Power storage device

Abstract

To equalize a charging rate of each battery in a power storage device including a plurality of batteries connected in series with each other.SOLUTION: A power storage device (PS) is provided with a cell balance circuit (CV) for individually discharging a plurality of batteries (B) connected in series, and a control unit (Cn) for controlling the operation of the cell balance circuit (CV) and the operation of a charger (CH) provided outside the power storage device (PS) and for supplying power to the plurality of batteries (B), wherein the control unit (Cn) controls the operation of the cell balance circuit (CV) so that the cell (B) to be subjected to cell balance processing is discharged until no battery (B) having a relatively large amount of charge remains, and then repeatedly controls the operation of the charger (CH) so that at least one battery (B) is charged to a full state.SELECTED DRAWING: Figure 1

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 comprising a plurality of batteries connected in series, the energy storage device comprising: a cell balancing circuit that discharges the plurality of batteries individually; and a control unit that controls the operation of the cell balancing circuit and the operation of a charger that is provided outside the energy storage device and supplies power to the plurality of batteries, wherein during cell balancing processing, the control unit repeatedly controls the operation of the cell balancing circuit so that a cell balancing target battery among the plurality of batteries is discharged until a battery with a relatively large charge amount is depleted, and then controls the operation of the charger so that at least one battery of the plurality of batteries is fully charged.

[0008] In this way, by controlling the operation of the cell balancing circuit so that the cell balancing target battery is discharged during cell balancing processing, the charge level of the cell balancing target battery can be reduced. Then, by controlling the operation of the charger so that at least one battery is fully charged, the charge level of the battery identified as the cell balancing target battery is increased by the amount of the reduced charge level. This also increases the charge levels of other batteries, thereby increasing the number of batteries with relatively high charge levels. By alternately repeating the process of controlling the operation of the cell balancing circuit so that the cell balancing target battery is discharged and the process of controlling the charger so that at least one battery is fully charged, the charge levels of all batteries can be relatively high, eliminating any batteries with relatively high charge levels and thereby equating the charge levels of all batteries. In other words, in a power storage device including multiple batteries connected in series, the charging rates of each battery can be equalized.

[0009] Furthermore, during execution of the cell balancing process, the control unit may repeatedly control the operation of the cell balancing circuit so that the voltage of the cell balancing target battery becomes equal to or lower than a predetermined voltage until the battery with a relatively high charge level is depleted, and then control the operation of the charger so that at least one battery of the plurality of batteries becomes fully charged, wherein the predetermined voltage may be the minimum value of the open circuit voltage of the battery when a range of change in open circuit voltage per unit state of charge is equal to or greater than a predetermined value on a curve showing the correspondence relationship between the state of charge and open circuit voltage of the battery on a two-dimensional coordinate system.

[0010] This makes it possible to equalize the charging rates of the batteries even in a configuration in which batteries having SOC-OCV characteristics with a plateau region are connected in series.

[0011] The control unit may be configured to execute the cell balancing process each time a plurality of charging processes are completed.

[0012] This makes it possible to prevent the cell balancing process from being performed too frequently even when the charge levels of each battery are relatively equal, thereby reducing the discomfort felt by the user when the charging process is performed again for the cell balancing process after the charging process has ended.

[0013] The control unit may be configured to determine the charge amounts of the plurality of batteries using the voltages of the plurality of batteries at the time of polarization elimination after the charging process for the plurality of batteries has been completed.

[0014] This allows the charge amount of the battery to be determined with high accuracy, and therefore allows the charge rates of the batteries to be equalized with high accuracy.

[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 and second assembled batteries. [Effects of the Invention]

[0017] 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]

[0018] [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 related to cell balancing processing. [Figure 4] FIG. 1 is a diagram showing an example of SOC-OCV characteristics. [Figure 5] FIG. 10 is a diagram for explaining cell balancing processing when each battery has SOC-OCV characteristics with a plateau region. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, the embodiments will be described in detail with reference to the drawings.

[0020] FIG. 1 is a diagram illustrating an example of a power storage device according to an embodiment.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] The storage unit Str is configured by a RAM (Random Access Memory) or a ROM (Read Only Memory), and stores thresholds and the like, which will be described later.

[0035] 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.

