Power storage device

Abstract

In an electricity storage device including a plurality of batteries connected in series, the charging rates of the batteries are equalized. [Solution] The energy storage device PS is configured to include series-connected batteries Ba1 to Ba3 and batteries Bb1 to Bb3, cell balance circuits CVa and CVb that individually discharge the batteries Ba1 to Ba3 and batteries Bb1 to Bb3, and a control unit Cnt that controls the operation of the cell balance circuits CVa and CVb so that, when cell balancing processing is performed, at least one cell balancing target battery B', including battery B that satisfies the full charge condition among batteries Ba1 to Ba3 and batteries Bb1 to Bb3, is discharged by an amount equivalent to the charging rate SOCc.

Description

[Technical Field]

[0001] The present invention relates to an electricity storage device including a plurality of batteries connected in series. [Background technology]

[0002] In order to prevent the usable capacity of the entire power storage device from being limited, some power storage devices discharge each battery individually so that the charging rates (the ratio [%] of the remaining capacity of the batteries to the fully charged capacity of the batteries) of the series-connected batteries are equalized. Related technology is disclosed in Patent Document 1.

[0003] Generally, in batteries such as lithium iron phosphate batteries that have SOC-OCV characteristics with a plateau region in which the amount of change in open-circuit voltage per unit charge rate is relatively small, if the actual charge rate is within the plateau region, it may be difficult to uniquely determine the charge rate from the open-circuit voltage.

[0004] Therefore, in the above-mentioned energy storage device, when equalizing the charging rates of batteries having SOC-OCV characteristics with a plateau region, it becomes difficult to uniquely determine the charging rate from the open circuit voltage, making it difficult to estimate the amount of deviation between each charging rate, and there is a risk that it will not be possible to equalize the charging rates of each battery. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2021-36734 Summary of the Invention [Problem to be solved by the invention]

[0006] An object according to one aspect of the present invention is to equalize the charging rates of the batteries in an electricity storage device including a plurality of batteries connected in series. [Means for solving the problem]

[0007] One aspect of the present invention is an energy storage device that includes a plurality of batteries connected in series, a cell balancing circuit that individually discharges the plurality of batteries, and a control unit that controls the operation of the cell balancing circuit during cell balancing processing so that at least one cell balancing target battery, including a battery that satisfies a full charge condition, among the plurality of batteries is discharged at a predetermined charge rate.

[0008] As a result, the charging rates of the cell balancing target batteries decrease by a predetermined amount when cell balancing is performed, and batteries other than the cell balancing target batteries are more likely to be identified as cell balancing target batteries in subsequent charging processes.By repeatedly performing cell balancing and charging, the charging rates of all batteries can be made to converge within a certain range.In other words, in an energy storage device having multiple batteries connected in series, the charging rates of the batteries can be made equal.

[0009] The predetermined charging rate may be equal to or greater than the maximum difference between the maximum and minimum charging rates of the plurality of batteries that occur at regular intervals.

[0010] Furthermore, when the control unit determines that the charging rates of the plurality of batteries are equalized to a certain extent and when a plurality of cell balancing target batteries are identified, the control unit may be configured to reduce the predetermined charging rate corresponding to each of the cell balancing target batteries in accordance with the voltage of each of the cell balancing target batteries.

[0011] This makes it possible to prevent the charging rates of the cells being balanced from being lowered more than necessary, even when the charging rates of the batteries are equalized to a certain extent. This makes it possible to prevent the difference between the maximum and minimum charging rates of the batteries from diverging, and makes it possible to accurately equalize the charging rates of the batteries.

[0012] The control unit may be configured not to execute the next cell balancing process until a predetermined time has elapsed after the cell balancing process has been executed, even if there is a battery that satisfies a full charge condition.

[0013] Each of the plurality of batteries may be a lithium iron phosphate battery.

[0014] The control unit may also be configured to identify the cell balancing target battery based on voltages of the plurality of batteries when a predetermined time has elapsed since the charging process for the plurality of batteries ended.

[0015] The power storage device may also include a first assembled battery in which some of the plurality of batteries are connected in series, and a second assembled battery in which the remaining plurality of batteries are connected in series, and the control unit may be configured to perform the cell balancing process separately for the first assembled battery and the second assembled battery.

[0016] This allows the voltage of the entire first assembled battery to approach the voltage of the entire second assembled battery, thereby preventing an increase in the reflux current flowing between the first assembled battery and the second assembled battery.

[0017] The control unit may be configured to execute the cell balancing process on a plurality of the cell balancing target batteries.

[0018] The cell balance circuit may be a passive cell balance circuit. [Effects of the Invention]

[0019] According to the present invention, in an electricity storage device including a plurality of batteries connected in series with each other, the charging rates of the batteries can be equalized. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a diagram illustrating an example of a power storage device according to an embodiment. [Figure 2] FIG. 10 is a diagram for explaining constant current / constant voltage charging control. [Figure 3] 10 is a flowchart illustrating an example of an operation of a control unit when performing cell balancing processing. [Figure 4] FIG. 10 is a diagram illustrating an example of a method for identifying a cell balancing target battery. [Figure 5] FIG. 10 is a diagram showing an example of the correspondence relationship between the number of charging processes when charging processes and cell balancing processes are repeatedly executed alternately and the difference between the maximum and minimum charging rates of each battery. [Figure 6] FIG. 1 is a diagram showing an example of SOC-OCV characteristics. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, the embodiments will be described in detail with reference to the drawings.

