JP6327175B2 - Power storage device - Google Patents

Description

  The present invention relates to a power storage device including a plurality of batteries connected in series.

  In recent years, with the advent of electric vehicles and smart grids, batteries with high battery capacity and high output have been demanded. Such a battery can be increased in voltage by connecting a plurality of cells in series.

  Cell voltage balance technology that corrects voltage deviation between battery cells in order to maximize the performance of each battery cell in a power storage device including an assembled battery in which a plurality of battery cells are connected in series It has been known. Specifically, for example, a method of matching the voltage balance in the assembled battery by connecting a resistance to each battery cell and discharging other battery cells in accordance with the battery cell determined as a reference is known.

  For example, the prior art described in Patent Document 1 discloses a technique for suppressing deterioration due to exceeding the upper limit voltage by changing the charging start voltage depending on the magnitude of the internal resistance of each battery.

  Moreover, in the prior art described in Patent Document 2, a balance target voltage value is calculated according to the battery capacity difference between the battery cell defined as a reference and each battery cell, and the calculated balance target voltage value is calculated for each battery cell. Adjust the voltage. The balance target voltage value is calculated so that the remaining charge capacity at the end of charging of the assembled battery and the remaining discharge capacity at the end of discharging substantially coincide. As a result, a technique is disclosed that suppresses voltage variations between batteries so as to maximize the performance of the battery with the lowest performance, both during charging and discharging.

JP 2009-232659 A JP 2013-169055 A

However, although the prior art described in Patent Document 1 can suppress deterioration during charging with a large current due to variations in resistance caused by long-term use, the battery capacity varies due to variations in chargeable / dischargeable battery capacity. There is a problem that the capacity of can not be maximized.
Although the prior art described in Patent Document 2 controls the voltage so as to maximize the performance of the battery having the smallest battery capacity, it deteriorates by exceeding the upper and lower limit voltages when charging and discharging a large current. Since the input / output characteristics are degraded due to the limitation of the upper limit voltage and the lower limit voltage by the upper system, there is still room for improvement.

  The present invention has been made in order to solve the above-mentioned problems, and its main object is to provide a battery capacity capable of charging / discharging the battery with the smallest battery capacity in a power storage device including a plurality of batteries connected in series. By controlling the input / output characteristics of the power storage device while maintaining it, it is possible to optimize the balance between input and output not only during battery production but also when used for a long period of time.

  The present invention provides a power storage device including a plurality of batteries connected in series, wherein the plurality of batteries include a first battery and a second battery, and the battery capacity of the first battery is greater than the battery capacity of the second battery. And the slope of the voltage with respect to the charge amount of the first battery is larger than the slope of the voltage with respect to the charge amount of the second battery.

  According to the above configuration, a plurality of batteries are connected in series in the power storage device, and the plurality of batteries include the first battery and the second battery. The battery capacity of the first battery is larger than the battery capacity of the second battery, and the slope of the voltage with respect to the charge amount of the first battery is larger than the slope of the voltage with respect to the charge amount of the second battery.

  If the battery capacity of all the batteries is substantially the same as in the prior art and the battery has only a small slope of the voltage with respect to the charge amount without using such a configuration, one or more batteries can be charged or By discharging, it is possible to suppress exceeding the upper limit or lower limit voltage allowed by the system by changing the voltage. However, since the slope of the voltage with respect to the charge amount is uniformly small, the decrease in the battery capacity that can be charged and discharged compared with the improvement of the input / output characteristics is compared with the battery configuration in which the slope of the voltage with respect to the charge amount is uniformly large. Become bigger. On the other hand, when the battery capacities of all the batteries are substantially the same as in the past and the slope of the voltage with respect to the charge amount is uniformly large, the voltage change due to charging / discharging is large, and the width of the charge state that can be input and output is narrow It becomes. On the other hand, like the first battery having the above-described configuration, the battery has a battery capacity larger than the battery capacity of the second battery, and includes a battery whose voltage gradient with respect to the charge amount is larger than that of the second battery. By controlling the charge amount of the first battery, it is possible to control the voltage without reducing the capacity of the second battery, and thus to control the input / output characteristics constrained by the upper and lower system voltages. Become. In particular, the effect is high when the upper limit voltage during charging and the lower limit voltage during discharging are biased, or when the resistance increase during charging or discharging is biased. This configuration is superior in terms of physique, weight, and material cost compared to the case where the capacity of all batteries is increased.