[0036] <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.

[0037] <Example of charging process> When the control unit Cnt receives a charging instruction sent from outside the power storage device PS, it starts a normal charging process, turns on relays Rea, Reb, and ReCh, and turns off relay ReLo to enable 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-Ba3 and batteries Bb1-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.

[0038] 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 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.

[0039] 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.

[0040] Next, at time t1, when voltage Va1, among 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 Fig. 2(b), voltage Va1 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 Fig. 2(a).

[0041] 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.

[0042] The control unit Cnt may be configured to perform constant current charging control when the charging process is executed.

[0043] In this case, first, the control unit Cnt starts constant current charging control at time t0, and sends a current instruction indicating a constant current Ic to the charger Ch for each control period from time t0 to time t1. Then, from time t0 to time t1, the constant current Ic flows through the battery pack BPa, and from time t0 to time t1, the voltages Va1 to Va3 gradually increase.

[0044] Then, at time t1, when voltage Va1, among voltages Va1 to Va3, first reaches or exceeds the constant voltage Vc, control unit Cnt determines that battery Ba1 has reached a fully charged state, sets the current command to zero, and ends constant current charging control.

[0045] Alternatively, the control unit Cnt may be configured to perform constant voltage charging control when the charging process is executed.

[0046] In this case, first, the control unit Cnt starts constant voltage charging control at time t1, and then gradually decreases the current command for each control period so that the voltage Va1 is maintained at the constant voltage Vc from time t1 to time t2. Then, the voltage Va1 is maintained at the constant voltage Vc from time t1 to time t2, and the current Ia flowing through the battery pack BPa gradually decreases from time t1 to time t2.

[0047] 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 the battery Ba1 has reached a fully charged state, sets the current command to zero, and terminates the constant voltage charging control.

[0048] <An example of cell balancing processing> When performing cell balancing processing, the control unit Cnt alternately controls the operation of the cell balancing circuit CV so that the cell balancing target battery B' among the batteries Ba1 to Ba3 and the batteries Bb1 to Bb3 is discharged, and controls the operation of the charger Ch so that at least one battery B is fully charged, thereby gradually aligning the charging rates of the batteries Ba1 to Ba3 and the batteries Bb1 to Bb3.

[0049] FIG. 3 is a flowchart showing an example of the operation of the control unit Cnt related to the cell balancing process.

[0050] First, when the charging process ends (step Stp1: Yes), the control unit Cnt starts cell balancing and determines whether or not there is at least one battery B among the batteries Ba1 to Ba3 and the batteries Bb1 to Bb3 that has a relatively large charge amount, that is, whether or not there is a battery B that has a charge amount that is significantly larger than the other batteries B (step Stp2). The charge amount is defined as the voltage or charging rate of the battery B. The control unit Cnt is also assumed to be able to uniquely determine the charging rate of the battery B from the voltage of the battery B based on a characteristic that indicates the correspondence relationship between the charging rate and voltage of the battery B. In this case, if there is at least one battery B that has a relatively large charge amount, the control unit Cnt determines that the charging rates of the batteries Ba1 to Ba3 and the batteries Bb1 to Bb3 are not equalized, and if there is no battery B that has a relatively large charge amount, the control unit Cnt determines that the charging rates of the batteries Ba1 to Ba3 and the batteries Bb1 to Bb3 are equalized.

[0051] For example, if at least one of the differences ΔVmin between the voltage of each of the batteries Ba1-Ba3 and Bb1-Bb3 and the minimum voltage Vmin among the voltages of the batteries Ba1-Ba3 and Bb1-Bb3 is greater than a threshold Vth1, the control unit Cnt determines that there is at least one battery B with a relatively high charge amount.On the other hand, if all of the differences ΔVmin are equal to or less than the threshold Vth1, the control unit Cnt determines that there is no battery B with a relatively high charge amount.

[0052] Alternatively, if at least one of the differences ΔVave between the voltage of each of the batteries Ba1 to Ba3 and the batteries Bb1 to Bb3 and the average value Vave of the voltages of each of the batteries Ba1 to Ba3 and the batteries Bb1 to Bb3 is greater than the threshold value Vth2, the control unit Cnt determines that there is at least one battery B with a relatively high charge amount.On the other hand, if all the differences ΔVave are equal to or less than the threshold value Vth2, the control unit Cnt determines that there is no battery B with a relatively high charge amount.