[0022] FIG. 1 is a diagram illustrating an example of a power storage device according to an embodiment.

[0023] 1 is mounted on a vehicle such as an electric forklift or an electric car, and supplies power to a load Lo such as a traction motor or electrical equipment mounted on the vehicle. In addition, power output from a charger Ch provided outside the vehicle is supplied to the power storage device PS.

[0024] The power storage device PS also includes a battery pack BPa (first battery pack), a battery pack BPb (second battery pack), cell balance circuits CVa and CVb, voltage detection units Sva1 to Sva3, voltage detection units Svb1 to Svb3, current detection units Sia and Sib, relays Rea, Reb, ReCh, and ReLo, a memory unit Str, and a control unit Cnt.

[0025] The battery pack BPa includes batteries Ba1-Ba3 connected in series, and the battery pack BPb includes batteries Bb1-Bb3 connected in series. Each of the batteries Ba1-Ba3 and Bb1-Bb3 is composed of one or more secondary batteries, such as lithium iron phosphate batteries or ternary lithium ion batteries. The batteries Ba1-Ba3 and Bb1-Bb3 have different rates of decrease per unit time in their charge rates (the percentage [%] of battery B's remaining capacity relative to its full charge capacity) due to self-discharge, and different rates of decrease per unit time in their full charge capacities due to aging. The difference between the maximum and minimum charge rates when neither charging nor discharging is occurring increases over time. In other words, the charge rates of the batteries Ba1-Ba3 and Bb1-Bb3 vary due to self-discharge and aging. Furthermore, when the battery packs BPa and BPb are not distinguished, they are simply referred to as the battery pack BP, and when the batteries Ba1-Ba3 and Bb1-Bb3 are not distinguished, they are simply referred to as the batteries B. When the cell balance circuits CVa and CVb are not distinguished, they are simply referred to as the cell balance circuits CV. When the voltage detection units Sva1-Sva3 and voltage detection units Svb1-Svb3 are not distinguished, they are simply referred to as the voltage detection units Sv. Furthermore, the number of battery packs BP connected in parallel to each other in the power storage device PS is not particularly limited and may be three or more. Furthermore, the number of batteries B connected in series to each other in the battery pack BP is not particularly limited and may be two or four or more.

[0026] The relays Rea, Reb, ReCh, and ReLo are each composed of a semiconductor relay or an electromagnetic relay, and the control unit Cnt controls the conductive and cutoff states. For example, when the relays Rea, Reb, ReCh, and ReLo are each in the cutoff state, the battery packs BPa and BPb are not connected in parallel with each other, power cannot be supplied from the battery packs BPa and BPb to the load Lo, and power cannot be supplied from the charger Ch to the battery packs BPa and BPb. When the relays Rea and Reb switch from the cutoff state to the conductive state from this state, the battery packs BPa and BPb are connected in parallel with each other. Furthermore, when the relay ReLo switches from the cutoff state to the conductive state, power can be supplied from the battery packs BPa and BPb to the load Lo, and power can be regenerated from the load Lo to the battery packs BPa and BPb. Alternatively, when the relay ReCh switches from the cutoff state to the conductive state, power can be supplied from the charger Ch to the battery packs BPa and BPb.

[0027] The current detection unit Sia is configured with a shunt resistor or the like, detects the current Ia flowing through the battery pack BPa (batteries Ba1 to Ba3), and sends the detected value of the current Ia to the control unit Cnt.

[0028] The current detection unit Sib is configured with a shunt resistor or the like, detects the current Ib flowing through the battery pack BPb (batteries Bb1 to Bb3), and sends the detected current Ib to the control unit Cnt. When there is no particular distinction between the current detection units Sia and Sib, they are simply referred to as the current detection unit Si. It is also assumed that the power storage device PS is provided with the same number of current detection units Si as the battery pack BP.

[0029] That is, the positive terminal of battery Ba1 is connected to the positive terminal of battery Bb1, the negative terminal of battery Ba1 is connected to the positive terminal of battery Ba2, and the negative terminal of battery Ba2 is connected to the positive terminal of battery Ba3. The negative terminal of battery Ba3 is connected to the negative terminal of battery Bb3 via current detection unit Sia, relay Rea, relay Reb, and current detection unit Sib. The positive terminal of battery Bb3 is connected to the negative terminal of battery Bb2, and the positive terminal of battery Bb2 is connected to the negative terminal of battery Bb1. The connection point between the positive terminal of battery Ba1 and the positive terminal of battery Bb1 is connected to the positive terminal of load Lo, and the connection point between relay Rea and relay Reb is connected to the negative terminal of load Lo via relay ReLo. Furthermore, when the power storage device PS and the charger Ch are connected to each other via the charging cable Ca, the connection point between the positive terminal of the battery Ba1 and the positive terminal of the battery Bb1 is connected to the positive terminal of the charger Ch, and the connection point between the relay Rea and the relay Reb is connected to the negative terminal of the charger Ch via the relay ReCh.

