JP5495319B2 - Power storage device - Google Patents

Description

  The present invention interconverts alternating current power and direct current power, and uses the direct current power to store electric energy in a battery cell or an assembled battery formed by connecting a plurality of battery cells in series. The present invention relates to a power storage device that discharges electrical energy from a battery.

2. Description of the Related Art Conventionally, there are power storage devices that store electric energy in a battery cell such as a lithium ion battery and release the electric energy from the battery cell. This power storage device includes voltage conversion means including an AC-DC bidirectional converter that mutually converts an AC voltage and a DC voltage.
In general, an AC-DC bidirectional converter composed of a full bridge circuit is used as voltage conversion means for performing bidirectional voltage conversion (see, for example, Patent Document 1).

  An AC-DC bidirectional converter composed of a full bridge circuit is required to make the DC voltage higher than a certain value with respect to the AC voltage. For this reason, in general, a DC voltage (number of batteries in series) is determined in consideration of the AC voltage, a transformer that generates an AC voltage in consideration of the lower limit of the DC voltage is used, or the DC voltage is changed to a voltage suitable for the AC voltage. A technique such as using a DC-DC converter that steps up and down is used.

In general, a battery cell such as a lithium ion battery has a characteristic that the cell voltage changes with respect to SOC (State of Charge (remaining capacity)) as shown in FIG.
As shown in the figure, the battery cell has a first region in which the rate of change of the cell voltage relative to the change in SOC is relatively small, and a second region in which the rate of change is larger than the first region.
When the battery cell is used in the second region where the SOC is small, the cell voltage is rapidly lowered, so that there is a risk of overdischarge or deterioration of the cell. For this reason, it is reasonable to use the battery cell only in the first region without using it in the second region, and a power storage device designed as such has been put to practical use (for example, see Patent Document 2). .

In the power storage device, when an AC-DC bidirectional converter configured with a full bridge circuit is used, the lower limit of the DC voltage to be considered can be set high by using the battery cell only in the first region. For this reason, the AC voltage of the AC-DC bidirectional converter can be set high, and the AC current can be reduced.
Furthermore, by reducing the alternating current, a switching element with a lower capacity, a smaller size, and a lower cost can be used, and a rational design with less waste compared to the case of using up to the second region is possible.

Further, in an assembled battery in which battery cells such as lithium ion batteries are connected in series, when used for a long time, the SOC of each battery cell varies due to a slight difference in self-discharge. When the variation occurs, the usable SOC area is narrowed, and thus it is necessary to correct the variation. For this reason, when using an assembled battery generally, the function which corrects the dispersion | variation in SOC is added to BMS (Battery Management System).
In BMS, the SOC of the battery cell cannot be directly detected. Therefore, by detecting the cell voltage of the battery cell having a certain correlation with the SOC and correcting the variation, the variation in the SOC is corrected ( For example, see Patent Document 3).

JP 2002-272121 A JP-A-2005-65352 JP 2004-173345 A

  By the way, since battery cells, such as a lithium ion battery, have correlation with SOC and a cell voltage as shown in FIG. 1, the 2nd area | region where the change of the cell voltage of a battery cell is relatively large with respect to the change of SOC. If the variation in the cell voltage of the battery cell is corrected, the process can be performed with higher accuracy and speed.

  However, it is used only in the first region when using voltage conversion means that requires the DC voltage to be higher than a certain value with respect to the AC voltage, such as an AC-DC bidirectional converter composed of a full bridge circuit. If it is designed on the assumption that this is done, voltage conversion cannot be performed normally in the second region.

  For this reason, in order for the voltage conversion means to perform voltage conversion normally even in the second region, it is necessary to design the voltage conversion means so that the lower limit of the DC voltage is included in the second region. However, with such a design, it is necessary to use components with a large current rating as compared with the voltage conversion means used only in the first region, so that design waste is increased and the size and cost are inferior. It will be.