[0053] Alternatively, if the difference ΔV between the maximum voltage Vmax and the minimum voltage Vmin among the voltages of the batteries Ba1 to Ba3 and the batteries Bb1 to Bb3 is greater than the threshold Vth3, the control unit Cnt determines that there is at least one battery B with a relatively large charge amount.On the other hand, if the difference ΔV is equal to or less than the threshold Vth3, the control unit Cnt determines that there is no battery B with a relatively large charge amount.

[0054] The voltage of battery B at the end of the charging process used to determine whether there is at least one battery B with a relatively large charge amount may be, for example, the voltage of battery B immediately before the end of the charging process (e.g., the closed-circuit voltage measured by the voltage detection unit Sv immediately before time t2 shown in FIG. 2(b)), the voltage of battery B immediately after the end of the charging process (e.g., the open-circuit voltage measured by the voltage detection unit Sv immediately after time t2 shown in FIG. 2(b)), or the voltage of battery B at the time of polarization elimination after the end of the charging process (e.g., the open-circuit voltage measured by the voltage detection unit Sv at the time of polarization elimination at time t3 shown in FIG. 2(b)). If the voltage of battery B immediately before the end of the charging process is used as the voltage of battery B at the end of the charging process, cell balancing can be performed taking into account variations in the internal resistance of battery B, thereby improving the accuracy of equalizing the state of charge of each battery B. Furthermore, if the voltage of battery B at the time of polarization elimination after the end of the charging process is used as the voltage of battery B at the end of the charging process, cell balancing can be performed taking into account the variation in polarization of battery B, thereby improving the accuracy of equalizing the charging rate of each battery B.

[0055] The control unit Cnt may also be configured to determine whether there is at least one battery B with a relatively large charge amount based on the charge rate of the battery B at the end of the charging process. In this case, for example, the control unit Cnt determines the charge rate of the battery B at the end of the charging process using the integrated value of the current flowing through the battery B.

[0056] In the flowchart shown in FIG. 3, when the control unit Cnt determines that there is no battery B with a relatively large charge amount (step Stp2: No), it ends the cell balancing process.

[0057] On the other hand, if the control unit Cnt determines that there is at least one battery B with a relatively high charge level (step Stp2: Yes), it controls the operation of the cell balancing circuit CV so that the cell balancing target battery B', which is the battery B with a relatively high charge level, discharges until its voltage reaches a predetermined voltage (step Stp3). Note that the predetermined voltage is a voltage lower than the voltage of a fully charged battery B, and is the voltage of battery B that allows the charging rate of battery B to be uniquely determined based on the characteristics that indicate the correspondence relationship between the charging rate and voltage of battery B.

[0058] For example, the control unit Cnt identifies the battery B, of the batteries Ba1 to Ba3 and the batteries Bb1 to Bb3, whose voltage at the end of the charging process is closest to the constant voltage Vc, as the cell balancing target battery B'.

[0059] Alternatively, the control unit Cnt identifies, as the cell balancing target battery B', one of the batteries Ba1 to Ba3 and the batteries Bb1 to Bb3 whose voltage at the end of the charging process is within an arbitrary voltage range based on a constant voltage Vc.

[0060] The voltage of battery B at the end of the charging process used to identify the cell balancing target battery B' may be, for example, the voltage of battery B immediately before the end of the charging process, the voltage of battery B immediately after the end of the charging process, or the voltage of battery B at the time of polarization elimination after the end of the charging process.

[0061] The control unit Cnt may also be configured to identify the cell balancing target battery B' based on the charging rate of the battery B at the end of the charging process. In this case, for example, the control unit Cnt determines the charging rate of the battery B at the end of the charging process using the integrated value of the current flowing through the battery B.

[0062] Next, the control unit Cnt controls the operation of the charger Ch so that at least one battery B among the batteries Ba1 to Ba3 and the batteries Bb1 to Bb3 is fully charged (step Stp4). For example, in step Stp4, the control unit Cnt performs the constant current / constant voltage charging control, the constant current charging control, or the constant voltage charging control.