[0030] The voltage detection units Sva1 to Sva3 are each composed of an IC (Integrated Circuit) or the like. The voltage detection unit Sva1 detects the voltage of battery Ba1 and sends the detected value, voltage Va1, to the control unit Cnt using CAN (Controller Area Network) communication or the like. The voltage detection unit Sva2 detects the voltage of battery Ba2 and sends the detected value, voltage Va2, to the control unit Cnt using CAN communication or the like. The voltage detection unit Sva3 detects the voltage of battery Ba3 and sends the detected value, voltage Va3, to the control unit Cnt using CAN communication or the like. Furthermore, the voltage detection unit Sva1 is driven by power supplied from battery Ba1, the voltage detection unit Sva2 is driven by power supplied from battery Ba2, and the voltage detection unit Sva3 is driven by power supplied from battery Ba3. Therefore, if there is variation in the current consumption of each of the voltage detection units Sva1 to Sva3, there is a possibility that the driving of the voltage detection units Sva1 to Sva3 will cause variation in the charging rates of the batteries Ba1 to Ba3.

[0031] The voltage detection units Svb1 to Svb3 are each composed of an IC or the like. The voltage detection unit Svb1 detects the voltage of battery Bb1 and sends the detected value, voltage Vb1, to the control unit Cnt using CAN communication or the like. The voltage detection unit Svb2 detects the voltage of battery Bb2 and sends the detected value, voltage Vb2, to the control unit Cnt using CAN communication or the like. The voltage detection unit Svb3 detects the voltage of battery Bb3 and sends the detected value, voltage Vb3, to the control unit Cnt using CAN communication or the like. Furthermore, the voltage detection unit Svb1 is driven by power supplied from battery Bb1, the voltage detection unit Svb2 is driven by power supplied from battery Bb2, and the voltage detection unit Svb3 is driven by power supplied from battery Bb3. Therefore, if there is variation in the current consumption of each of the voltage detection units Svb1 to Svb3, there is a possibility that the driving of the voltage detection units Svb1 to Svb3 will cause variation in the charging rates of the batteries Bb1 to Bb3.

[0032] The cell balancing circuit CVa includes resistors Ra1-Ra3 and switches SWa1-SWa3. That is, the cell balancing circuit CVa is a passive cell balancing circuit that individually discharges the batteries Ba1-Ba3 by causing the output power of the batteries Ba1-Ba3 to be consumed by the resistors Ra1-Ra3. The switches SWa1-SWa3 are configured, for example, with MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) and are controlled between a cutoff state and a conductive state by a control unit Cnt.

[0033] That is, one terminal of resistor Ra1 is connected to the positive terminal of battery Ba1, the other terminal of resistor Ra1 is connected to one terminal of switch SWa1, and the other terminal of switch SWa1 is connected to one terminal of resistor Ra2 and the junction between the negative terminal of battery Ba1 and the positive terminal of battery Ba2. The other terminal of resistor Ra2 is connected to one terminal of switch SWa2, and the other terminal of switch SWa2 is connected to one terminal of resistor Ra3 and the junction between the negative terminal of battery Ba2 and the positive terminal of battery Ba3. The other terminal of resistor Ra3 is connected to one terminal of switch SWa3, and the other terminal of switch SWa3 is connected to the negative terminal of battery Ba3. For example, when switches SWa1 and SWa2 are in a conducting state and switch SWa3 is in a cutoff state, resistor Ra1 is connected in parallel to battery Ba1 and resistor Ra2 is connected in parallel to battery Ba2, causing batteries Ba1 and Ba2 to discharge and reducing the charging rates of batteries Ba1 and Ba2. Furthermore, when switch SWa3 is in a conducting state and switches SWa1 and SWa2 are in a cutoff state, resistor Ra3 is connected in parallel to battery Ba3, causing battery Ba3 to discharge and reducing the charging rate of battery Ba3.

[0034] Like the cell balance circuit CVa, the cell balance circuit CVb is a passive cell balance circuit that includes resistors Rb1-Rb3 and switches SWb1-SWb3 and individually discharges the batteries Bb1-Bb3. The switches SWb1-SWb3 are configured, for example, with MOSFETs and are controlled between a cutoff state and a conduction state by a control unit Cnt.

[0035] That is, one terminal of resistor Rb1 is connected to the positive terminal of battery Bb1, the other terminal of resistor Rb1 is connected to one terminal of switch SWb1, and the other terminal of switch SWb1 is connected to one terminal of resistor Rb2 and the junction between the negative terminal of battery Bb1 and the positive terminal of battery Bb2. Also, the other terminal of resistor Rb2 is connected to one terminal of switch SWb2, and the other terminal of switch SWb2 is connected to one terminal of resistor Rb3 and the junction between the negative terminal of battery Bb2 and the positive terminal of battery Bb3. Also, the other terminal of resistor Rb3 is connected to one terminal of switch SWb3, and the other terminal of switch SWb3 is connected to the negative terminal of battery Bb3. For example, when switches SWb1 and SWb2 are in a conducting state and switch SWb3 is in a cutoff state, resistor Rb1 is connected in parallel to battery Bb1 and resistor Rb2 is connected in parallel to battery Bb2, causing batteries Bb1 and Bb2 to discharge and lowering the charging rates of each of batteries Bb1 and Bb2.Furthermore, when switch SWb3 is in a conducting state and switches SWb1 and SWb2 are in a cutoff state, resistor Rb3 is connected in parallel to battery Bb3, causing battery Bb3 to discharge and lowering the charging rate of battery Bb3.

