BMU 5.

  The master BMU 5 monitors and controls each storage battery 3 and each BMU 4 in an integrated manner. The master BMU 5 transmits a charge / discharge command to the PCS management control unit 1 based on a charge / discharge request sent from the outside. In this embodiment, the master BMU 5 receives a charge / discharge request sent from the outside. For example, the PCS management control unit 1 receives a charge / discharge request sent from the outside, and the master BMU 5 However, you may make it send the information regarding the state of each storage battery 3 to the PCS management control part 1. FIG.

  Next, one structural example of the storage battery 3 is shown in FIG. In the configuration example shown in FIG. 2, the storage battery 3 has a configuration in which a plurality of battery packs 6 are connected in series, and the battery pack 6 corresponds to a “battery unit” recited in the claims.

  Each battery pack 6 includes a plurality of battery blocks 7 in which a plurality of battery cells are connected in parallel. In addition, the battery block 7 is not limited to the structural example shown in FIG. 2, A single battery cell may be sufficient. The plurality of battery blocks 7 are connected in series within the battery pack 6.

  A temperature sensor 8 that detects the temperature of the battery block 7 is provided for each battery block 7. The detection result of each temperature sensor 8 is output to the battery pack controller 9. The battery pack 9 may include a state detection unit that detects the state of each battery block other than the temperature or the state of the battery pack. The state detection unit detects, for example, the voltage value of each battery block 7, and also detects the current value and voltage value between the + and-electrodes of the battery pack, the remaining capacity of the battery pack, and uses the detected data as the battery pack. Output to the control unit 9. The remaining capacity of the battery pack is obtained from the integrated value of the charge / discharge current flowing in the battery pack, and is a calculation formula or table showing the relationship between the predetermined open circuit voltage (OCV) of the battery pack and the remaining capacity. Can be obtained by referring to. The battery pack control unit 9 transmits the detection result of each temperature sensor 8 and the detection data acquired from the state detection unit as battery data to the BMU 4 (see FIG. 1) via the communication unit 10. For example, the BMU 4 may recognize the average temperature of the battery block 7 provided in one battery pack 6 as the temperature of the battery pack 6, and the maximum temperature of the battery block 7 provided in one battery pack 6. May be recognized as the temperature of the battery pack 6, and the lowest temperature of the battery block 7 provided in one battery pack 6 may be recognized as the temperature of the battery pack 6.

  A parallel circuit of a battery pack heating unit 11 having a resistance and a battery pack bypass circuit 12 having a switch is connected in parallel to the series circuit of the plurality of battery blocks 7. Further, the battery pack 6 includes switches SW1 and SW2 that are complementarily turned on / off based on an instruction from the communication unit 10. When the switch SW1 is ON, a series circuit of the plurality of battery blocks 7 is selected, and the battery block 7 can be charged / discharged. On the other hand, when the switch SW2 is ON, a parallel circuit of the battery pack warming unit 11 and the battery pack bypass circuit 12 is selected. If the switch of the battery pack bypass circuit 12 is ON, a plurality of battery blocks 7 are connected in series. If the circuit is bypassed by the battery pack bypass circuit 12 and the switch of the battery pack bypass circuit 12 is OFF, the resistance of the battery pack heating unit 11 generates heat and the battery pack 6 is heated. The resistance of the battery pack heating unit 11 may be a resistor provided exclusively for heating, and also serves as a resistance of a balance discharge unit that performs balance discharge for reducing the charge amount difference between the battery packs 6. It may be a combination of both.

  Another configuration example of the storage battery 3 is shown in FIG. In the configuration example shown in FIG. 3, the storage battery 3 has a configuration in which a plurality of battery packs 13 are connected in series, and a later-described battery block 14 corresponds to a “battery unit” recited in the claims. The storage battery 3 is not limited to the configuration example shown in FIG. 3 and may be a single battery pack 13.