[0063] Then, the control unit Cnt repeatedly executes steps Stp3 and Stp4 until it determines that there is no battery B with a relatively large charge amount, and when it determines that there is no battery B with a relatively large charge amount (step Stp2: No), it ends the cell balancing process.

[0064] That is, when performing cell balancing processing, the control unit Cnt controls the operation of the cell balancing circuit CV so that the cell balancing target battery B' is discharged until the battery B with a relatively large charge amount is depleted, and then repeats the process of controlling the operation of the charger Ch so that at least one battery B is fully charged.

[0065] In this way, during cell balancing, controlling the operation of the cell balancing circuit CV so that the cell balancing target battery B', a battery B with a relatively high charge level, is discharged, thereby reducing the charge level of the cell balancing target battery B'. Then, controlling the operation of the charger Ch so that at least one battery B is fully charged not only increases the charge level of the battery B identified as the cell balancing target battery B' by the amount of the reduced charge level, but also increases the charge levels of the other batteries B, thereby increasing the number of batteries B with a relatively high charge level. By alternately repeating the process of controlling the operation of the cell balancing circuit CV so that the cell balancing target battery B' is discharged and the process of controlling the operation of the charger Ch so that at least one battery B is fully charged, the charge levels of all batteries B can be relatively high, eliminating any batteries B with a relatively high charge level and thereby equating the charge levels of all batteries B. In other words, in a power storage device including multiple batteries B connected in series, the charging rates of the batteries B can be equalized.

[0066] Here, for example, it is assumed that battery B is composed of a lithium iron phosphate ion battery.

[0067] Fig. 4 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. 4 represents the state of charge, and the vertical axis represents the voltage. The solid line in Fig. 4 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.

[0068] The SOC-OCV characteristics shown in Figure 4 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 change in open-circuit voltage per unit charge rate is smaller than a predetermined value, and a steep region that indicates the range of charge rates when the change in open-circuit voltage per unit charge rate is equal to or greater than the predetermined value. The minimum value of the unused region is 0%, the maximum value of the unused region is smaller than the minimum value of the plateau region, the maximum value of the plateau region is smaller than the minimum value of the steep region, and the maximum value of the steep region is 100%.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 5(a) to 5(e) are diagrams illustrating cell balancing when each battery B has an SOC-OCV characteristic with a plateau region. The horizontal axis of the two-dimensional coordinate system shown in FIGS. 5(a) to 5(e) represents batteries Ba1 to Ba3 and Bb1 to Bb3, and the vertical axis represents voltage. The points shown in FIGS. 5(a) to 5(e) represent the voltages of batteries Ba1 to Ba3 and Bb1 to Bb3. For example, the points shown in FIG. 5(a) represent the voltages of batteries Ba1 to Ba3 and Bb1 to Bb3 at the end of the charging process, with the voltage of battery Ba1 > the voltage of battery Ba2 > the voltage of battery Ba3 > the voltage of battery Bb3 > the voltage of battery Bb2 > the voltage of battery Bb1. In the example shown in FIG. 5(a), the difference ΔVmin between the voltages of batteries Ba2 and Bb1 is greater than the threshold value Vth1. 5(a), the voltages of batteries Ba3, Bb1 to Bb3 are each lower than a predetermined voltage Vα, and the voltages of batteries Ba1 and Ba2 are each equal to or higher than the predetermined voltage Vα. The predetermined voltage Vα is set to be less than a constant voltage Vc. The predetermined voltage Vα is set to a value determined based on parameters relating to the environment in which the power storage device PS is used, such as temperature and the output current of the charger Ch, and is set to a voltage corresponding to the minimum charging rate in the steep region, for example.

[0073] When each battery B has an SOC-OCV characteristic with a plateau region, the control unit Cnt determines that there is at least one battery B with a relatively high charge level if at least one difference ΔVmin is greater than the threshold value Vth1, and at least one voltage among the voltages of batteries Ba1-Ba3 and batteries Bb1-Bb3 is smaller than a predetermined voltage Vα and the remaining voltages are equal to or greater than the predetermined voltage Vα.On the other hand, the control unit Cnt determines that there is no battery B with a relatively high charge level if all differences ΔVmin are equal to or less than the threshold value Vth1, and the voltages of batteries Ba1-Ba3 and batteries Bb1-Bb3 are equal to or greater than the predetermined voltage Vα.