[0036] The storage unit Str is configured with a RAM (Random Access Memory) or a ROM (Read Only Memory), and stores the cell balancing target battery B' (to be described later) and the like.

[0037] The control unit Cnt is configured by a processor or a programmable device (an FPGA (Field Programmable Gate Array) or a PLD (Programmable Logic Device)), and performs power supply processing, charging processing, cell balancing processing, and the like.

[0038] <An example of power supply processing> When the control unit Cnt receives a power supply start instruction sent from outside the power storage device PS (for example, a control unit that controls the running of the vehicle), it starts a power supply process and turns on relays Rea, Reb, and ReLo and turns off relay Rech, thereby enabling power to be supplied from the assembled batteries BPa and BPb to the load Lo. Furthermore, during the power supply process, the control unit Cnt estimates the overall charging rate of the assembled batteries BPa and BPb and displays the estimated charging rate on a display (not shown) or the like, thereby informing the user of the state of the power storage device PS. When the control unit Cnt receives a power supply end instruction sent from outside the power storage device PS, it turns off relays Rea, Reb, ReLo, and ReCh, thereby enabling power to be supplied from the assembled batteries BPa and BPb to the load Lo and thereby terminating the power supply process.

[0039] <Example of charging process> When the control unit Cnt receives a charging instruction sent from outside the power storage device PS, it starts the charging process and switches relays Rea, Reb, and ReCh to a conductive state and relay ReLo to a cutoff state, thereby enabling power to be supplied from the charger Ch to the battery packs BPa and BPb, and then sends a current instruction to the charger Ch. The charger Ch outputs a current to the power storage device PS according to the current instruction sent from the power storage device PS. Furthermore, when at least one battery B among batteries Ba1 to Ba3 and batteries Bb1 to Bb3 reaches a fully charged state, the control unit Cnt sets the current instruction to zero and terminates the charging process. For example, the control unit Cnt performs constant-current constant-voltage charging control during the charging process.

[0040] Fig. 2 is a diagram illustrating constant-current, constant-voltage charging control. The horizontal axis of the two-dimensional coordinate system shown in Fig. 2(a) represents time, and the vertical axis represents current. The solid line in Fig. 2(a) shows an example of the current Ia flowing through the battery pack BPa during constant-current, constant-voltage charging control. The horizontal axis of the two-dimensional coordinate system shown in Fig. 2(b) represents time, and the vertical axis represents voltage. The solid line in Fig. 2(b) shows an example of the voltage Va1 of battery Ba1, the dashed line in Fig. 2(b) shows an example of the voltage Va2 of battery Ba2, and the dashed line in Fig. 2(b) shows an example of the voltage Va3 of battery Ba3. In the example shown in FIG. 2(b), the charging rates of batteries Ba1 to Ba3 vary over a certain period of time Tc due to variations in the natural discharge amounts of batteries Ba1 to Ba3, variations in the full charge capacities of batteries Ba1 to Ba3, and variations in the current consumption of voltage detection units Sva1 to Sv3, so that at the start time t0 of the constant current / constant voltage charging control, voltage Va1 is greater than voltage Va2, and voltage Va2 is greater than voltage Va3.

[0041] First, the control unit Cnt starts constant-current / constant-voltage charging control at time t0, and sends a current instruction indicating a constant current Ic to the charger Ch for each control period (a period shorter than the period from time t0 to time t1, for example, a clock period of the control unit Cnt) from time t0 to time t1. Then, as shown in Fig. 2(a), the constant current Ic flows through the battery pack BPa from time t0 to time t1, and as shown in Fig. 2(b), the voltages Va1 to Va3 gradually increase from time t0 to time t1.

[0042] Next, at time t1, when voltage Va1, of voltages Va1 to Va3, first becomes equal to or exceeds constant voltage Vc, control unit Cnt gradually decreases the current command for each control period so that voltage Va1 is maintained at constant voltage Vc from time t1 to time t2. Then, as shown in Figure 2(b), battery Ba1 is maintained at constant voltage Vc from time t1 to time t2, and current Ia flowing through battery pack BPa gradually decreases from time t1 to time t2 as shown in Figure 2(a).

[0043] Then, at time t2, when the current Ia flowing through the battery pack BPa becomes equal to or less than the termination current If, the control unit Cnt determines that battery Ba1 is fully charged, sets the current command to zero, and terminates the constant-current / constant-voltage charging control. As shown in FIG. 2(a), when the current Ia flowing through the battery pack BPa becomes zero at time t2, as shown in FIG. 2(b), at time t2, voltage Va1 drops by the voltage obtained by multiplying the internal resistance of battery Ba1 by the termination current If, voltage Va2 drops by the voltage obtained by multiplying the internal resistance of battery Ba2 by the termination current If, and voltage Va3 drops by the voltage obtained by multiplying the internal resistance of battery Ba3 by the termination current If. Thereafter, voltages Va1 to Va3 gradually decrease from time t2 until time t3, when the polarization of batteries Ba1 to Ba3 is eliminated.