  Each battery pack 13 includes a plurality of battery blocks 14 in which a plurality of battery cells are connected in parallel. In addition, the battery block 14 is not limited to the structural example shown in FIG. 3, A single battery cell may be sufficient. The plurality of battery blocks 14 are connected in series when each switch SW3 described later is ON.

  A temperature sensor 15 that detects the temperature of the battery block 14 is provided for each battery block 14. The detection result of each temperature sensor 15 is output to the battery pack control unit 16. The battery pack 13 may include a state detection unit that detects the state of each battery block other than the temperature or the state of the battery pack. For example, the state detection unit detects the voltage value of each battery block 14, detects the current value and voltage value between the + and-electrodes of the battery pack, the remaining capacity of the battery pack, and uses the detected data as the battery pack. Output to the control unit 16. The battery pack control unit 16 transmits the detection result of each temperature sensor 15 and the detection data acquired from the state detection unit as battery data to the BMU 4 (see FIG. 1) via the communication unit 17.

  A parallel circuit of a battery block heating unit 18 having resistance and a battery block bypass circuit 19 having a switch is connected in parallel to each battery block 14. The battery pack 13 includes switches SW3 and SW4 that are complementarily turned on / off based on instructions from the communication unit 17 for each battery block 14. When the switch SW3 is ON, the corresponding battery block 14 is selected, and the corresponding battery block 14 can be charged / discharged. On the other hand, when the switch SW4 is ON, a parallel circuit of the corresponding battery block warming unit 18 and the corresponding battery block bypass circuit 19 is selected, and if the switch of the corresponding battery block bypass circuit 19 is ON, the corresponding When the corresponding battery block 14 is bypassed by the corresponding battery block bypass circuit 19 and the switch of the corresponding battery block bypass circuit 19 is OFF, the resistance of the corresponding battery block heating unit 18 generates heat, and the corresponding battery block 14 Is warmed. The resistance of the battery block heating unit 18 may be a resistor provided exclusively for heating, and also serves as a resistance of a balance discharge unit that performs balance discharge that reduces the charge amount difference between the battery blocks 14. It may be a combination of both.

  Here, the example of arrangement | positioning of the principal part of the electric power supply system shown in FIG. 1 provided with the storage battery 3 of the structure shown in FIG. 2 is shown in FIG. 4, and the electric power supply system shown in FIG. 1 provided with the storage battery 3 of the structure shown in FIG. An example of the arrangement of the main parts is shown in FIG. In the arrangement examples of FIGS. 4 and 5, the storage battery 3, BMU 4, PCS 2, and the like are stored in the rack 20. Further, FIG. 6 shows a configuration example of a main part of the BMU 4.

  As shown in FIG. 6, the BMU 4 includes a temperature difference measurement recording unit 21 and a heating control unit 22.

  In the arrangement example shown in FIG. 4, the temperature difference measurement recording unit 21 classifies the battery pack 6 into a group G1 on the outside of the rack and a group G2 on the center of the rack, and the batteries sent from the communication unit 10 of the battery pack 6 are sent. Using the temperature data of the block 7, the temperature difference of the battery pack 6 between the group G1 outside the rack and the group G2 at the center of the rack is measured and recorded. Based on the temperature difference of the battery pack 6 between the group G1 on the outside of the rack and the group G2 on the center of the rack recorded in the temperature difference measurement recording unit 21, the heating control unit 22 is set for each group before starting charging. The heating start time is determined, and the switch SW1, the switch SW2, and the switch of the battery pack bypass circuit 12 are controlled via the communication unit 10 of the battery pack 6 according to the determination. Note that if the heating start time before the start of charging coincides with the charging start time, the heating amount becomes zero.

