JP2009159768A - Voltage equalizer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a voltage equalizer of a battery pack that equalizes the voltage of each unit secondary battery. <P>SOLUTION: The voltage equalizer that equalizes each voltage of the battery pack includes: individual discharge resistors having the same resistance values, which are provided separately corresponding to each unit secondary battery and are wired so that one end each may connect with the positive electrode of a corresponding unit secondary battery and the other end each may connect with the negative electrodes of the corresponding unit secondary battery; each discharge switch, which is inserted between the positive electrode of each unit secondary battery and each discharge resister; a current sensor, which is provided in a current supply path L that supplies the battery pack with a charge current so as to detect the magnitude of the charge current; and a control circuit, which perform an operation of equalizing the voltages of individual unit secondary batteries constituting the above battery pack all at once by switching on the individual discharge units all at once. The control circuit concerned performs the above equalizing operation by switching on the above individual discharge switches all at once while the value of the above charge current I is between two thresholds that are set in a constant voltage region in which charge is performed with the constant voltage. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a voltage equalizing apparatus.

Conventionally, an assembled battery in which a plurality of unit secondary batteries are connected in series to obtain a predetermined high voltage is widely known. For example, as disclosed in Patent Document 1 below, this type of assembled battery is provided with a discharge circuit composed of a discharge resistor corresponding to each unit secondary battery, and a unit secondary battery with a high voltage is replaced with a unit with a low voltage. It discharges according to the secondary battery, and the battery voltage of each unit secondary battery is equalized.
Japanese Patent Laid-Open No. 08-19188

As for the above, as a premise to equalize the battery voltage, first, the battery voltage of each unit secondary battery constituting the assembled battery is individually measured, and whether or not there is a difference in the battery voltage of each unit secondary battery It is necessary to measure. Such measurement generally requires a CPU or the like (in other words, requires digital processing), and the configuration of the apparatus becomes large.
The present invention has been completed based on the above circumstances, and provides a battery pack voltage equalization apparatus capable of equalizing the battery voltage of each unit secondary battery with a relatively simple configuration. The purpose is to provide.

  The present invention provides a voltage equalizing device for equalizing each battery voltage of an assembled battery formed by connecting a plurality of unit secondary batteries in series, and charging the assembled battery with a constant current at the initial stage of charging. The constant current-constant voltage charging is performed until the charging is completed after the total voltage of the assembled battery exceeds the set voltage or any of the unit secondary batteries exceeds the set voltage. In the type, each unit secondary battery is provided individually and connected so that one end is connected to the positive electrode of the corresponding unit secondary battery and the other end is connected to the negative electrode of the corresponding unit secondary battery. Each discharge switch having the same resistance value and each discharge switch interposed between the positive electrode of each unit secondary battery and each discharge resistance, or between the negative electrode of each unit secondary battery and each discharge resistance And provided in a current supply path for supplying a charging current to the assembled battery. A current sensor that detects the magnitude of the charging current, and a control that executes an equalizing operation for simultaneously equalizing the battery voltages of the unit secondary batteries constituting the assembled battery by simultaneously turning on the discharge switches. And a control circuit configured to charge the current value of the charging current supplied to the assembled battery through the current supply path at the constant voltage based on a detection signal output from the current sensor. Detecting whether the current is between two current threshold values set in the voltage region, and performing the equalization operation by simultaneously turning on the discharge switches in a section where the current value of the charging current is between the current threshold values However, it has characteristics.

-As an embodiment of this invention, it is preferable to set it as the following structures.
The control circuit includes two comparators to which two reference voltages corresponding to the two current thresholds are respectively applied to one input terminal, and a detection signal of the current sensor is commonly input to the other input terminal, When the outputs of these comparators do not match, the discharge switches are turned on all at once, and the equalization is executed. Otherwise, the discharge switches are turned off. In this way, the control circuit can be configured with only analog electronic components such as a comparator, and the entire voltage equalization apparatus can be simplified.

-In the case where the unit secondary battery is a lithium ion secondary battery, corresponding to the subsequent areas including the change area at the end of charging in which the battery voltage tends to rise with a steep slope with respect to the increase in the charging rate. The two current threshold values are set, and the equalization is performed in the subsequent areas including the change area. In this way, battery voltage imbalance that can occur particularly in the change region can be equalized.