[0044] <An example of cell balancing processing> At the end of the charging process, the control unit Cnt updates at least one cell balancing target battery B', including the battery B that satisfies the full charge condition, and then controls the operation of the cell balancing circuits CVa and CVb so that the updated cell balancing target battery B' is discharged by the charging rate SOCc (predetermined charging rate).

[0045] FIG. 3 is a flowchart showing an example of the operation of the control unit Cnt when performing cell balancing processing.

[0046] First, when the control unit Cnt determines that the charging process of the battery pack BP has ended (step Stp1: Yes), it identifies a new cell balancing target battery B' from among the batteries Ba1 to Ba3 and the batteries Bb1 to Bb3, and updates the cell balancing target battery B' by overwriting the cell balancing target battery B' stored in the memory unit Str with the identified cell balancing target battery B' (step Stp2).

[0047] Fig. 4 is a diagram illustrating an example of a method for identifying the cell balancing target battery B'. Note that the horizontal axis of the two-dimensional coordinate system shown in Fig. 4 represents batteries Ba1-Ba3 and Bb1-Bb3, and the vertical axis represents voltage. The points shown in Fig. 4 represent the voltages (closed circuit voltages) of batteries Ba1-Ba3 and Bb1-Bb3 at the end of the charging process. Furthermore, voltage Vα is set to be less than a constant voltage Vc, and voltage Vα is set to a value determined based on, for example, parameters related to the environment in which power storage device PS is used (such as temperature and the output current of charger Ch).

[0048] For example, at the end of the charging process, the control unit Cnt identifies, among the batteries Ba1 to Ba3 and the batteries Bb1 to Bb3, battery Ba1 as the battery B whose voltage is closest to the constant voltage Vc and satisfies the full charge condition, and battery Ba2 whose voltage is within the range from voltage Vα to the constant voltage Vc, as the new cell balancing target battery B'.

[0049] Next, in the flowchart shown in FIG. 3 , after executing step Stp2, the control unit Cnt controls the operation of the cell balancing circuits CVa and CVb so as to discharge the updated cell balancing target battery B′ stored in the memory unit Str by the amount corresponding to the charging rate SOCc (step Stp3). For example, when batteries Ba1 and Ba2 are stored in the memory unit Str as the updated cell balancing target battery B′, the control unit Cnt transitions the switches Sa1 and Sa2 from the cutoff state to the conduction state and leaves the switches Sa3 and Sb1 to Sb3 in the cutoff state. This allows current to flow from batteries Ba1 and Ba2 to resistors Ra1 and Ra2, thereby discharging batteries Ba1 and Ba2. Furthermore, when the voltage of battery Ba1 drops to a voltage lower by the amount corresponding to the charging rate SOCc, the control unit Cnt transitions the switch Sa1 from the conduction state to the cutoff state. When the voltage of battery Ba2 drops to a voltage lower by the amount corresponding to the charging rate SOCc, the control unit Cnt transitions the switch Sa2 from the conduction state to the cutoff state. For example, the control unit Cnt refers to information (SOC-OCV characteristics) that is stored in advance in the memory unit Str and indicates the correspondence between the state of charge of battery B and the open circuit voltage of battery B (the voltage of battery B when no current is flowing), and determines the state of charge that corresponds to the measured voltage of battery B. This allows the state of charge of each of batteries Ba1 and Ba2 to be reduced by the state of charge SOCc.

[0050] The control unit Cnt may execute step Stp2 immediately after executing step Stp2, or may execute step Stp3 at any timing after executing step Stp2. For example, the control unit Cnt may be configured to execute step Stp3 after executing step Stp2, such as when a cell balancing start instruction is received from the user or when a predetermined time has elapsed.

[0051] The charging rate SOCc may be set to a value equal to or greater than the maximum deviation in the charging rate of each battery B that occurs every certain time Tc due to variations in the natural discharge amount of the battery B, variations in the full charge capacity of the battery B, and variations in the current consumption of the voltage detection unit Sv, and may be set to a value equal to or greater than the difference between the maximum and minimum values ​​of the charging rates of each battery B when the certain time Tc has elapsed since the charging rates of each battery B were identical. The certain time Tc may be set to, for example, the interval (one day, one week, one month, etc.) between charging processes when the charging process is periodically repeated.

[0052] Furthermore, an upper limit may be set in advance for the charging rate SOCc so as not to exceed the allowable value of the charging rate of battery B. The allowable value of the charging rate of battery B is, for example, the charging rate corresponding to one segment when one or more segments corresponding to the charging rate of power storage device PS are displayed on a display (not shown).

[0053] In this manner, in the cell balancing process of this embodiment, the cell balancing target battery B' is updated each time the charging process is completed, and then, at any timing, the charging rate of the updated cell balancing target battery B' is reduced by the charging rate SOCc.