It is a system configuration figure concerning one embodiment. It is a figure which shows the input / output characteristic of the battery cell concerning one Embodiment. It is a figure which shows the voltage with respect to the charge amount of the battery cell concerning a comparative example. It is a figure which shows the voltage with respect to the charge amount of the battery cell concerning one Embodiment. It is the table | surface which compared ratio of the input-output electric power of one Embodiment and a comparative example. It is the figure which showed the input / output characteristic after deterioration of a battery cell.

  Hereinafter, an embodiment in which a power storage device according to the present invention is applied to an auxiliary battery for a vehicle will be described with reference to the drawings.

  As shown in FIG. 1, the assembled battery 10 serves as a power source for auxiliary machines and the like mounted on the vehicle. The assembled battery 10 is a series connection body of battery cells (unit cells) as “unit batteries”, and the terminal voltage thereof is a predetermined voltage (for example, 12 V) suitable for driving the auxiliary machine. In the present embodiment, the number of battery cells constituting the assembled battery 10 is three. In the present embodiment, each of these battery cells is hereinafter referred to as i-th battery cell Ci (i = 1 to 3). In addition, as shown in FIG. 2, the battery capacity of the first battery cell C1 (corresponding to the first battery) is the other second battery cell C2 (corresponding to the second battery) or the third battery cell C3 (corresponding to the second battery). This corresponds to approximately twice the battery capacity. In addition, the slope of the voltage with respect to the charge amount of the first battery cell C1 is greater than the slope of the voltage with respect to the charge amount of the battery cells C2 and C3. Incidentally, in this embodiment, a lithium ion secondary battery is used as the battery cell.

  The signal line L (i + 1) is connected to the positive terminal of the i-th battery cell Ci, and the signal line Li is connected to the negative terminal of the i-th battery cell Ci. That is, except for the signal lines L1 and L4, the signal line on the negative terminal side of the battery cell on the high potential side and the signal line on the positive terminal side of the battery cell on the low potential side among the adjacent battery cells are shared. ing.

  The voltage between the terminals of the i-th battery cell Ci is controlled by the control circuit 12 (discharge depth control means) via the signal lines Li, L (i + 1) and the i-th low-pass filter RCi configured to include a resistor and a capacitor. Is taken in. Here, the i-th low-pass filter RCi is provided to remove high-frequency noise superimposed on the voltage signal and increase the detection accuracy of the voltage across the terminals of the i-th battery cell Ci.

  The i-th battery cell Ci is connected in parallel with an i-th Zener diode ZDi for avoiding an overvoltage being applied to the i-th battery cell Ci. More specifically, the cathode of the i-th Zener diode ZDi is connected to the signal line L (i + 1), and the anode of the i-th Zener diode ZDi is connected to the signal line Li.

  Both ends of the i-th battery cell Ci can be connected to both ends of the capacitor 16 and the voltage sensor 24 via a converter 14 including an i-th p-side switching element Spi and an i-th n-side switching element Sni. Yes. Specifically, the capacitor 16 and the voltage sensor 24 are connected in parallel, the first electric path Lα is connected to one end of the capacitor 16 and the voltage sensor 24, and the second electric path is connected to the other end. Lβ is connected. The path connecting the positive terminal of the i-th battery cell Ci and the first electrical path Lα is provided with an i-th p-side switching element Spi that opens and closes this path, and the negative terminal of the i-th battery cell Ci. And a second electrical path Lβ are provided with an i-th n-side switching element Sni that opens and closes the path.

  In the present embodiment, a pair of N-channel MOSFETs whose sources are short-circuited with each other are used as the switching elements Spi and Sni. Here, the sources are short-circuited in order to facilitate the on operation (close operation) or the open operation (off operation) of the pair of N-channel MOSFETs. In other words, since the N-channel MOSFET is turned on / off by the gate voltage that is the gate potential with respect to the source, the potentials of the sources of the pair of N-channel MOSFETs can be made the same by short-circuiting the sources. The operation can be performed by a single opening / closing operation signal (voltage signal).