  According to the present invention, the discharge switches corresponding to the unit secondary batteries are turned on all at once to equalize the battery voltages of the unit secondary batteries at the same time. With such equalization, it is not necessary to individually measure the battery voltage of each unit battery, and each battery voltage of each unit secondary battery, which has been essential in the previous voltage equalization apparatus, is detected individually. A function can be abolished. Further, according to the present invention, the current value (level) of the charging current is monitored by the current sensor, and the battery voltage of each unit secondary battery is equalized in a constant voltage region near the end of charging. Therefore, it is optimal as the operation timing of the equalization operation, and the battery voltage can be equalized efficiently.

An embodiment of the present invention will be described with reference to FIGS.
In this embodiment, the battery pack B formed by connecting three unit secondary batteries (lithium ion secondary batteries) E1 to E3 in series is charged by the charger 20, and the negative electrode of the battery pack B is grounded. While being connected to GND, the positive electrode is connected to the charger 20 through the current supply path L.

  And using the voltage equalization apparatus U of this invention, the battery voltage of each lithium ion secondary battery E1-E3 is equalized, and each lithium ion secondary battery E1-E3 is the same battery voltage without a voltage difference. It can be charged with.

  As will be described in detail below, the voltage equalizing apparatus U is mainly composed of discharge resistors R1 to R3, discharge switches SW1 to SW3, a current sensor 30, and a switch control circuit 40.

  As shown in FIG. 1, the lithium ion secondary batteries E1 to E3 are provided with discharge resistors R1 to R3, respectively. Each discharge resistance R1-R3 is connected so that one end is connected to the positive electrode of each corresponding lithium ion secondary battery E1-E3 and the other end is connected to the negative electrode of corresponding lithium ion secondary battery E1-E3.

  More specifically, the discharge resistor R1 is connected so that one end is connected to the positive electrode of the lithium ion secondary battery E1 and the other end is connected to the negative electrode of the lithium ion secondary battery E1, and the discharge resistor R2 is connected to the lithium ion secondary battery E2. One end is connected to the positive electrode and the other end is connected to the negative electrode of the lithium ion secondary battery E2. The discharge resistor R3 is connected so that one end is connected to the positive electrode of the lithium ion secondary battery E3 and the other end is connected to the negative electrode of the lithium ion secondary battery E3. The discharge resistances R1 to R3 are all set to the same resistance value.

  Further, discharge switches SW1 to SW3 are inserted between the discharge resistors R1 to R3 and the positive electrodes of the corresponding lithium ion secondary batteries E1 to E3.

  All of these three discharge switches SW1 to SW3 are configured by P-channel field effect transistors. The gates of the discharge switches SW1 to SW3 are connected to the power supply side via a pull-up resistor (not shown), and the discharge switches SW1 to SW3 are always controlled to be in an off state.

  A common signal line L1 is electrically connected to the gates of the discharge switches SW1 to SW3. The signal line L1 is provided with an N-channel field effect transistor (hereinafter simply referred to as an FET) 41 with the source S connected to the ground GND and the drain D connected to the switch side.

  The FET 41 constitutes an output stage of the switch control circuit 40 described below. When the gate G of the FET 41 is set to H level, the FET 41 is turned on and the line voltage of the signal line L1 becomes L level. The discharge switches SW1 to SW3 described above can be turned off all at once.

  The switch control circuit 40 includes a pair of comparators 51 and 55 at the input stage. The comparators 51 and 55 compare the level of the input signal with the level of the reference voltage and output a binary signal (H level / L level signal) according to the comparison result. Reference voltages Va and Vb (a specific setting method of the reference voltage will be described later) are given to the positive terminal (corresponding to “one input terminal” of the present invention).

  On the other hand, the detection signal Sr of the current sensor 30 is applied as an input signal to each minus side terminal (corresponding to “the other input terminal” of the present invention) of the comparators 51 and 55. The current sensor 30 includes a shunt resistor (not shown) inserted into the current supply path L, an amplifier (not shown) that amplifies and outputs the voltage across the shunt resistance, and the like, and is connected to the assembled battery B through the current supply path L. A voltage signal (detection signal Sr) proportional to the current value of the supplied charging current I is output.