[0054] As a result, when performing cell balancing, the charging rate of the cell balancing target battery B' can be made closer to the charging rate of the batteries B other than the cell balancing target battery B'. Furthermore, by preventing a decrease in the charging rate of the batteries B other than the cell balancing target battery B' when performing cell balancing, the possibility that the batteries B other than the cell balancing target battery B' will be updated as the cell balancing target battery B' in subsequent charging processes can be increased. Therefore, by repeatedly performing charging and cell balancing, the cell balancing target battery B' and the batteries B other than the cell balancing target battery B' are periodically replaced, allowing the charging rate of each battery B to fluctuate within a certain range, and the difference ΔSOC between the maximum and minimum charging rates of each battery B to converge within a certain range. In other words, in a power storage device PS including multiple batteries B connected in series, the charging rates of the batteries B can be made equal.

[0055] Fig. 5 is a diagram showing an example of the relationship between the number of charging processes and the difference ΔSOC between the maximum and minimum charging rates of each battery B when charging processes and cell balancing processes are alternately and repeatedly executed. The horizontal axis of the two-dimensional coordinate system shown in Fig. 5 indicates the number of charging processes, and the vertical axis indicates the charging rate. Each point shown in Fig. 5 indicates the relationship between the number of charging processes and the difference ΔSOC. The charging rate SOCmin is set to be smaller than the charging rate SOCmax.

[0056] 5, the difference ΔSOC is relatively large before the charging process and the cell balancing process are repeatedly executed alternately (charging process: 0 times), and the difference ΔSOC gradually decreases as the number of charging processes increases from 1 to 6. Thereafter, the difference ΔSOC converges within a certain range from the charging rate SOCmin to the charging rate SOCmax as the number of charging processes increases from 7 to 50.

[0057] As a comparative example of this embodiment, consider a case where the charging rates of all batteries B are equalized at once using the SOC-OCV characteristics. In this case, each battery B needs to be discharged so that the open-circuit voltages of the batteries B match each other, which raises concerns that the charging rates of the batteries B cannot be equalized during charging or discharging. Furthermore, in order to accurately measure the open-circuit voltage, it is necessary to wait until the polarization that occurs in the battery B after charging or discharging is eliminated, which raises concerns that it takes a long time to equalize the charging rates. Thus, when the charging rates of all batteries B are equalized at once using the SOC-OCV characteristics, opportunities for equalizing the charging rates are limited.

[0058] On the other hand, in the cell balancing process of the embodiment, after updating the cell balancing target battery B', discharging of the cell balancing target battery B' is started at any timing, and the cell balancing target battery B' is discharged by the charging rate SOCc, so the cell balancing process can be performed whether the battery B is charging or discharging. This allows the frequency of cell balancing to be increased, and the charging rates can be equalized by limiting the current flowing from the battery B to the resistor R during cell balancing, which in turn allows the power storage device PS to be configured with less expensive components, thereby reducing the manufacturing cost of the power storage device PS.

[0059] Also, for example, it is assumed that battery B is composed of a lithium iron phosphate ion battery.

[0060] Fig. 6 is a diagram showing an example of the SOC-OCV characteristics of battery B, which is a lithium iron phosphate ion battery. The horizontal axis of the two-dimensional coordinate system shown in Fig. 6 represents the state of charge, and the vertical axis represents the voltage. The solid line in Fig. 6 shows an example of the SOC-OCV characteristics (curve) that show the correspondence between the state of charge and the open circuit voltage of battery B.

[0061] The SOC-OCV characteristics shown in Figure 6 have an unused region that indicates the range of charge rates when use of Battery B is prohibited, a plateau region that indicates the range of charge rates when the range of change in open-circuit voltage per unit charge rate is relatively small, and a steep region that indicates the range of charge rates when the range of change in open-circuit voltage per unit charge rate is relatively large. The minimum value of the unused region is 0%, the maximum value of the unused region is a value smaller than the minimum value of the plateau region, the maximum value of the plateau region is a value smaller than the minimum value of the steep region, and the maximum value of the steep region is 100%.

[0062] Thus, there is a concern that it may be difficult to uniquely estimate the charge rate from the open circuit voltage for Battery B, which has SOC-OCV characteristics with a plateau region.

[0063] As a comparative example of this embodiment, consider a case where the charging rates of all batteries B are equalized at once using the SOC-OCV characteristics, and each battery B has an SOC-OCV characteristic with a plateau region. In this case, if the charging rate corresponding to the measured open circuit voltage is within the plateau region, it becomes difficult to estimate the deviation amount of each charging rate, which raises the concern that the charging rates of each battery B cannot be equalized.

[0064] On the other hand, the cell balancing process of this embodiment can equalize the charging rates of the batteries B without estimating the amount of deviation between the charging rates, and therefore can equalize the charging rates of the batteries B even if the batteries B have SOC-OCV characteristics with a plateau region. In other words, the energy storage device PS of this embodiment is suitable for equalizing the charging rates of batteries B that have SOC-OCV characteristics with a plateau region, such as lithium iron phosphate batteries.

[0065] The present invention is not limited to the above-described embodiments, and various improvements and modifications can be made without departing from the spirit and scope of the present invention.

[0066] <Variation 1> In the above embodiment, when multiple cell balancing target batteries B' are identified during cell balancing processing, the configuration is such that all of the cell balancing target batteries B' are uniformly discharged by an amount corresponding to the charging rate SOCc. However, if the charging rates of the individual batteries B are equalized to a certain extent, the configuration may be such that each of the multiple cell balancing target batteries B' is discharged by an amount corresponding to a different charging rate SOCc.