  The side of the capacitor 16 to which the first electrical path Lα is connected is connected to one end of the connector 18 via the first cutoff switching element Q1. The other end of the capacitor 16 is connected to the other end of the connector 18 via the second cutoff switching element Q2 on the side where the second electrical path Lβ is connected. Here, the connector 18 is an output terminal for outputting the voltage across the capacitor 16 to the external load 20. For example, another battery, an in-vehicle auxiliary machine (generator, electrical component), or the like is connected to the connector 18. In the present embodiment, the first cutoff switching element Q1 and the second cutoff switching element Q2 are connected to each other in the same manner as the i-th p-side switching element Spi and the i-th n-side switching element Sni. Is used as a pair of N-channel MOSFETs.

  The control circuit 12 is mainly composed of a microcomputer, and takes in the voltage between the terminals of the i-th battery cell Ci, takes in the detected value of the voltage sensor 24 for detecting the voltage of the assembled battery 10 as a whole, and connects from the capacitor 16 to the connector. The detection value of the current sensor 22 that detects the load current flowing to the external load 20 via 18 is acquired. The control circuit 12 also turns on / off the i-th p-side switching element Spi and the i-th n-side switching element Sni via the i-th drive circuit DUi corresponding to the i-th battery cell Ci, The cutoff switching element Q1 and the second cutoff switching element Q2 are turned on / off.

  In the power storage device 100 configured as described above, the charging process can be performed individually for each battery cell. For example, when the charging process of the first battery cell C1 is performed, the second and third p-side switching elements Sp2 to Sp3 and the second and third n-side switching elements Sn2 to Sn3 are turned off. The first p-side switching element Sp1 and the first n-side switching element Sn1 are turned on. As a result, a closed circuit including the capacitor 16, the first electrical path Lα, the first p-side switching element Sp1, the first battery cell C1, the first n-side switching element Sn1, and the second electrical path Lβ is formed. The electric energy stored in the capacitor 16 is discharged and the first battery cell C1 is charged.

  FIG. 3 shows the transition of the voltage with respect to the charge amount when the slopes of the voltages with respect to the battery capacity and the charge amount of the three battery cells are equal to each other. In FIG. 3, “1 series” indicates a voltage with respect to the amount of charge by a single second (third) battery cell C2 (C3), and “2 series” indicates two second (third) battery cells. The voltage with respect to the charge amount when C2 (C3) is connected in series is shown, and “3 series” is the charge amount when three second (third) battery cells C2 (C3) are connected in series. The voltage is shown.

  In such a configuration, since any second (third) battery cell C2 (C3) has the same input / output characteristics, the voltage width at which any second (third) battery cell C2 (C3) can be input / output is It stays in the same range. For this reason, the influence of the voltage of one second (third) battery cell C2 (C3) on the voltage of the assembled battery 10 (the entire assembled battery) is small. For example, the voltage of the assembled battery 10 in “3 series” Is difficult to control with the voltage of one second (third) battery cell C2 (C3).

  FIG. 4 is a diagram illustrating a voltage with respect to a charge amount of the battery cell according to the present embodiment. In the figure, “1 series” indicates the voltage with respect to the single charge amount of the battery cells C2 and C3, and “2 series” indicates the voltage with respect to the charge amount when the battery cells C2 and C3 are connected in series. ing. "3 series" has shown the voltage with respect to the charge amount of the assembled battery 10 whole (battery cell C1-C3). Further, in FIG. 4, the “1 series” and “2 series” charge amounts at the time of “high voltage” are larger than the “1 series” and “2 series” charge amounts at the time of “low voltage”. However, it shows that the battery cells C2 and C3 are used in a range (second predetermined range) in which the charge amount of the first battery cell C1 is large.

  In the present embodiment, the battery capacity of the first battery cell C1 is greater than the battery capacity of the battery cells C2 and C3, and the slope of the voltage with respect to the charge amount is greater than the slope of the voltage with respect to the charge amount of the battery cells C2 and C3. ing. Thereby, the voltage change depending on the charge amount of the first battery cell C1 is larger than the voltage change depending on the charge amount of the battery cells C2 and C3. As a result, as shown in FIG. 4, the influence on the voltage of the assembled battery 10 due to the change in the voltage of the first battery cell C1 is larger than the influence on the voltage of the assembled battery 10 due to the change in the voltage of the battery cells C2 and C3. Become. Therefore, in FIG. 3, it is necessary to change the voltage while changing the charge amount to allow a reduction in the chargeable / dischargeable battery capacity, whereas in the assembled battery 10, the battery is controlled by controlling the charge amount of C <b> 1. The voltage can be changed without a decrease in capacity.