  In the arrangement example shown in FIG. 5, since the battery packs 13 are all arranged outside the rack, grouping is not performed on the battery pack 13 but on the battery block 14 (see FIG. 3). Specifically, the temperature difference measurement recording unit 21 classifies the battery block 14 into a group outside the battery pack and a group at the center of the battery pack, and the battery block 14 sent from the communication unit 17 of the battery pack 13 Using the temperature data, the temperature difference of the battery block 14 between the group outside the battery pack and the group at the center of the battery pack is measured and recorded. Based on the temperature difference of the battery block 14 between the group outside the battery pack and the group at the center of the battery pack recorded in the temperature difference measurement recording unit 21, the heating control unit 22 pre-charges for each group. The heating start time is determined, and the switches SW3, SW4, and the battery block bypass circuit 19 are controlled via the communication unit 17 of the battery pack 13 according to the determination. Note that if the heating start time before the start of charging coincides with the charging start time, the heating amount becomes zero.

  Next, the temperature transition of the battery unit in the storage battery charging device according to the present invention will be described. FIG. 7 is a time chart showing the temperature and charge amount of each battery in the storage battery charging device according to the present invention. In the time chart regarding the temperature of the battery unit, the solid line indicates the temperature of the battery unit (for example, the battery pack 6 belonging to the group G2 at the center of the rack shown in FIG. 4) of the group whose temperature is likely to increase during charging, and the dotted line is the charge The temperature of the battery unit (for example, the battery pack 6 belonging to the group G1 outside the rack shown in FIG. 4) of the group in which the temperature is difficult to rise is shown. Here, a case where charging is performed periodically once a day will be described, but the charging cycle is not limited to one day. Further, the charging cycle may vary.

Temperature difference measuring recording unit 21, PCS2 battery unit of the group is difficult temperature rises during charging the battery unit of the group easily rise temperature during charging at time t _CH to switch to constant voltage charging mode of charging from the constant current charging Measure and record the temperature difference D (day).

Heating control unit 22, a constant temperature difference D ~ in the charging start time t _CS Considering warming (day), the temperature difference the temperature difference measuring recording unit 21 was measured and recorded the day before D (DAY- 1) and the steady temperature difference D ~ (day-1) calculated by the heating control unit 22 on the previous day, and is calculated by the following equation (1). In the following formula (1), α is a fixed constant obtained experimentally.
D ~ (day) = α × D (day-1) + D ~ (day-1) (1)

Then, the heating control unit 22 matches the heating start time of the battery unit of the group whose temperature is likely to rise during charging with the charging start time _tCS, and sets the heating amount of the battery unit of the group whose temperature is likely to rise during charging. and zero, the heating start time t _W of the battery unit of the group difficult to increase the temperature calculated by the following equation (2) during charging. Incidentally, the following (2) t _CS in the formula is the charging start time set in the habitual charging start time or scheduled user specifies, t _{1 deg} is a battery unit of the group is difficult temperature rises during charging This is the time taken to raise the temperature by 1 ° C. by heating. The t _1DEG may be calculated experimentally, for example, and the heating control unit 22 may hold it in the form of a lookup table.
t _W = t _{CS −D˜} (day) × t 1 _DEG (2)

The warming control unit 22 controls the switches in the battery pack based on the above calculation result. As shown in FIG. 8, when the warming start time t _W is reached from the state where charging is suspended, charging is in progress. in to start the warming of the battery unit of the group difficult to increase the temperature is, or becomes the charging start time t _CS, or, the temperature of each battery unit of even during the heating not have to become the charging start time t _CS each battery If either exceeds a target temperature of the units (the temperature in the heating start time t _W (a value obtained by adding a D ~ (day) × t _1DEG in 2)), to start the charging to stop heating The battery is charging. When the charging is completed, the charging is stopped again.

  Compared to the case where the pre-charging is not performed as in the temperature transition of the battery unit in the conventional storage battery charging device shown in FIG. 9, in the storage battery charging device according to the present invention, the above-described heating control unit By the operation of 22, the temperature difference between the battery unit of the group whose temperature is likely to rise during charging and the battery unit of the group whose temperature is difficult to rise during charging is reduced after charging. Therefore, it is possible to suppress variation in performance (for example, the amount of power that can be used and the degree of deterioration) of a plurality of battery units.