  On the other hand, a DC-DC converter is provided on the DC side of the AC-DC bidirectional converter so that it can be used in the second region and a part having a large current rating is avoided. There is a method of stepping up and down the DC voltage. However, even in such a configuration, the system becomes complicated and the size and cost are inferior.

  In view of the above problems, it is an object of the present invention to provide a power storage device with a simple configuration that can convert voltage even when a DC voltage is lowered to a low voltage region with a minimum size and cost.

In order to solve the above-described problems, a power storage device according to the present invention is a power storage device that can convert a reference AC voltage into a DC voltage and store electrical energy in at least one battery cell. And a voltage converting means for converting the transformed AC voltage into a DC voltage, the voltage transforming means being either a primary winding or a secondary winding. A transformer having a first tap and a second tap drawn from positions with different numbers of turns on either side, and a switch for switching the connection so that either the first tap or the second tap is selected. has, connecting the switch to the first tap, the transformer means, as well as transforms the reference alternating voltage to the first transformer AC voltage, the voltage conversion means, a first transformer AC voltage When the switch is connected to the second tap while the first DC voltage is converted, the transforming means transforms the reference AC voltage into the second transformed AC voltage whose voltage value is lower than the first transformed AC voltage, and the voltage converting means. However, the second transformed AC voltage is converted into a second DC voltage having a voltage value lower than that of the first DC voltage, and the battery cell has a relative change rate of the cell voltage with respect to the remaining capacity in the change characteristic of the cell voltage with respect to the remaining capacity. The first region is smaller than the first region, and the remaining capacity is smaller than the first region. The first region and one end of the second region When the cell voltage at the boundary is the lower limit setting voltage during normal use and the cell voltage when the remaining capacity at the other end of the second region is zero is the battery cell minimum voltage, the lower limit of the first DC voltage is the lower limit during normal use Setting Is set to or higher than the voltage, the lower limit of the second DC voltage, the battery cell minimum voltage or more and characterized in that it is set lower than normally used when a lower limit set voltage.

According to this configuration, it is possible to generate two transformer AC voltages having different voltage values from the reference AC voltage with a simple configuration, and by switching the connection between the tap and the switch, the transformer means can convert the voltage converter means to the voltage converter means. One of the two transformed AC voltages can be supplied.
As a result, a DC voltage over a wide range can be output from the reference AC voltage. Thus, with a minimum size and cost, voltage conversion is possible even when the direct current voltage is lowered to a low voltage region, and a power storage device with a simple configuration can be provided.
According to this configuration, when the switch is connected to the first tap, the lower limit of the first DC voltage is set to be equal to or higher than the normal use lower limit setting voltage, and when the switch is connected to the second tap, The lower limit of the DC voltage is set to be equal to or higher than the battery cell minimum voltage and lower than the lower limit setting voltage during normal use. For this reason, when the battery cell is used in the first region, the switch is connected to the first tap, and the lower limit of the first DC voltage input to the voltage conversion means or the first DC voltage output from the voltage conversion means. The lower limit can be made higher than the lower limit set voltage during normal use. As a result, the input / output AC voltage can be set higher than the voltage conversion means used up to the second region, and a rational design with little waste is possible. On the other hand, when the switch is connected to the second tap, the voltage can be normally applied even in the second region without greatly impairing the design rationality of the voltage conversion means that is assumed to be used only in the first region. Conversion can be performed.

Further, in the above power storage device, the voltage conversion means is an AC-DC bidirectional converter configured with a full bridge circuit, and at least one battery cell is a battery pack formed by connecting a plurality of battery cells in series. A voltage detection means for detecting a cell voltage of each battery cell constituting the assembled battery, and a balance means for performing a balance operation for charging and discharging each battery cell to reduce a voltage difference between the battery cells. The balance means is provided until the voltage difference detected by the voltage detection means exceeds an allowable variation range and the switch is connected to the second tap so that the total voltage of the assembled battery becomes lower than the lower limit set voltage during normal use. It is preferable that the balance operation is performed after electric energy is discharged from each battery cell.