  Control of the voltage of the power storage device 100 by the control circuit 12 will be described. In FIG. 4, a predetermined range in which the charge amount of the first battery cell C1 is close to the lower limit, that is, a predetermined range in which the voltage of the battery pack 10 is low is set as the first predetermined range, and the charge amount of the first battery cell C1 is A predetermined range close to the upper limit, that is, a predetermined range in which the voltage of the assembled battery 10 becomes a high voltage is set as the second predetermined range.

  Here, for example, it is assumed that the charge amount of the first battery cell C1 is currently in the first predetermined range, and that the state is within the second predetermined range from this state. In this case, the first battery cell C1 alone is charged to reduce the depth of discharge of the first battery cell C1, and control is performed so that the charge amount of the first battery cell C1 falls within the second predetermined range. Thereby, the voltage of the 1st battery cell C1 can be raised, and by extension, the voltage of the assembled battery 10 (power storage device 100) can also be raised.

  On the other hand, when the charge amount of the first battery cell C1 is currently within the second predetermined range, and when it is within the first predetermined range from this state, the following control is performed. By charging the battery cells C2 and C3 in a state where the charge amount of the assembled battery 10 is reduced (for example, the SOC of the first battery cell C1 is 50%, the SOC of the battery cells C2 and C3 is 0%), The depth of discharge of the battery cells C2 and C3 is reduced, and the first battery cell C1 is not charged. By this control, the charge amount of the first battery cell C1 is relatively decreased with respect to the charge amounts of the battery cells C2 and C3. As a result, electric power is supplied to the in-vehicle auxiliary equipment connected to the connector 18 without charging the first battery cell C1, so that the voltage of the first battery cell C1 is also reduced due to the decrease in the SOC of the first battery cell C1. As a result, the voltage of the assembled battery 10 also decreases.

  Note that the voltage range of the power storage device 100 is adjusted by adjusting the amount of charge of the first battery cell C1 by performing the above-described control so as to be within the upper limit voltage and the lower limit voltage allowed by the system in which the power storage device 100 is mounted. Is controlled.

  With the configuration described above, the power storage device 100 according to the present embodiment has the following effects.

  By including a battery having a battery capacity larger than the battery capacity of the battery cells C2 and C3 and having a voltage gradient with respect to the charge amount larger than that of the battery cells C2 and C3, like the first battery cell C1, By controlling the voltage of the first battery having a large battery capacity by controlling the depth of discharge or the like, it is possible to control the voltage of the power storage device 100 and thus to control the input / output characteristics of the power storage device 100.

  The control circuit 12 controls the discharge depth of the first battery cell C1 individually, so that the charge amount of the first battery cell C1 is controlled to be within the first predetermined range or the second predetermined range. For example, it is assumed that the amount of charge of the first battery cell C1 is currently in the first predetermined range, and the state falls within the second predetermined range from this state. In this case, the first battery cell C1 alone is charged to reduce the depth of discharge of the first battery cell C1, and the charge amount of the first battery cell C1 is controlled to fall within the second predetermined range. . Thereby, the voltage of the 1st battery cell C1 can be raised, and by extension, the voltage of the assembled battery 10 (power storage device 100) can also be raised.

  -The charge amount of the 1st battery cell C1 is changed so that the voltage of the electrical storage apparatus 100 may become between an upper limit voltage and a lower limit voltage. For this reason, the voltage of the electrical storage device 100 is stored so as to be between the upper limit voltage and the lower limit voltage through a change in the charge amount of the first battery cell C1, and charging / discharging within the range can be handled.

  When the battery capacity of the first battery cell C1 is close to the battery capacity of the battery cells C2 and C3, it depends on the voltage change depending on the charge amount of the first battery cell C1 and the charge amount of the battery cells C2 and C3. The difference from the voltage change is reduced. In addition, there is less room for changing the charge amount of the first battery cell C1 relative to the charge amounts of the battery cells C2 and C3. For this reason, the change in the voltage of the first battery cell C1 does not significantly affect the voltage of the power storage device 100, and it may be difficult to control the voltage of the power storage device 100 only by controlling the voltage of the first battery cell C1. There is sex. On the other hand, the voltage capacity of the first battery cell C1 can be widened by setting the battery capacity of the first battery cell C1 to approximately twice the battery capacity of the battery cells C2 and C3 as in this embodiment. be able to. Thereby, the influence on the voltage of power storage device 100 due to the change in voltage of first battery cell C1 is increased, and the voltage of power storage device 100 can be controlled only by controlling the voltage of first battery cell C1.