  The output terminals of both the comparators 51 and 55 are connected to the power supply side via the pull-up resistor Rp, and the output terminal of the comparator 51 is connected to the source S of the P-channel field effect transistor 61. The output terminal 55 is connected to the source G of the P-channel field effect transistor 61. Hereinafter, the P-channel field effect transistor 61 is simply referred to as FET 61.

  The drain D of the FET 61 is connected to the gate G of the FET 41 described above. Further, a gate bias resistor R4 is provided between the gate and the source of the FET 41, and a gate bias resistor R6 is provided between the gate and the source of the FET 61.

  Next, the charging operation performed by the charger 20 will be described. The charger 20 performs constant current-constant voltage (CCCV) charge control and charges the assembled battery B. The constant current-constant voltage charging control means that the charging operation proceeds at a constant current Io in the initial stage of charging, and then any one of the lithium ion secondary batteries E1 to E3 reaches a predetermined set voltage, or lithium ion When the total voltage of the secondary batteries E1 to E3 reaches a predetermined set voltage, after that, the charging operation is advanced at the constant voltage Vo to fully charge each of the lithium ion secondary batteries E1 to E3 (for example, the charging end voltage 4 .2V).

The constant current-constant voltage control type charger 20 described above includes a power supply circuit, a voltage detection circuit that monitors the battery voltage of each unit secondary battery, and an output of the power supply circuit based on the detection result of the voltage detection circuit. For example, the following patent document discloses.
-JP2003-23736

  In order to explain the characteristics of the battery, in FIG. 2, the battery voltage of the battery is plotted on the vertical axis, and the charging rate (SOC; State of Charge) is plotted on the horizontal axis. A characteristic diagram is shown. As shown in the figure, the lithium ion secondary battery E has a characteristic in which the rate of change in the battery voltage with respect to the increase in the charging rate is small and the slope is flat in the region where the charging rate is more than 10% to 80%. (Hereinafter, flat region).

  On the other hand, the region exceeding 80% shows a change in which the battery voltage rises with a steep slope with respect to the increase in the charging rate (hereinafter referred to as a change region). In this embodiment, the voltage at the boundary where the flat region changes to the change region is about 4V, and a voltage value exceeding this voltage (for example, about 4.2V) is set as the set voltage for switching the charge control. It is. Therefore, after the transition from the flat region to the change region, the charging control is switched at a timing when the voltage reaches about 4.2 V, and the constant current region is shifted to the constant voltage region.

  Also, in FIG. 3, the transition of the current value of the charging current I when the unit secondary battery applied to the present embodiment, that is, the lithium ion secondary battery, is charged using the charger 20 is shown on the horizontal axis. It takes time to show. As shown in the figure, in the constant voltage region, the charging current I droops with time. This is because the battery voltage of the lithium ion secondary battery gradually increases with the progress of charging, and the potential difference with respect to the constant voltage Vo gradually decreases.

  Moreover, the magnitude of the constant voltage Vo is set to the voltage value (4.2 V) of the battery voltage at the time of full charge, and when the lithium ion secondary battery E reaches full charge, the charging current drooping with time The size is set to be almost zero.

  In the present embodiment, two current threshold values Ia and Ib are determined corresponding to this constant voltage region (in other words, the change region shown in FIG. 2). That is, the current threshold value Ia is set to a value that is smaller than the current value Io at the time of constant current charging by a difference value α, and the current threshold value Ib is a value that is larger by a difference value α than the current value at the time of charging completion (almost zero). Is set to

  When the charging current I equal to the current threshold value Ia flows through the current supply path L, the voltage level of the detection signal Sr output from the current sensor 30 is set as the reference voltage Va of the comparator 51, and the current The voltage level of the detection signal Sr output from the current sensor 30 when the charging current I equal to the magnitude of the threshold value Ib flows through the current supply path L is set as the reference voltage Vb of the comparator 55.

  With such a setting, as described below, the battery voltage of each of the lithium ion secondary batteries E1 to E3 is set in a region (constant voltage region) after the change region after the flat region shown in FIG. The balancing operation for balancing is executed.

Next, the circuit operation of the voltage equalizing apparatus U will be described.
When charging of the assembled battery B is started, a constant current having a current value Io is supplied from the charger 20 to the assembled battery B through the current supply path L at the initial stage of charging. At this time, a detection signal Sr corresponding to the level of the current value Io is output from the current sensor 30 and input to the minus terminals of both the comparators 51 and 55, respectively.