According to this configuration, before performing the balance operation, the switch is connected to the second tap, and electric energy is released from each battery cell until the cell voltage of each battery cell is included in the second region. Thereby, voltage conversion can be performed in a low voltage region (second region) suitable for correcting the SOC variation between the battery cells.

  The battery cell of the above power storage device is a lithium ion battery.

  According to the present invention, in a power storage device capable of mutually converting a reference AC voltage and a DC voltage and storing electric energy in a battery cell, the DC voltage is reduced to a low voltage region with a minimum size and cost. Even if the voltage is lowered, voltage conversion is possible, and a power storage device with a simple structure can be provided.

It is a figure which shows the change characteristic of the cell voltage with respect to SOC. It is a circuit diagram which shows the structure of the electrical storage apparatus which concerns on 1st Embodiment. It is a circuit diagram which shows the structure of an AC-DC bidirectional converter. It is a circuit diagram which shows the structure of the electrical storage apparatus which concerns on 2nd Embodiment. It is a figure which shows the change characteristic of the voltage of an assembled battery with respect to SOC. It is a circuit diagram which shows the structure of the electrical storage apparatus which concerns on 2nd Embodiment used with a wind power generator. It is a figure which shows the 1st method of correcting the variation in the cell voltage of the battery cell contained in the assembled battery in 2nd Embodiment. It is a figure which shows the 2nd method of correcting the variation in the cell voltage of the battery cell contained in the assembled battery in 2nd Embodiment.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

[First Embodiment]
As shown in FIG. 2, the power storage device 1 according to the present embodiment includes a transformation means 3, a voltage conversion means 4, a voltage detection means 5, and a control means 6. Electric energy is stored (charged) and discharged (discharged) from the battery cell 2 in order to regenerate the electric energy on the AC power source 7 side.

The battery cell 2 is made of, for example, a lithium ion battery, and has a characteristic that the cell voltage changes with respect to the SOC (remaining capacity) as shown in FIG.
As shown in the figure, the battery cell 2 includes a first region in which the rate of change of the cell voltage relative to the change in the SOC is relatively small, a rate of change that is greater than the first region, and an SOC that is greater than that in the first region. There is a small second region.

As shown in FIG. 2, the transforming means 3 includes a transformer 10 having a first tap 8 and a second tap 9 on the primary side, and a reference AC voltage supplied from an external AC power source 7 as a first tap 8 or a second tap. 9 and a switch 11 such as a relay or an FET that is input to the transformer 10 via 9. The transformer 3 can change the transformation ratio by switching the switch 11.
Specifically, when the switch 11 is connected to the first tap 8 during charging, the transformer 3 transforms the reference AC voltage to the first AC voltage, and the switch 11 is connected to the second tap 9. If so, the reference AC voltage is transformed to the second transformer AC voltage.
On the other hand, when the switch 11 is connected to the first tap 8 during regeneration, the transformer 3 transforms the first transformed AC voltage to the reference AC voltage, and when connected to the second tap 9, the second transformer Transform AC voltage to reference AC voltage.
In the case where the switch 11 is connected to the first tap 8 and the case where the switch 11 is connected to the second tap 9, the former has fewer turns of the primary winding. For this reason, the voltage value of the second transformed AC voltage generated on the secondary side of the transformer 10 is lower than that of the first transformed AC voltage.

The voltage conversion means 4 performs voltage conversion between an AC voltage and a DC voltage, and is required to make the DC voltage higher than a certain value with respect to the AC voltage. The voltage converting means 4 converts the first transformed AC voltage into the first DC voltage or converts the second transformed AC voltage into a second DC voltage lower than the first DC voltage during charging.
On the other hand, the voltage conversion means 4 converts the first DC voltage into a first transformed AC voltage and converts the second DC voltage into a second transformed AC voltage during regeneration.