[0074] Alternatively, if each battery B has an SOC-OCV characteristic with a plateau region, the control unit Cnt determines that there is at least one battery B with a relatively high charge level if at least one difference ΔVave is greater than the threshold value Vth2 and at least one voltage among the voltages of batteries Ba1-Ba3 and batteries Bb1-Bb3 is smaller than a predetermined voltage Vα and the remaining voltages are equal to or greater than the predetermined voltage Vα.On the other hand, the control unit Cnt determines that there is no battery B with a relatively high charge level if all differences ΔVave are equal to or less than the threshold value Vth2 and the voltages of batteries Ba1-Ba3 and batteries Bb1-Bb3 are equal to or greater than the predetermined voltage Vα.

[0075] Alternatively, if each battery B has an SOC-OCV characteristic with a plateau region, the control unit Cnt determines that there is at least one battery B with a relatively high charge level if the difference ΔV is greater than the threshold Vth3 and at least one voltage of the batteries Ba1-Ba3 and Bb1-Bb3 is less than the predetermined voltage Vα and the remaining voltages are equal to or greater than the predetermined voltage Vα.On the other hand, if the difference ΔV is equal to or less than the threshold Vth3 and the voltages of the batteries Ba1-Ba3 and Bb1-Bb3 are equal to or greater than the predetermined voltage Vα, the control unit Cnt determines that there is no battery B with a relatively high charge level.

[0076] Also, for example, if each battery B has an SOC-OCV characteristic with a plateau region, the control unit Cnt selects a battery B whose voltage is equal to or higher than a predetermined voltage Vα as the cell balancing target battery B'.

[0077] In this way, when each battery B has an SOC-OCV characteristic with a plateau region, the control unit Cnt first starts cell balancing after the end of the charging process, and if the difference ΔVmin between the voltage of battery Ba2 and the voltage of battery Bb1 is greater than the threshold value Vth1, as shown in Figure 5(a), and if the voltages of batteries Ba3, Bb1 to Bb3 are smaller than a predetermined voltage Vα and the voltages of batteries Ba1 and Ba2 are equal to or greater than the predetermined voltage Vα, it determines that there is at least one battery B with a relatively large charge amount, and identifies batteries Ba1 and Ba2 whose voltages are equal to or greater than the predetermined voltage Vα (i.e., whose voltages are within the voltage range from the predetermined voltage Vα to a constant voltage Vc) as the cell balancing target batteries B'.

[0078] Next, as shown in FIG. 5(b), the control unit Cnt turns on switches SWa1 and SWa2 until the voltages of batteries Ba1 and Ba2, the cell balancing target battery B', fall below a predetermined voltage Vα. Switches SWa3 and SWb1 to SWb3 are maintained in a cutoff state, preventing the voltages of batteries Ba3 and Bb1 to Bb3 from decreasing. In the example shown in FIG. 5(b), the voltages of batteries Ba1 and Ba2 are within a voltage range corresponding to the steep region, allowing the state of charge of batteries Ba1 and Ba2 to be uniquely determined from the voltages of batteries Ba1 and Ba2. Therefore, by discharging batteries Ba1 and Ba2 until the voltages of batteries Ba1 and Ba2 fall below the predetermined voltage Vα, the charge levels of batteries Ba1 and Ba2 can be reduced by a desired amount.

[0079] Next, the control unit Cnt executes a charging process on the batteries Ba1-Ba3 and Bb1-Bb3, thereby increasing the voltages of the batteries Ba1-Ba3 and Bb1-Bb3, as shown in FIG. 5(c). For example, among the batteries Ba1 and Ba2 identified as the cell balancing target battery B' in FIG. 5(b), the charge amount of the battery Ba1 decreases. Therefore, the charge amounts of the batteries Ba1 and Ba2 as well as the batteries Ba3 and Bb1-Bb3 increase, resulting in an increase in the voltages of the batteries Ba1-Ba3 and Bb1-Bb3 in FIG. 5(c). In the example shown in FIG. 5(c), at the end of the charging process, the voltage of the battery Ba1 is approximately equal to the voltage of the battery Ba2 > the voltage of the battery Ba3 > the voltage of the battery Bb3 > the voltage of the battery Bb2 > the voltage of the battery Bb1. Also, in the example shown in FIG. 5(c), the difference ΔVmin between the voltages of the batteries Bb3 and Bb1 is greater than the threshold Vth1. In the example shown in FIG. 5(c), it is assumed that the voltages of the batteries Bb1 and Bb2 are lower than the predetermined voltage Vα, and the voltages of the batteries Ba1 to Ba3 and Bb3 are equal to or higher than the predetermined voltage Vα.

