Electricity storage system and electricity storage unit

Abstract

Control units included in a plurality of electricity storage units connected in parallel each acquire the discharge voltages of electricity storage packs included in the other electricity storage units by communication from the other control units, and adaptively control the outputs of DC / DC converters included in the electricity storage units including the control units so that the discharge voltages of the plurality of electricity storage packs included in the plurality of electricity storage units approach each other.

Description

Energy storage system and energy storage unit

[0001] The present disclosure relates to a power storage system in which a plurality of power storage units are connected in parallel to a DC bus, and to the power storage units.

[0002] In recent years, to improve power conversion efficiency within data centers, organizations such as the Open Compute Project (OCP) have proposed distributed backup systems that install backup power supplies in each server rack. This method requires a high output power density of several kW to several tens of kW for a short period of time, such as a few minutes, which places a heavy burden on the batteries.

[0003] For this reason, parallel systems are often constructed in which multiple battery modules are connected in parallel and each battery module is transformed via a DC / DC converter to output power. Each DC / DC converter connected in parallel detects its own output current, shares its output value with the other DC / DC converters through communication, and controls its own output current so that the output current of all the DC / DC converters is uniform (hereinafter referred to as current share control in this specification).

[0004] However, in a parallel system using multiple battery units, it is difficult to align all of the battery units with the same characteristics, such as the output current accuracy of the battery units, the conversion efficiency of the DC / DC converters in the battery units, the internal resistance of the battery packs, and the degree of battery pack degradation. These variations can cause variations in the discharge current of the battery packs.

[0005] In the current sharing control described above, even if the battery characteristics are the same, voltage variations can occur between battery packs due to errors in the output accuracy of the battery units, and the voltage variations can increase. For example, even if the battery packs have the same level of degradation, SOC (State of Charge), and DC / DC converter discharge efficiency, differences in output current between battery units can increase the discharge current of a battery pack included in a battery unit with a high output current, causing a drop in the voltage of that battery pack. This voltage drop accelerates the increase in the discharge current of that battery pack, creating a vicious cycle. In this case, the parallel system's capabilities are not fully utilized. For example, if one battery unit runs out of remaining capacity and is removed from the parallel system, the current burden on the remaining battery units increases, raising the risk of the remaining battery units successively shutting down due to overcurrent.

[0006] Patent Literature 1 discloses a method for determining power allocation so that a power storage unit with a relatively low degree of degradation allocates a relatively large amount of power to the power storage unit in order to suppress performance variations among the multiple power storage units that make up an in-vehicle power supply system. Because the degree of degradation is not a value that changes instantaneously, this method suppresses performance variations among the multiple power storage units over the long term.

[0007] On the other hand, in applications such as backup systems in data centers, replacing a battery unit can significantly change the capacity difference and characteristic variations between battery units. Because control and data are required to calculate the degree of deterioration, and the degree of deterioration itself is an index that captures long-term changes, the above method has difficulty quickly tracking such large environmental changes between battery units.

[0008] JP 2010-4667 A

[0009] According to one aspect of the present disclosure, there is provided a power storage system including a plurality of power storage units connected in parallel to a DC bus to which a load is connected. Each of the plurality of power storage units includes a power storage pack, a DC / DC converter for charging and discharging the power storage pack, and a control unit for controlling the DC / DC converter. The control unit acquires, via communication, discharge voltages of the power storage packs included in other power storage units from other control units, and adaptively controls the output of the DC / DC converter included in a power storage unit among the plurality of power storage units that includes the power storage pack so that the discharge voltages of the power storage packs included in the plurality of power storage units approach each other.

[0010] Any combination of the above components, and conversion of the expression of the present disclosure into an apparatus, system, method, computer program, etc., are also valid aspects of the present disclosure.

[0011] According to the present disclosure, it is possible to suppress variations in discharge end timing among a plurality of power storage packs included in a power storage unit connected in parallel.