As shown in FIG. 1, the cell voltage at the boundary between the first region and one end of the second region is the lower limit set voltage during normal use, and the cell voltage when the SOC at the other end of the second region is zero is the battery cell. When the minimum voltage is set, the lower limit of the first DC voltage is set to be equal to or higher than the lower limit set voltage during normal use (included in the first region), and the lower limit of the second DC voltage is higher than the battery cell minimum voltage and It is set to be lower than the lower limit set voltage during use (included in the second region).
Note that the battery cell 2 is overdischarged when the cell voltage becomes lower than the lowest battery cell voltage.

As the voltage conversion means 4, for example, a three-phase AC-DC bidirectional converter 4A configured with a full bridge circuit as shown in FIG. 3 is used. As shown in the figure, the three-phase AC-DC bidirectional converter 4A includes AC switches S1 to S6 that rectify AC voltage, a capacitor C1 that smoothes the voltage that has passed through the AC switches S1 to S6, and noise. It consists of capacitors C2 to C4 and coils L1 to L3 that absorb.
The switching of the AC switches S1 to S6 is performed by the control means 6. The AC-DC bidirectional converter 4A operates as an AC-DC converter during charging, and operates as a DC-AC inverter during regeneration.

  The voltage detection means 5 detects the cell voltage of the battery cell 2 and is connected to the battery cell 2 in parallel.

The power storage device 1 normally has a battery under the control of the control means 6 so that the cell voltage of the battery cell 2 does not become lower than the normal use lower limit setting voltage (3.7 V in this embodiment) shown in FIG. The cell 2 is charged / discharged. For this reason, the voltage conversion means 4 is designed on the assumption that the battery cell 2 is charged and discharged in the first region.
Therefore, compared with the voltage conversion means designed in consideration of charging / discharging not only in the first area but also in the second area, the voltage conversion means 4 in the present embodiment has a lower limit of the DC voltage to be considered (first Therefore, a rational design with little waste can be achieved without increasing the size and cost.

  Furthermore, since the power storage device 1 includes the transformer unit 3, the AC voltage input to the voltage converter unit 4 can be lowered by transforming the reference AC voltage supplied from the external AC power source 7. Thereby, even when the battery cell 2 is charged / discharged in the second region, the power storage device 1 has the design rationality of the voltage conversion means 4 on the assumption that the battery cell 2 is used only in the first region. The voltage conversion can be normally performed even in the second region without significant loss.

  Next, in order to transport the battery cell 2 and the power storage device 1, the battery cell 2 is intentionally discharged to lower the cell voltage of the battery cell 2 to the second region, and after transportation, the cell voltage is returned to the first region again. The method of returning will be described.

In the power storage device 1, the battery cell 2 is normally charged and discharged so that the cell voltage of the battery cell 2 does not become lower than the normal use lower limit setting voltage, and therefore the switch 11 is connected to the first tap 8. Therefore, first, the switch 11 is switched and connected to the second tap 9 to discharge the battery cell 2.
Alternatively, while discharging the battery cell 2, the switch 11 is switched and connected to the second tap 9 before the cell voltage of the battery cell 2 becomes lower than the use lower limit setting voltage.
The discharge is performed by a method of regenerating the electric energy stored in the battery cell 2 to the AC power source 7 side.

Then, the battery cell 2 is discharged until the cell voltage of the battery cell 2 reaches a predetermined voltage value in the second region, for example, SOC 5% (3.5 V in this embodiment).
After the discharge is completed, the battery cell 2 and the power storage device 1 are transported.

When the cell voltage of the battery cell 2 is returned to the first region after transportation, the battery cell 2 is charged using the second DC voltage output from the voltage conversion means 4 while the switch 11 is connected to the second tap 9. Let
When the cell voltage of the battery cell 2 returns to the first region, the switch 11 is switched to connect to the first tap 8.