  -Originally, in a power storage device including an assembled battery composed only of battery cells having a small voltage gradient with respect to the charge amount such as battery cells C2 and C3, the voltage of the entire assembled battery is reduced because the voltage gradient with respect to the charge amount is small. It is difficult to detect the amount of charge based on it. However, the voltage with respect to the charge amount of the assembled battery 10 as a whole can be obtained by making the slope of the voltage with respect to the charge amount of the first battery cell C1 larger than the slope of the voltage with respect to the charge amount of the battery cells C2 and C3 as in this embodiment. And the charge amount based on the voltage of the entire assembled battery 10 can be easily detected. Furthermore, it is possible to calculate the charge amounts of the battery cells C2 and C3 corresponding to the detected voltage of the power storage device 100.

  -When the first battery cell C1 having a large battery capacity and the battery cells C2 and C3 having a small battery capacity are connected in series, the input / output power of the battery is smaller than that of an assembled battery in which battery cells having the same battery capacity are connected in series. The adjustment range is improved. As a specific example, as shown in FIG. 5, when the first battery cell C1 having a large battery capacity and C2 having a small battery capacity are connected in series (Example 1), the second battery cell C2 having a small battery capacity is used. The comparison with the case where two are connected in series (comparative example) is given. At this time, in Example 1, the SOC (State of charge) of the second battery cell C2 having a small battery capacity is fixed at 30%. The amount of change in power when the SOC of the first battery cell C1 is charged from 10% to 30% is detected. In the comparative example, the SOC of one second battery cell C2 is fixed at 30%, and the amount of power change when the SOC of the other first battery cell C1 is charged from 10% to 30% is detected. . Here, the output ratio in FIG. 5 is a battery cell that can be output at a voltage of 4 V or more for 10 seconds when the SOC of the battery cell to be charged (for example, the first battery cell C1 in Example 1 is 10%) is 10%. It shows how much the output power of the entire battery cell that can be output at a voltage of 4 V or more for 10 seconds is improved when the SOC of the battery cell to be charged is 30% as compared with the overall output power. The input ratio is the SOC of the battery cell to be charged compared to the input power of the entire battery cell that can be input at a voltage of 7.5 V or less for 10 seconds when the SOC of the battery cell to be charged is 10%. Shows how much the input power of the entire battery cell that can be charged with a voltage of 7.5 V or less for 10 seconds at 30% is improved. Here, in order to facilitate the comparison, the input ratio is set to −15% in the first embodiment and the comparative example. At this time, it was found that the output ratio of Example 1 was improved as compared with the comparative example.

  In addition, the said embodiment can also be changed and implemented as follows.

  -In the said embodiment, the lithium ion secondary battery was applied to battery cell C1-C3. About this, it is not limited to a lithium ion secondary battery, You may connect a different kind of battery in series. Note that the order of series can be arbitrarily changed in consideration of the thermal design, physique, and weight of the battery.

  The battery capacity of the first battery cell C1 was approximately twice the battery capacity of the battery cells C2 and C3. In this regard, if the battery capacity of the first battery cell C1 is larger than the battery capacity of the battery cells C2 and C3, it need not be limited to approximately twice.

  In the above embodiment, three battery cells are connected in series. About this, you may be comprised with four or more battery cells. Even with this configuration, the same operations and effects as in the above-described embodiment can be achieved.

  In the above embodiment, the charge amount of the first battery cell C1 is currently in the second predetermined range, and when the charge amount falls within the first predetermined range from this state, the battery cells C2 and C3 are charged. The first battery cell C1 was not charged. In this regard, a configuration in which the above control is not performed can be adopted as long as the circuit can discharge the battery cells individually. Specifically, the charge amount of the first battery cell C1 may be decreased by discharging the first battery cell C1 whose charge amount is within the second predetermined range alone, and may be within the first predetermined range.