[0012] FIG. 1 is a diagram illustrating the configuration of a battery system according to an embodiment. FIG. 2 is a flowchart illustrating the operation of the battery system according to an embodiment. FIGS. 3(a) and 3(b) are diagrams illustrating specific examples of the discharge voltage Vbat, discharge current Ibat, output voltage Vbus, and output current Ibus when there is characteristic variation in the converter module. FIGS. 4(a) and 4(b) are diagrams illustrating specific examples of the discharge voltage Vbat, discharge current Ibat, output voltage Vbus, and output current Ibus when there is characteristic variation in the battery module.

[0013] FIG. 1 is a diagram illustrating the configuration of a battery system 1 according to an embodiment. In data centers, as a countermeasure against power supply failures, a backup battery system 1 is generally used to secure a power source until a generator (e.g., a diesel generator) starts up in the event of a power outage. Note that the system may be configured to back up data to another server in the event of a power outage, and even in such cases, it is important to secure a power source long enough to complete the data evacuation.

[0014] The battery system 1 includes a plurality of battery units 10 connected in parallel to a DC bus 2. In this embodiment, an example is assumed in which eight battery units 10 are connected in parallel. The number of parallel connections is determined depending on the power required by the loads 3. The DC bus 2 is connected to the loads 3 that are to be backed up in the event of a power outage. The loads 3 are a general term for servers and storage devices.

[0015] In this embodiment, the rack accommodates eight battery units 10a to 10h and a plurality of servers and storage devices as loads 3. A bus bar is installed on the rear of the rack as a DC bus 2, and the plurality of servers and storage devices accommodated in the rack can receive power from the bus bar.

[0016] Each battery unit 10 includes a battery module 11 and a converter module 14. The battery module 11 includes a battery pack 12 and a battery control unit 13. The converter module 14 includes a DC / DC converter 15 and a control unit 16. The battery control unit 13 and the control unit 16 may be integrated into one unit.

[0017] The battery pack 12 includes multiple cells connected in series. The cells may be lithium-ion battery cells, nickel-metal hydride battery cells, lead-acid battery cells, etc. In the following description, we will assume an example in which lithium-ion battery cells (nominal voltage: 3.6-3.7 V) are used.

[0018] The battery pack 12 is connected to a DC / DC converter 15 for charging and discharging the battery pack 12. The DC / DC converter 15 is connected between the battery pack 12 and the DC bus 2. The DC / DC converter 15 controls the voltage or current of DC power supplied from a commercial power system to the DC bus 2 via an inverter, and can charge the battery pack 12. When used as a backup for the load 3, the battery pack 12 basically stands by in a nearly fully charged state. The DC / DC converter 15 can also control the voltage or current of DC power discharged from the battery pack 12 and output it to the DC bus 2.

[0019] The battery control unit 13 manages the state of the battery pack 12. The battery control unit 13 is configured, for example, by combining an analog front-end IC and a microcontroller. Note that an ASIC (Application Specific Integrated Circuit) may be used instead of the analog front-end IC.

[0020] The analog front-end IC measures and monitors the voltage of each cell in the battery pack 12, the current flowing through the battery pack 12, and the temperature of the battery pack 12, and transmits the measurement data to the microcontroller. The analog front-end IC can measure the current flowing through the battery pack 12 using a shunt resistor and a differential amplifier. A Hall element may be used instead of the shunt resistor. The analog front-end IC can measure the temperature of the battery pack 12 by dividing the output of a thermistor installed in the battery pack 12.

[0021] The microcontroller obtains the total voltage of each battery pack by dividing the voltage across the battery pack 12 using resistors. The total voltage may be calculated by adding up the voltages of the individual cells in the battery pack 12.

[0022] The battery control unit 13 transmits battery information such as the voltage, current, temperature, and SOC of the battery pack 12 to the control unit 16. The battery control unit 13 and the control unit 16 in the battery unit 10 are connected via serial communication (for example, Modbus).