In the embodiment described above, the temperature difference measuring recording unit 21, during charging the battery unit of the group easily rise temperature during charging at time t _CH of PCS2 switches to the constant voltage charging mode of charging from the constant current charging Although the temperature difference D (day) with the battery unit of the group that is difficult to rise in temperature is measured and recorded, the temperature difference D (day) is the battery in the group that easily rises during charging at the charge end time _tCE . You may change into the temperature difference between the unit and the battery unit of the group in which the temperature does not easily rise during charging.

  Moreover, you may make it the heating control part 22 equip with the external temperature detection part which measures or estimates the external temperature of the electric power supply system shown in FIG. For example, the outside air temperature detecting unit uses the temperature data of the battery block 7 sent from the communication unit 10 of the battery pack 6 classified into the group G1 outside the rack in the arrangement example shown in FIG. May be configured to estimate.

  Then, from the viewpoint of preventing the temperature of the battery unit from becoming too high, when the outside air temperature measured or estimated by the outside air temperature detection unit is equal to or higher than the first predetermined value, the heating control unit 22 controls the heating. It is desirable to stop.

  Further, in the present embodiment, the heating amount of the battery unit of the group whose temperature is likely to rise during charging is set to zero, but the outside air temperature measured or estimated by the outside air temperature detection unit is smaller than the first predetermined value. 2. If the value is less than the predetermined value of 2, from the viewpoint of suppressing efficiency decrease due to low temperature, the battery unit of the group whose temperature is likely to rise during charging is also heated, and the heating amount of the battery unit of the group whose temperature is likely to rise during charging May be smaller than the heating amount of a battery unit of a group in which the temperature does not easily rise during charging. In addition, the difference of the heating amount between groups can be easily realized by changing the heating start time for each group, for example.

  In the present embodiment, the battery units are classified into two groups, but may be classified into three or more. When classified into three or more, heating control becomes complicated, but finer heating control is possible, and temperature variation within the same group can be suppressed.

1 PCS management control unit 2 PCS
3 Storage battery 4 BMU
5 Master BMU
6, 13 Battery pack 7, 14 Battery block 8, 15 Temperature sensor 9, 16 Battery pack control unit 10, 17 Communication unit 11 Battery pack heating unit 12 Battery pack bypass circuit 18 Battery block heating unit 19 Battery block bypass circuit 20 Rack 21 Temperature difference measurement recording unit 22 Heating control unit SW1 to SW4 Switch 100 Load 200 Power system

Claims (6)

A storage battery in which a plurality of battery units are connected in series;
Classifying the plurality of battery units into at least two groups, and measuring and recording a temperature difference of the battery units between the groups from a past charge history; and a temperature difference measurement recording unit;
A heating unit capable of heating the battery unit for each group;
Based on the temperature difference of the battery units between the groups recorded in the temperature difference measurement recording unit, the temperature difference of the battery units between the groups after charging is reduced. A charging device for a storage battery, comprising: a heating control unit that determines a heating amount before starting charging by a heating unit and controls the heating unit.   2. The storage battery charging device according to claim 1, wherein the heating unit is capable of heating the battery unit for each battery unit, and has a resistor connected in parallel to the battery unit.   The storage battery charging device according to claim 2, further comprising a bypass circuit that is installed for each battery unit and can bypass the battery unit.   The heating control unit determines a heating start time by the heating unit for each group based on a temperature difference of the battery unit between the groups recorded in the temperature difference measurement recording unit. The charging device for a storage battery according to any one of claims 1 to 3. An outside air temperature detection unit that measures or estimates the outside air temperature of the charging device,
The control of the heating unit by the heating control unit is stopped when the outside temperature of the charging device measured or estimated by the outside air temperature detection unit is a predetermined value or more. The storage battery charging device according to any one of the above.   The storage battery charging device according to claim 2, wherein the resistance of the heating unit also serves as a resistance of a balance discharge unit that performs balance discharge that reduces a charge amount difference between the battery units.

Images