  In the above embodiment, the control circuit 12 controls the voltage of the power storage device 100 by performing control to increase the charge amount of the first battery cell C1 or the battery cells C2 and C3. In this regard, the above control is not performed when the amount of charge (hereinafter referred to as the discharge rate) decreased by self-discharge per unit time of the first battery cell C1 is larger than the discharge rate of the battery cells C2 and C3. A configuration can also be adopted. The discharge rate is increased by using a material that stores electricity by forming an electric double layer, such as activated carbon, as compared with a material into which lithium ions such as graphite are inserted. At this time, the self-discharge is a change in battery capacity when the battery cell is left at 25 ° C. for about one week. In addition, self-discharge includes local battery reaction in the battery cell, and the size can be controlled by changing the temperature and voltage. It can be adjusted. In such a configuration, when self-discharge occurs over time, the charge amount of the first battery cell C1 decreases faster than the charge amounts of the battery cells C2 and C3. And when the charge amount of battery cell C2, C3 is returned to the value before self-discharge by the charge performed after self-discharge, the charge amount of the 1st battery cell C1 is smaller than the value before self-discharge. Here, when the charge amount of the first battery cell C1 is initially within the second predetermined range, the charge amount of the first battery cell C1 is increased from the second predetermined range by the self-discharge and charging as described above. Gradually shift to one predetermined range side. Therefore, the input / output characteristics of power storage device 100 can be changed over time without performing the above control.

  For example, as shown in FIG. 6, the above-mentioned another example is a case where the usage area of the positive electrode charge amount increases with deterioration as in the case where lithium iron phosphate is used for the positive electrode and graphite (graphite) is used for the negative electrode. When the low positive charge area of the large positive electrode becomes unusable due to deterioration and the output resistance of the battery cell is reduced, the output characteristics are reduced by lowering the voltage when the input / output resistance balance is biased. The balance can be optimized by improving the input characteristics.

  Moreover, in the said another example, the voltage of the electrical storage apparatus 100 after deterioration of battery capacity is used with a high voltage in order to ensure required discharge capacity. For this reason, the voltage of power storage device 100 after deterioration is approaching an upper limit voltage that can be tolerated by a system including power storage device 100. On the other hand, as described above, the charge amount of the first battery cell C <b> 1 decreases due to self-discharge with time, and thus the voltage of the power storage device 100 decreases. Thus, in order to ensure the discharge capacity which can be used by deterioration, even when using it in a high charge state, it is possible to reduce a voltage and to secure input power by having the said structure.

  In the above embodiment, the control circuit 12 controls the voltage of the power storage device 100 by performing control to increase the charge amount of the first battery cell C1 or the battery cells C2 and C3. About this, when the discharge rate of the 1st battery cell C1 is smaller than the discharge rate of battery cell C2, C3, the structure which does not implement the said control is also employable. In such a configuration, when self-discharge occurs over time, the charge amount of the first battery cell C1 decreases later than the charge amounts of the battery cells C2 and C3. And when the charge amount of battery cell C2, C3 is returned to the value before self-discharge by the charge performed after self-discharge, the charge amount of the 1st battery cell C1 is larger than the value before self-discharge. Here, when the charge amount of the first battery cell C1 is initially within the first predetermined range, the charge amount of the first battery cell C1 is increased from the first predetermined range by the self-discharge and the charge as described above. (2) Gradually shift to a predetermined range. Therefore, the input / output characteristics of power storage device 100 can be changed over time without performing control for changing the charge amount of first battery cell C1.

C1 ... first battery cell, C2 ... second battery cell, C3 ... third battery cell, 10 ... assembled battery, 100 ... power storage device.

Claims (3)

In a power storage device (10, 100) including a plurality of batteries (C1 to C3) connected in series,
The plurality of batteries include a first battery (C1) and a second battery (C2, C3),
Larger than the battery capacity before Symbol first battery cell capacity of the second battery,
Much larger than the slope of the voltage for charging the amount of inclination of a voltage to the pre-Symbol charge amount of the first battery corresponds to the amount of charge of the first battery of the second battery,
By controlling the depth of discharge of the first battery individually, the discharge depth control means (12) controls the charge amount of the first battery so as to be within the first predetermined range near the lower limit or the second predetermined range near the upper limit. ) power storage device, characterized in that it comprises a. The power storage device according to claim 1, characterized in that the charge amount of the first battery to be between voltage and upper limit voltage and lower limit voltage of the electric storage device changes. 3. The power storage device according to claim 1, wherein the battery capacity of the first battery is approximately twice the battery capacity of the second battery.

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