[0023] The control unit 16 is configured by, for example, a microcontroller. The control unit 16 acquires the output voltage Vbus and the output current Ibus of the DC / DC converter 15. The control unit 16 can measure the output voltage Vbus of the DC / DC converter 15 using a voltage dividing resistor and a differential amplifier. The control unit 16 can measure the output current Ibus of the DC / DC converter 15 using a shunt resistor and a differential amplifier. Note that a Hall element or a CT sensor may be used instead of the shunt resistor.

[0024] In the current share control described above, the control unit 16 performs feedback control so that the measured value of the output current Ibus is maintained at a target value (e.g., 50 A). When the measured value of the output current Ibus is lower than the target value, the control unit 16 generates a current command value for increasing the output power of the DC / DC converter 15, and when the measured value of the output current Ibus is higher than the target value, the control unit 16 generates a current command value for decreasing the output power of the DC / DC converter 15. The DC / DC converter 15 performs switching operation in response to a drive signal based on the generated current command value. As the current command value increases, the duty ratio increases, and as the current command value decreases, the duty ratio decreases.

[0025] Due to an error in the measurement system for the output current Ibus, the output current Ibus value of a certain battery unit 10 may be measured as lower than the actual current value. In this case, under current sharing control, the control unit 16 of that battery unit 10 controls the DC / DC converter 15 to obtain more power from the battery pack 12. As a result, more discharge current flows from that battery pack 12 than from the other battery packs 12, and the voltage of that battery pack 12 drops faster than the voltage of the other battery packs 12. This further increases the discharge current of that battery pack 12, further decreasing the voltage of that battery pack 12, creating a vicious cycle.

[0026] Furthermore, if the conversion efficiency of the DC / DC converter 15 of one battery unit 10 is lower than that of the DC / DC converter 15 of another battery unit 10, the control unit 16 of that battery unit 10 will control the DC / DC converter 15 to obtain more power from the battery pack 12. This also results in the vicious cycle described above.

[0027] This embodiment proposes a control method that prevents this vicious cycle. In this embodiment, a plurality of control units 16a to 16h included in each of a plurality of battery units 10a to 10h are connected by a communication line 20. For example, a Controller Area Network (CAN) or Ethernet (registered trademark) can be used for communication between the plurality of control units 16a to 16h. Note that, if noise requirements are met, the plurality of control units 16a to 16h can also be connected by wireless communication.

[0028] When the operation of the battery system 1 starts due to a power outage or the like, the control unit 16 acquires, via communication, the discharge voltage Vbat and discharge current Ibat of the battery packs 12 included in the other battery units 10, and the output voltage Vbus and output current Ibus of the DC / DC converter 15 from the other control units 16. Of these, the discharge voltage Vbat of the battery pack 12 is essential data. The discharge current Ibat of the battery pack 12, and the output voltage Vbus and output current Ibus of the DC / DC converter 15 are for confirmation purposes and can be omitted.

[0029] The control unit 16 adaptively controls the output of the DC / DC converter 15 included in the battery unit 10 so that the discharge voltages Vbat of the battery packs 12a to 12h included in the battery units 10a to 10h approach each other. This will be explained in detail below.

[0030] The control unit 16 performs feedback control so that the measured value of the output voltage Vbus is maintained at a target value (e.g., 50 V). When the measured value of the output voltage Vbus is lower than the target value, the control unit 16 generates a voltage command value for increasing the output power of the DC / DC converter 15, and when the measured value of the output voltage Vbus is higher than the target value, the control unit 16 generates a voltage command value for decreasing the output power of the DC / DC converter 15. The DC / DC converter 15 performs switching operation in response to a drive signal based on the generated voltage command value. The larger the voltage command value, the higher the duty ratio, and the smaller the voltage command value, the lower the duty ratio.