[0080] Next, after the charging process is completed, as shown in FIG. 5(c), if the difference ΔVmin between the voltage of battery Bb3 and the voltage of battery Bb1 is greater than threshold value Vth1, and if the voltages of batteries Bb1 and Bb2 are lower than predetermined voltage Vα and the voltages of batteries Ba1 to Ba3 and Bb3 are equal to or higher than predetermined voltage Vα, the control unit Cnt determines that there is at least one battery B with a relatively large charge amount, and identifies battery Ba1 to Ba3 and Ba3 whose voltage is equal to or higher than predetermined voltage Vα (within the voltage range from predetermined voltage Vα to constant voltage Vc) as the cell balancing target battery B'.

[0081] Next, as shown in Figure 5(d), the control unit Cnt turns on the switches SWa1-SWa3 and SWb3 until the voltages of the cells Ba1-Ba3 and Bb3, which are the cell balancing target cell B', fall below the predetermined voltage Vα. Note that the switches SWb1 and SWb2 are kept off, preventing the voltages of the cells Bb1 and Bb2 from decreasing.

[0082] Next, the control unit Cnt executes a charging process on batteries Ba1-Ba3 and batteries Bb1-Bb3, thereby increasing the voltages of batteries Ba1-Ba3 and batteries Bb1-Bb3, as shown in Fig. 5(e). For example, among batteries Ba1-Ba3 and Bb3 identified as cell balancing target battery B' in Fig. 5(d), the charge amount of battery Ba1 decreases, and not only the charge amounts of batteries Ba1-Ba3 and Bb3 but also the charge amounts of batteries Bb1 and Bb2 increase, resulting in an increase in the voltages of batteries Ba1-Ba3 and batteries Bb1-Bb3 in Fig. 5(e). In the example shown in Fig. 5(e), at the end of the charging process, the voltage of battery Ba1 ≈ the voltage of battery Ba2 ≈ the voltage of battery Ba3 ≈ the voltage of battery Bb3 > the voltage of battery Bb2 > the voltage of battery Bb1. 5(e), the difference ΔVmin between the voltages of batteries Ba1 and Bb1, the difference ΔVmin between the voltages of batteries Ba2 and Bb1, the difference ΔVmin between the voltages of batteries Ba3 and Bb1, the difference ΔVmin between the voltages of batteries Bb3 and Bb1, and the difference ΔVmin between the voltages of batteries Bb2 and Bb1 are each assumed to be less than or equal to threshold value Vth1. Also, in the example shown in FIG. 5(c), the voltages of batteries Ba1 to Ba3 and Bb1 to Bb3 are assumed to be greater than or equal to predetermined voltage Vα.

[0083] Then, after the charging process is completed, if all differences ΔVmin are equal to or less than the threshold value Vth1 and the voltages of the batteries Ba1-Ba3 and Bb1-Bb3 are equal to or greater than the predetermined voltage Vα, as shown in Fig. 5(e), the control unit Cnt determines that there is no battery B with a relatively large charge amount and terminates the cell balancing process. In the example shown in Fig. 5(e), the voltages of the batteries Ba1-Ba3 and Bb1-Bb3 are within the voltage range corresponding to the steep region, and the state of charge of the batteries Ba1-Ba3 and Bb1-Bb3 can be uniquely determined from the voltages of the batteries Ba1-Ba3 and Bb1-Bb3. Therefore, if the voltages of the batteries Ba1-Ba3 and Bb1-Bb3 are equalized, it can be determined that the state of charge of the batteries Ba1-Ba3 and Bb1-Bb3 are equalized.