  The voltage level of the detection signal Sr is higher than the voltage level of the reference voltage Va given to the plus terminal of the comparator 51 and the voltage level of the reference voltage Vb given to the plus terminal of the comparator 55. Therefore, at this time, the outputs of both the comparators 51 and 55 are the outputs that coincide at the L level.

  Then, the FET 61 becomes almost zero and Vgs is turned off. As a result, the FET 41 is also almost zero in Vgs and is turned off. From the above, since the gates of the discharge switches SW1 to SW3 maintain the H level state, the discharge switches SW1 to SW3 are all turned off.

  As long as there is no change in the outputs of the comparators 51 and 55, each of the discharge switches SW1 to SW3 maintains the off state. Charging will proceed. As the charging progresses, each of the lithium ion secondary batteries E1 to E3 constituting the assembled battery B is flat after the charging voltage shown in FIG. The battery voltage of each of the lithium ion secondary batteries E1 to E3 gradually increases.

  Thereafter, the battery voltage of each of the lithium ion secondary batteries E1 to E3 substantially reaches a level that passes through the flat region. When the battery voltage of any one of the lithium ion secondary batteries E1 to E3 exceeds 4V that is the boundary voltage between the flat region and the change region and reaches 4.2V that is the set voltage, this is detected by the charger 20. The

  Then, the charger 20 switches control from constant current control to constant voltage control, and thereafter performs charging at the constant voltage Vo. Immediately after this switching, since the current value of the charging current is Io, each of the discharge switches SW1 to SW3 is still kept off.

  Now, when the charging control is switched to a constant voltage, as shown in FIG. 3, the magnitude of the charging current I gradually decreases with the progress of charging. As a result, the voltage level of the detection signal Sr output from the current sensor 30 also decreases.

  When the magnitude of the charging current I falls below the current threshold Ia, the voltage level of the detection signal Sr falls below the voltage level of the reference voltage Va applied to the plus terminal of the comparator 51, and the output of the comparator 51 is H Become a level.

  On the other hand, since the voltage level of the detection signal Sr is still higher than the voltage level of the reference voltage Vb given to the plus terminal of the comparator 55, the output of the comparator 55 remains at the L level. .

  As described above, when the outputs of the comparators 51 and 55 are in a mismatched state, the FET 61 becomes a predetermined voltage having a negative Vgs and is turned on. As a result, the FET 41 also has a positive voltage Vgs and is turned on. From the above, since the gates of the discharge switches SW1 to SW3 are switched from the H level state to the L level state, the discharge switches SW1 to SW3 are turned on all at once, and the equalizing operation (battery voltage Balance operation) is performed.

  Here, immediately before the discharge switches SW1 to SW3 are turned on all at once, the total voltage of the assembled battery B is 11.4V, the battery voltage of the first-stage lithium ion secondary battery E1 is 3.6V, and the second stage It is assumed that the battery voltage of the lithium ion secondary battery E2 is 3.8V and the battery voltage of the third stage lithium ion secondary battery E3 is 4V. In this case, when the discharge switches SW1 to SW3 are turned on all at once, as shown in FIG. 5, a voltage obtained by sharing the total voltage 11.4V according to the resistance ratio is applied to the discharge resistors R1 to R3. Moreover, since the resistance values of the discharge resistors R1 to R3 are all set to the same value, the discharge resistors R1 to R3 are eventually divided into three equal parts of the total voltage 11.4V, that is, 3. A voltage of 8V is applied.

  Then, as a result of the current If flowing through the path shown in FIG. 5, the third-stage lithium ion secondary battery E3 having a higher battery voltage than the average voltage 3.8V is discharged, and on the contrary, the average voltage 3 The first-stage lithium ion secondary battery E1 having a lower battery voltage than .8V is charged. In addition, neither discharging nor charging is performed on the second-stage lithium ion secondary battery E2 equal to the average voltage of 3.8 V (in other words, the current If flows so as to bypass the lithium ion secondary battery E2).

  From the above, after the discharge switches SW1 to SW3 are turned on all at once, the lithium ion secondary batteries E1 to E3 are balanced, and the difference in battery voltage gradually decreases.