As described above, in the power storage device 1, the cell voltage of the battery cell 2 can be lowered to the second region by connecting the switch 11 to the second tap 9 and discharging the battery cell 2.
Furthermore, in the power storage device 1, the battery cell 2 can be charged while the switch 11 is connected to the second tap 9, thereby returning the cell voltage of the battery cell 2 to the first region. In other words, the power storage device 1 performs normal voltage conversion even in the second region without greatly impairing the design rationality of the voltage conversion means 4 that is assumed to be used only in the first region by switching the switch. It can be carried out.

[Second Embodiment]
As shown in FIG. 4, the power storage device 21 according to the present embodiment uses an AC power supply 27 for the assembled battery 22 in which a plurality (three in the present embodiment) of battery cells 2A, 2B, and 2C are connected in series. Electric energy is stored (charged), and is discharged (discharged) from the assembled battery 22 in order to regenerate the electric energy on the AC power supply 27 side.

  The basic configuration of the power storage device 21 is the same as that of the power storage device 1 according to the first embodiment except for the voltage detection means 25 and the voltage balance means 28.

The voltage detection means 25 detects the voltage of the assembled battery 22 and the cell voltage for each of the battery cells 2A, 2B, 2C included in the assembled battery 22. The voltage detection means 25 is provided with notification means for outputting an alarm signal to notify the user when the voltage difference between the cell voltages of the battery cells 2A, 2B, and 2C exceeds an allowable variation range. .
Note that when the allowable variation range is exceeded, for example, when the voltage difference between the maximum value and the minimum value of the cell voltages of the battery cells 2A, 2B, and 2C is 0.1 V or more, or the battery cell 2A, This is the case when the cell voltage of at least one of the battery cells 2A, 2B, and 2C deviates by 0.1 V or more from the average value of the cell voltages of 2B and 2C.

  The voltage balance means 28 performs a balance operation for correcting variations in the cell voltages of the battery cells 2A, 2B, and 2C. Specifically, the voltage balance means 28 charges / discharges the battery cells 2A, 2B, and 2C to reduce the cell voltage difference between the battery cells 2A, 2B, and 2C.

By the way, each battery cell 2A, 2B, 2C contained in the assembled battery 22 has the characteristic that a cell voltage changes with respect to SOC, as shown in FIG. For this reason, as shown in FIG. 5, the assembled battery 22 has the characteristic that a voltage changes with respect to SOC as a whole.
In power storage device 21, the voltage of assembled battery 22 is the sum of the cell voltages of battery cells 2A, 2B, and 2C.

In the power storage device 21, control is usually performed so that the voltage of the assembled battery 22 does not become lower than the normal use lower limit setting voltage that is a voltage at the boundary between the first region and one end of the second region shown in FIG. 5. The battery pack 22 is charged and discharged using the first DC voltage under the control of the means 26. For this reason, the voltage conversion means 24 is designed on the assumption that the assembled battery 22 is used in the first region.
Therefore, as compared with the voltage conversion means designed in consideration of being used not only in the first area but also in the second area, the voltage conversion means 24 in the present embodiment has a lower limit of the DC voltage to be considered (first Since the lower limit of the DC voltage becomes high, a rational design with little waste is possible.

  Furthermore, since the power storage device 21 includes the transformer means 23, the AC voltage input to the voltage converter 24 can be lowered by transforming the reference AC voltage supplied from the external AC power supply 27. As a result, even when the assembled battery 22 is charged and discharged in the second region, the power storage device 21 has the design rationality of the voltage conversion means 24 that is assumed to be used only in the first region. The voltage conversion can be normally performed even in the second region without significant loss.

  Next, a method for correcting variations in cell voltage generated between the battery cells 2A, 2B, and 2C of the assembled battery 22 will be described with reference to FIGS.