[0067] For example, when the control unit Cnt estimates that the charging rates of the batteries B are equalized to a certain degree during cell balancing and multiple cell balancing target batteries B' are identified, the control unit Cnt reduces the charging rate SOCc corresponding to each cell balancing target battery B' in accordance with the voltage of each cell balancing target battery B'. For example, the control unit Cnt determines that the charging rates of the batteries B are equalized to a certain degree when the differences between the voltages of the batteries B and the minimum voltages of the batteries B are all equal to or less than a threshold, or when the differences between the maximum and minimum voltages of the batteries B are all equal to or less than a threshold, or when the differences between the voltages of the batteries B and the average voltages of the batteries B are all equal to or less than a threshold. Alternatively, the control unit Cnt determines that the charging rates of the batteries B are equalized to a certain degree when the voltages of the batteries B are all within a voltage range corresponding to the steep region shown in FIG. 6.

[0068] 4 , when the difference between the voltage of battery Ba1 (the maximum value among the voltages of each battery B) and the voltage of battery Bb1 (the minimum value among the voltages of each battery B) is equal to or less than a threshold value and batteries Ba1 and Ba2 are identified as cell balancing target batteries B′, the charging rate SOCc of battery Ba1 is reduced by an amount corresponding to the voltage of battery Ba1 (e.g., a charging rate (0.05%) corresponding to the difference (0.05V) between the constant voltage Vc and the voltage of battery Ba1), and the charging rate SOCc of battery Ba2 is reduced by an amount corresponding to the voltage of battery Ba2 (e.g., a charging rate (0.5%) corresponding to the difference (0.5V) between the constant voltage Vc and the voltage of battery Ba2). In this case, the control unit Cnt controls the operation of the cell balancing circuits CVa and CVb during cell balancing processing so that battery Ba1 is discharged by an amount corresponding to a charging rate SOCc that is 0.05% lower than battery Ba1, and battery Ba2 is discharged by an amount corresponding to a charging rate SOCc that is 0.5% lower than battery Ba2.

[0069] As a result, even if the charging rates of the batteries B are equalized to a certain extent, the charging rate of each cell balancing target battery B' can be prevented from being reduced more than necessary, thereby preventing the difference between the maximum and minimum charging rates of each battery B from diverging, and enabling the charging rates of each battery B to be equalized with high precision.

[0070] <Variation 2> In the above embodiment, at any timing after the end of the charging process, the new cell balancing target battery B' is identified as the battery B whose voltage is closest to the constant voltage Vc and satisfies the full charge condition, and as the battery B whose voltage is within the range from voltage Vα to the constant voltage Vc other than the battery B that satisfies the full charge condition. However, the configuration may also be such that at any timing after the end of the charging process, only the battery B whose voltage is closest to the constant voltage Vc and satisfies the full charge condition is identified as the new cell balancing target battery B'.

[0071] Even with this configuration, the charging rate of the cell balancing target battery B' can be made closer to the charging rate of the batteries B other than the cell balancing target battery B' during cell balancing processing. Furthermore, by preventing a decrease in the charging rate of the batteries B other than the cell balancing target battery B' during cell balancing processing, the possibility that the batteries B other than the cell balancing target battery B' will be updated as the cell balancing target battery B' in subsequent charging processes can be increased. Therefore, by repeatedly performing charging and cell balancing processing, the cell balancing target battery B' and the batteries B other than the cell balancing target battery B' are periodically replaced, allowing the charging rate of each battery B to fluctuate within a certain range, and the difference ΔSOC between the maximum and minimum charging rates of each battery B to converge within the certain range. In other words, in a power storage device PS including multiple batteries B connected in series, the charging rates of the batteries B can be equalized.

[0072] <Variation 3> In the above embodiment, the cell balancing process is executed at any timing after the end of the charging process, but the cell balancing process may be executed every time a predetermined time T has elapsed. For example, after the cell balancing process is executed, the control unit Cnt will not execute the next cell balancing process until the predetermined time T has elapsed, even if there is a battery B that satisfies the full charge condition.

[0073] Even with this configuration, the charging rate of the cell balancing target battery B' can be made closer to the charging rate of the batteries B other than the cell balancing target battery B' during cell balancing processing. Furthermore, by preventing a decrease in the charging rate of the batteries B other than the cell balancing target battery B' during cell balancing processing, the possibility that the batteries B other than the cell balancing target battery B' will be identified as the cell balancing target battery B' in subsequent charging processes can be increased. Therefore, by repeatedly performing charging and cell balancing processing, the cell balancing target battery B' and the batteries B other than the cell balancing target battery B' are periodically replaced, allowing the charging rate of each battery B to fluctuate within a certain range, and the difference ΔSOC between the maximum and minimum charging rates of each battery B to converge within the certain range. In other words, in a power storage device PS including multiple batteries B connected in series, the charging rates of the batteries B can be equalized.

[0074] <Variation 4> In the above embodiment, the cell balancing process is performed at any timing after the end of the charging process or every time a predetermined time T has elapsed. However, if it is determined that the charging rates of the batteries B have been equalized to a certain extent, the frequency of the cell balancing process may be reduced.