  This balance operation is performed in the section (2) in FIG. 3 in which the outputs of the comparators 51 and 55 are inconsistent, that is, in the present invention, the level of the current detected by the current sensor is the constant voltage. The section between the two current threshold values (here, Ia and Ib) set in the constant voltage region where the charging is performed at is continuously performed, and at the same time, the assembled battery B by the charger 20 The charging operation is advanced.

  Accordingly, the lithium ion secondary batteries E1 to E3 approach the full charge while the battery voltage is balanced, and the current value of the charging current I supplied from the charger 20 also decreases.

  When the magnitude of the charging current I falls below Ib (ie, the section (3) in FIG. 3), the voltage level of the detection signal Sr is the reference voltage Vb applied to the plus terminal of the comparator 55. The output of the comparator 51 and the output of the comparator 55 both become H level, and the outputs of the comparators 51 and 55 again coincide with each other.

  Then, the FET 61 becomes almost zero and Vgs is turned off. As a result, the FET 41 is also almost zero in Vgs and is turned off. From the above, the gates of the discharge switches SW1 to SW3 are switched from the L level to the H level, and the discharge switches SW1 to SW3 that have been turned on are all turned off. This completes the balance operation.

  At this time, each of the lithium ion secondary batteries E1 to E3 is almost fully charged, and since the balance operation has been executed until then, the battery voltages are all equalized. Thereafter, charging by the charger 20 is continued and the charging operation is completed when the charging current becomes almost zero. Thus, each of the lithium ion secondary batteries E1 to E3 is fully charged (charge end voltage 4.2V).

  As described above, according to the present embodiment, the voltage equalization device U operates during the charging operation by the charger 20 to balance the battery voltages of the lithium ion secondary batteries E1 to E3. It is possible to perform charging without any problems.

  In addition, in this embodiment, the discharge switches SW1 to SW3 corresponding to the lithium ion secondary batteries E1 to E3 are turned on all at once, so that the battery voltages of the lithium ion secondary batteries E1 to E3 are equalized all at once. ing. If the battery voltage is equalized by such a method, it is not necessary to individually measure the battery voltage of each of the lithium ion secondary batteries E1 to E3, and each unit secondary that has been indispensable in the conventional voltage equalization apparatus. The function to detect each battery voltage of a battery individually can be abolished.

  In addition, according to the present invention, the current value (level) of the charging current I is monitored by the current sensor 30, and the operation timing of the equalization operation is determined based on the current value of the charging current I. In addition, the switch control circuit 40 for controlling the operation timing is composed of only analog electronic components (in other words, discrete components) including the two comparators 51 and 55, and the voltage equalizing apparatus U is an extremely simple circuit. The cost can be reduced while ensuring the necessary performance of equalizing the battery voltage.

  In the present embodiment, the charger 20 performs constant current-constant voltage control to charge each of the lithium ion secondary batteries E1 to E3, but the change range of the lithium ion secondary battery E (the plateau at the end of charging) The constant voltage region corresponds to the region after (non-region) region. The values of the reference voltages Va and Vb applied to the comparators 51 and 55 are set in correspondence with the current value of the charging current I in the constant voltage range, and the equalization operation is performed by changing the lithium ion secondary battery E. The setting is performed in the constant voltage region after the region (more specifically, the region after the change region, see FIG. 3). Here, if the equalization operation is executed in the flat region shown in FIG. 2, the battery voltage of each battery is uniform to some extent in the flat region, and therefore, for example, the resistance values of R1 to R3 vary. If it is large, the battery voltage of each of the lithium ion secondary batteries E1 to E3 becomes unbalanced due to this variation or the like, and there is a possibility that charging / discharging is uselessly performed. In this respect, in this embodiment, the battery voltage is likely to vary, and the equalization operation is performed in the region after the change region that is closer to the end of charging than the flat region. E1 to E3 can be balanced efficiently. In particular, battery voltage imbalance that can occur in the change region can be equalized.

  In addition, the operating range in which the equalizing operation is executed can be easily changed by simply changing the magnitudes of the reference voltages Va and Vb applied to the comparators 51 and 55. This is also effective.

  In the present embodiment, the equalization operation is set to be executed in the section (2) in FIG. 3, but the section (2) is preferably set as long as possible. This is because if the interval for executing the balance operation is set to be short, depending on the size of the battery capacity and the variation in battery voltage, there is not enough time for balancing and it is assumed that the battery voltage cannot be equalized. Because.