The power storage device 21 shown in the figure is connected to the grid 29 and the wind power generator 30. Since the generated power fluctuates due to the change in wind speed, the wind power generator 30 needs to smooth the generated power. Specifically, the assembled battery 22 is charged when the generated power exceeds a predetermined output range, and the assembled battery 22 is discharged when the generated power falls below a predetermined output range.
Note that the smoothing of the generated power may cause the SOC of the assembled battery 22 to fluctuate greatly. Therefore, when smoothing is performed, the SOC of the assembled battery 22 is brought close to a predetermined SOC (capacity feedback target value). The capacity feedback control is also executed.
Therefore, as a method for correcting the variation in the cell voltage, a first method (see FIG. 7) in which the balance operation is executed in a state where the capacity feedback control is stopped, and a first method in which the balance operation is executed together with the capacity feedback control. There are two methods (see FIG. 8).

  In both of these methods, after the variation is detected, the cell voltages of the battery cells 2A, 2B, and 2C are lowered toward the capacity feedback target value by the capacity feedback control. Then, before the cell voltages of the battery cells 2A, 2B, and 2C are included in the second region, the switch 11 is switched and connected to the second tap 9 (tap switching).

In the first method, subsequently, as shown in FIG. 7, the minimum value of the cell voltages of the battery cells 2A, 2B and 2C is set to a preset lower limit voltage (for example, 3.0 V) in the second region. Until the value is reached.
When the minimum value reaches the lower limit voltage, the capacity feedback control is stopped, and the balance operation by the voltage balance means 28 is started (balance operation start). And after balance operation is performed for a while, the electric power of battery cell 2A, 2B, 2C is regenerated to the system | strain 29 (regeneration start), and the cell voltage of battery cell 2A, 2B, 2C is lowered | hung.
Then, this balancing operation and regeneration are repeated, so that the cell voltages of the battery cells 2A, 2B, and 2C substantially match in the vicinity of the lower limit voltage, and the variation is corrected.
Note that from the detection of variation to the start of the first balance operation, the cell voltage of the battery cells 2A, 2B, and 2C is lowered by regenerating the power of the battery cells 2A, 2B, and 2C in the system 29 instead of the capacity feedback control. In some cases.

On the other hand, in the second method, as shown in FIG. 8, the minimum value of the cell voltages of the battery cells 2A, 2B, and 2C does not fall below a lower limit voltage (for example, 3.0V) set in advance in the second region. Thus, it is lowered by the capacity feedback control.
Then, during the execution of the capacity feedback control, the balance operation by the voltage balance means 28 is started (balance operation start), and the variations in the cell voltages of the battery cells 2A, 2B, 2C are corrected.

As described above, in the power storage device 21, after the switch 11 is connected to the second tap 9, the cell voltages of the battery cells 2 </ b> A, 2 </ b> B, and 2 </ b> C are set to the second by capacity feedback control or power regeneration to the system 29. Can be lowered to the area.
Then, the balance operation is executed in the second region where the change rate of the cell voltage with respect to the change in the SOC is relatively large, thereby correcting the variation in the cell voltages of the battery cells 2A, 2B, and 2C more quickly. be able to.

  As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to the said embodiment.

  For example, the power storage devices 1 and 21 according to the above embodiment use the battery cells 2, 2 </ b> A, 2 </ b> B, and 2 </ b> C in which the first region and the second region exist in the change characteristics of the cell voltage with respect to the SOC. When 11 is connected to the first tap 8, a first transformed AC voltage is generated based on the reference AC voltage, and when connected to the second tap 9, the second voltage value is lower than the first transformed AC voltage. Although a two-stage switching method for generating a transformer AC voltage has been adopted, if a battery cell having three or more regions in the cell voltage change characteristics with respect to the SOC is used, a three-step or more switching method should be adopted. You can also.