[0075] For example, when the control unit Cnt determines that the charging rates of each battery B are equalized to a certain extent, it performs cell balancing processing each time the charging process is completed two or more times, or every time a predetermined time T' longer than the predetermined time T has elapsed.

[0076] This makes it possible to prevent the charging rate of the cell balancing target battery B' from being lowered more than necessary even when the charging rates of the batteries B are equalized to a certain extent, thereby preventing the difference between the maximum and minimum charging rates of the batteries B from diverging, and enabling the charging rates of the batteries B to be equalized with high precision.

[0077] <Variation 5> In the above embodiment, the cell balancing target battery B' is updated based on the voltage of each battery B at the end of the charging process. However, the cell balancing target battery B' may also be identified based on the voltage of each battery B when a predetermined time t has elapsed since the end of the charging process.

[0078] For example, the control unit Cnt identifies the battery Ba1, whose voltage is closest to the constant voltage Vc, as the cell balancing target battery B' among the voltages of the batteries Ba1 to Ba3 at time t3 shown in FIG. 2(b).

[0079] Even with this configuration, the charging rate of the cell balancing target battery B' can be made closer to the charging rate of the batteries B other than the cell balancing target battery B' during cell balancing processing. Furthermore, by preventing a decrease in the charging rate of the batteries B other than the cell balancing target battery B' during cell balancing processing, the possibility that the batteries B other than the cell balancing target battery B' will be identified as the cell balancing target battery B' in subsequent charging processes can be increased. Therefore, by repeatedly performing charging and cell balancing processing, the cell balancing target battery B' and the batteries B other than the cell balancing target battery B' are periodically replaced, allowing the charging rate of each battery B to fluctuate within a certain range, and the difference ΔSOC between the maximum and minimum charging rates of each battery B to converge within the certain range. In other words, in a power storage device PS including multiple batteries B connected in series, the charging rates of the batteries B can be equalized.

[0080] <Variation 6> In the above embodiment, the cell balancing process is performed on all the batteries Ba1 to Ba3 and Bb1 to Bb3, but the cell balancing process may be performed individually on the assembled batteries BPa and BPb.

[0081] This allows the voltage of the entire battery pack BPa to approach the voltage of the entire battery pack BPb, thereby suppressing an increase in the reflux current flowing between the battery packs BPa and BPb.

[0082] <Variation 7> In the above embodiment, the cell balancing circuits Cva and Cvb are configured as passive cell balancing circuits that consume the power of battery B through resistors. However, the cell balancing circuits Cva and Cvb may also be configured as active cell balancing circuits that transfer the power of the cell balancing target battery B' to any battery B other than the cell balancing target battery B'.

[0083] When the cell balance circuits CVa and CVb are configured using passive cell balance circuits, the circuit configuration of the cell balance circuits CVa and CVb can be made relatively simple, which makes it possible to reduce the manufacturing cost of the power storage device PS. [Explanation of symbols]

[0084] PS power storage device BPa, BPb assembled battery Cva, CVb cell balance circuit Sva1 to Sva3, Svb1 to Sv3 voltage detection section Sia, Sib current detection section Rea, Reb, ReCh, ReLo relays Str storage Cnt control unit

Claims

1. a plurality of batteries connected in series; a cell balance circuit that individually discharges the plurality of batteries; a control unit that controls the operation of the cell balancing circuit during cell balancing processing so that at least one cell balancing target battery, including a battery that satisfies a full charge condition, among the plurality of batteries is discharged at a predetermined charge rate; A power storage device comprising:

2. The power storage device according to claim 1, The predetermined charging rate is equal to or greater than the maximum difference between the maximum and minimum charging rates of the plurality of batteries that occur every certain time period. Energy storage device.

3. The power storage device according to claim 1, When the control unit determines that the charging rates of the plurality of batteries are equalized to a certain extent and when a plurality of cell balancing target batteries are identified, the control unit reduces the predetermined charging rate corresponding to each of the cell balancing target batteries in accordance with the voltage of each of the cell balancing target batteries. Energy storage device.

4. The power storage device according to claim 1, the control unit does not execute the next cell balancing process until a predetermined time has elapsed after the cell balancing process has been executed, even if there is a battery that satisfies a full charge condition. Energy storage device.

5. The power storage device according to claim 1, Each of the plurality of batteries is a lithium iron phosphate battery. Energy storage device.

6. The power storage device according to claim 1, The control unit identifies the cell balancing target battery based on voltages of the plurality of batteries when a predetermined time has elapsed since the charging process for the plurality of batteries was completed. Energy storage device.

7. The power storage device according to claim 1, a first assembled battery in which some of the plurality of batteries are connected in series; a second assembled battery in which the remaining plurality of batteries among the plurality of batteries are connected in series; Equipped with The control unit performs the cell balancing process individually for the first assembled battery and the second assembled battery. Energy storage device.

8. The power storage device according to claim 1, The control unit executes the cell balancing process for the plurality of cell balancing target batteries. Energy storage device.

9. The power storage device according to claim 1, The cell balance circuit is a passive cell balance circuit. Energy storage device.

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