[0084] That is, when each battery B has an SOC-OCV characteristic with a plateau region, the control unit Cnt is configured to control the operation of the cell balancing circuit CV so that the voltage of the cell balancing target battery B' becomes equal to or lower than a predetermined voltage Vα during cell balancing processing until there are no more batteries B with a relatively large charge amount, and then to repeatedly control the operation of the charger Ch so that at least one battery B among batteries Ba1 to Ba3 and batteries Bb1 to Bb3 is fully charged.

[0085] Thus, even in a configuration in which batteries B with SOC-OCV characteristics having a plateau region are connected in series, controlling the operation of the cell balancing circuit CV to discharge the cell balancing target battery B', which is a battery B with a relatively high charge level, during cell balancing can reduce the charge level of the cell balancing target battery B'. Then, controlling the operation of the charger Ch to fully charge at least one battery B not only increases the charge level of the battery B identified as the cell balancing target battery B' by the amount of the reduced charge level, but also increases the charge levels of the other batteries B, thereby increasing the number of batteries B with relatively high charge levels. By alternately repeating the process of controlling the operation of the cell balancing circuit CV to discharge the cell balancing target battery B' and the process of controlling the operation of the charger Ch to fully charge at least one battery B, the charge levels of all batteries B can be relatively high, eliminating any batteries B with a relatively high charge level and thereby equating the charge levels of all batteries B. In other words, even in a configuration in which batteries B with SOC-OCV characteristics having a plateau region are connected in series, the charging rates of the batteries B can be equalized.

[0086] 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.

[0087] <Variation 1> In the above embodiment, the cell balancing process is executed every time the charging process is completed, but the cell balancing process may be executed less frequently if it is determined that the charging rates of the batteries B have been equalized to a certain extent.

[0088] For example, when the control unit Cnt determines that the charge amounts of the batteries B are equal to a certain extent, it executes cell balancing processing each time multiple charging processes are completed.

[0089] This prevents the cell balancing process from being performed too frequently even when the charge levels of each battery B are relatively equal, thereby reducing the discomfort felt by the user when the charging process is performed again for the cell balancing process after the charging process has ended.

[0090] <Variation 2> 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.

[0091] 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.

[0092] <Variation 3> In the above embodiment, when multiple cell balancing target batteries B' are identified during cell balancing processing, each cell balancing target battery B' is discharged until its voltage drops to a predetermined voltage Vα. If the voltages of the cell balancing target batteries B' differ from one another, the charge amounts to be reduced also differ from one another. Alternatively, each cell balancing target battery B' may be discharged by a fixed charge amount.

[0093] <Variation 4> 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'.

[0094] 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]

[0095] 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 power storage device including a plurality of batteries connected in series, a cell balance circuit that individually discharges the plurality of batteries; a control unit that controls an operation of the cell balance circuit and an operation of a charger that is provided outside the power storage device and supplies power to the plurality of batteries; Equipped with The control unit, during cell balancing processing, controls the operation of the cell balancing circuit so that a cell balancing target battery among the plurality of batteries is discharged until the battery with a relatively large charge amount is depleted, and then repeats controlling the operation of the charger so that at least one battery of the plurality of batteries is fully charged. Energy storage device.

2. The power storage device according to claim 1, the control unit, during execution of the cell balancing process, repeatedly controls the operation of the cell balancing circuit so that the voltage of the cell balancing target battery becomes equal to or lower than a predetermined voltage until the battery with a relatively large charge amount is depleted, and then controls the operation of the charger so that at least one battery of the plurality of batteries is fully charged; The predetermined voltage is the minimum value of the open circuit voltage of the battery when the change in the open circuit voltage per unit charging rate is equal to or greater than a predetermined value on a curve showing the correspondence relationship between the charging rate and the open circuit voltage of the battery on a two-dimensional coordinate system. Energy storage device.

3. The power storage device according to claim 1, The control unit executes the cell balancing process each time a plurality of charging processes are completed. Energy storage device.

4. The power storage device according to claim 1, The control unit determines the charge amounts of the plurality of batteries using voltages of the plurality of batteries at the time of polarization elimination after the charging process for the plurality of batteries is completed. Energy storage device.

5. 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.

Images