  In the power storage devices 1 and 21, the battery cells 2, 2A, 2B, and 2C are discharged by a method of regenerating electrical energy stored in the AC power sources 7 and 27 (system 29). Instead of this, the discharging means may be connected, or the discharging means may be connected to the battery cells 2, 2A, 2B, and 2C to discharge the battery cells 2, 2A, 2B, and 2C.

  Further, in the power storage devices 1 and 21, the transformers 3 and 23 and the voltage converters 4 and 24 corresponding to the three phases are used, but those corresponding to the single phase may be used.

  In the transformer means 3 and 23, the primary side voltage is often higher than the secondary side voltage, so the first tap 8 and the second tap 9 are provided on the primary side of the transformer 10. When the secondary side voltage is higher, a tap may be provided on the secondary side.

  The battery cells 2, 2A, 2B, and 2C are not limited to lithium ion batteries, and various secondary batteries can be used.

  Further, the switch 11 and the voltage balance means 28 of the transformer means 3 and 23 may be controlled by the control means 6 and 26, or may be manually controlled from the outside.

1, 2 1 Power storage device 2, 2 A, 2 B, 2 C Battery cell 22 Battery assembly 3, 23 Transformer means 4, 24 Voltage converter means 5, 25 Voltage detector means 6, 26 Control means 7, 27 AC power supply 8 First tap 9 First 2 tap 10 transformer 11 switch 28 voltage balance means 29 system 30 wind turbine generator

Claims (3)

A power storage device capable of converting a reference AC voltage into a DC voltage and storing electrical energy in at least one battery cell,
Transforming means for transforming the reference alternating voltage into a transforming alternating voltage having a voltage value different from that of the reference alternating voltage;
Voltage conversion means for converting the transformed AC voltage into the DC voltage,
The transformer means includes either a transformer having a first tap and a second tap drawn from a position having a different number of turns in either the primary winding or the secondary winding, and any of the first tap and the second tap. And a switch for switching the connection so that the tap is selected,
When the switch is connected to the first tap, the transforming means transforms the reference AC voltage into a first transforming AC voltage, and the voltage converting means converts the first transforming AC voltage into a first DC voltage. While
When the switch is connected to the second tap, the transformer means transforms the reference AC voltage into a second transformer AC voltage having a voltage value lower than that of the first transformer AC voltage, and the voltage converter means Converting the second transformed AC voltage into a second DC voltage having a voltage value lower than that of the first DC voltage;
The battery cell includes a first region in which a change rate of the cell voltage with respect to the remaining capacity is relatively small in a change characteristic of the cell voltage with respect to the remaining capacity, the change rate is larger than the first region, and the remaining capacity is A second region smaller than the first region,
The cell voltage at the boundary between the first region and one end of the second region is defined as a normal use lower limit setting voltage, and the cell voltage at the other end of the second region is zero as the battery cell minimum voltage. When
The lower limit of the first DC voltage is set to be equal to or higher than the normal use lower limit setting voltage,
The power storage device , wherein a lower limit of the second DC voltage is set to be equal to or higher than the battery cell minimum voltage and lower than the normal use lower limit setting voltage . The voltage conversion means is an AC-DC bidirectional converter composed of a full bridge circuit,
The at least one battery cell is an assembled battery formed by connecting a plurality of battery cells in series,
Voltage detecting means for detecting a cell voltage of each battery cell constituting the assembled battery;
Balance means for performing a balance operation to charge and discharge each battery cell to reduce the voltage difference of the cell voltage between each battery cell;
The balance means is configured such that the voltage difference detected by the voltage detection means exceeds an allowable variation range, and the switch is connected to the second tap so that the cell voltage of each battery cell is set to the lower limit during normal use. The power storage device according to claim 1 , wherein the balance operation is performed after electric energy is released from each of the battery cells until the voltage becomes lower than a voltage . The power storage device according to claim 1 or 2, wherein the battery cell is a lithium ion battery .

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