  On the other hand, if the section (2) is set to be long, the difference value α is inevitably set to be small, but if this is made too small, depending on the measurement error of the current sensor 30, it may be a constant current region. Regardless, the voltage level of the detection signal Sr may be lower than the reference voltage Va of the comparator 51, and there is a possibility of causing a malfunction that unintentionally starts the equalization operation. Accordingly, the difference value α is set to be larger than the measurement error β of the current sensor 30 so that the equalization operation is reliably performed in the constant voltage region, and the equalization operation time is set to be as long as possible. It is best to do.

<Other embodiments>
The present invention is not limited to the embodiments described with reference to the above description and drawings. For example, the following embodiments are also included in the technical scope of the present invention.

  (1) In the above embodiment, the assembled battery is configured by connecting three unit secondary batteries E1 to E3 in series. However, the number of connection stages of the unit secondary batteries is limited to three. There may be four steps, five steps or more. Also, the type of battery is not limited to lithium ion secondary batteries. For example, other secondary batteries such as lead storage batteries, nickel metal hydride secondary batteries, nickel / cadmium secondary batteries, manganese dioxide / lithium secondary batteries, etc. It may be a battery.

The circuit diagram which shows the electrical constitution of the voltage equalization apparatus which concerns on one Embodiment of this invention. The figure which shows the charge characteristic of the lithium ion secondary battery The figure which shows the charge characteristic of the lithium ion secondary battery Chart summarizing output patterns of each electronic component Explanatory drawing explaining balance operation

Explanation of symbols

DESCRIPTION OF SYMBOLS 20 ... Charger 30 ... Current sensor 40 ... Switch control circuit (equivalent to "control circuit" of this invention)
41... N-channel field effect transistor 51... Comparator (corresponding to “comparator” of the present invention)
55. Comparator (corresponding to "comparator" of the present invention)
61 ... P channel type field effect transistor Ia ... Current threshold Ib ... Current threshold SW1-SW3 ... Discharge switch R1-R3 ... Discharge resistor Va ... Reference voltage Vb ... Reference voltage U ... Voltage equalization device

Claims (3)

A voltage equalizing device for equalizing each battery voltage of an assembled battery formed by connecting a plurality of unit secondary batteries in series,
The assembled battery is charged at a constant current at the beginning of charging, and after the total voltage of the assembled battery exceeds the set voltage or one of the unit secondary batteries exceeds the set voltage, the charging is completed. Is a constant current-constant voltage charge type that charges at a constant voltage,
The same resistance value is provided for each unit secondary battery, and is connected so that one end is connected to the positive electrode of the corresponding unit secondary battery and the other end is connected to the negative electrode of the corresponding unit secondary battery. Each discharge resistance,
Each discharge switch interposed between the positive electrode of each unit secondary battery and each of the discharge resistors, or between the negative electrode of each unit secondary battery and each of the discharge resistors,
A current sensor that is provided in a current supply path for supplying a charging current to the assembled battery and detects the magnitude of the charging current;
A control circuit that performs an equalization operation for simultaneously equalizing battery voltages of the unit secondary batteries constituting the assembled battery by simultaneously turning on the discharge switches,
Based on the detection signal output from the current sensor, the control circuit sets the current value of the charging current supplied to the assembled battery through the current supply path in a constant voltage region where charging is performed at the constant voltage. A voltage characterized by detecting whether the current is between two current thresholds, and performing the equalization operation by simultaneously turning on the discharge switches in a section where the current value of the charging current is between the current thresholds. Equalizing device. The control circuit includes two comparators to which two reference voltages corresponding to the two current thresholds are respectively applied to one input terminal, and a detection signal of the current sensor is commonly input to the other input terminal,
2. The apparatus according to claim 1, wherein when the outputs of the two comparators do not coincide with each other, the discharge switches are simultaneously turned on to execute the equalization, and otherwise, the discharge switches are turned off. The voltage equalization apparatus as described. In the unit secondary battery is a lithium ion secondary battery,
Each of the two current thresholds is set corresponding to a subsequent region including a change region at the end of charge, which indicates a tendency that the battery voltage increases with a steep slope with respect to an increase in the charge rate, and thereafter includes the change region. The voltage equalization apparatus according to claim 1, wherein the equalization is set to be executed in the area of the voltage.

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