[0031] When the discharge voltage Vbat of the battery pack 12 included in its own battery unit 10 is lower than the discharge voltages Vbat of all other battery packs 12 acquired from the other control units 16, the control unit 16 reduces the target value of the output voltage Vbus of its own DC / DC converter 15 by a predetermined value. While the discharge voltage Vbat of its own battery pack 12 is lower than the discharge voltages Vbat of all other battery packs 12, the control unit 16 reduces the target value of the output voltage Vbus of its own DC / DC converter 15 in steps of a predetermined step size. The predetermined value is determined by the designer based on the results of experiments and simulations. When the discharge voltage Vbat of its own battery pack 12 matches the discharge voltages Vbat of all other battery packs 12, the control unit 16 stops reducing the target value of the output voltage Vbus.

[0032] When the target value of the output voltage Vbus of the DC / DC converter 15 of a certain battery unit 10 decreases, the power supplied by that battery unit 10 to the load 3 decreases. This decrease in the power supplied to the load 3 is compensated for by a proportional increase in the output current Ibus of the other battery units 10.

[0033] When the output current Ibus of the DC / DC converter 15 exceeds the upper limit, the control unit 16 reduces, by a predetermined value, the target value of the output voltage Vbus of the DC / DC converter 15. The upper limit of the output current Ibus is set to α (for example, 0.7) times the overcurrent threshold of the output current Ibus of the DC / DC converter 15.

[0034] When the discharge current Ibat of the battery pack 12 included in its own battery unit 10 exceeds an upper limit, the control unit 16 reduces, by a predetermined value, the target value of the output voltage Vbus of its own DC / DC converter 15. The upper limit of the discharge current Ibat is set to β (for example, 0.7) times the overcurrent threshold value of the discharge current Ibat of the battery pack 12.

[0035] 2 is a flowchart illustrating the operation of the battery system 1 according to the embodiment. The control unit 16 of each of the battery units 10a-10h periodically acquires measurement data (discharge voltage Vbat, discharge current Ibat, output voltage Vbus, output current Ibus, and temperature Temp) of its own battery unit 10 and the other battery units 10 (S10). When a power outage in the commercial power grid is detected (Y in S11), the control unit 16 of each of the battery units 10a-10h compares the discharge voltage Vbat of the other battery packs 12 with the discharge voltage Vbat of its own battery pack 12 (S12).

[0036] If the discharge voltage Vbat of the battery pack 12 is lower than the discharge voltage Vbat of the other battery packs 12 (Y in S12), the control unit 16 compares the target value of the output voltage Vbus of the DC / DC converter 15 with the threshold value of the voltage Vbus of the DC bus 2 (S13). The threshold value of the voltage Vbus of the DC bus 2 is the lower limit value of the voltage Vbus of the DC bus 2 that is set based on the specifications required by the load 3.

[0037] If the target value of the output voltage Vbus is higher than the threshold value of the voltage Vbus of the DC bus 2 (Y in S13), the control unit 16 updates the target value of the output voltage Vbus of its own DC / DC converter 15 to a set value (S14). Specifically, the control unit 16 updates the target value of the output voltage Vbus to a value that is lower than the current value by a predetermined value.

[0038] In step S12, if the discharge voltage Vbat of the battery pack 12 is not lower than the discharge voltage Vbat of the other battery packs 12 (not the lowest state) (N in S12), the processes of steps S13-S14 are skipped. In step S13, if the target value of the output voltage Vbus is equal to or lower than the threshold value of the voltage Vbus of the DC bus 2 (N in S13), the process proceeds to step S20, which will be described later.

[0039] Each control unit 16 compares the discharge current Ibat of the battery pack 12 included in its own battery unit 10 with the threshold value (upper limit current value) of the discharge current Ibat (S15). If the discharge current Ibat of the battery pack 12 included in its own battery unit 10 is equal to or less than the threshold value of the discharge current Ibat (N in S15), the corresponding control unit 16 compares the output current Ibus of its own DC / DC converter 15 with the threshold value (upper limit current value) of the output current Ibus (S16). If the output current Ibus of its own DC / DC converter 15 is equal to or less than the threshold value of the output current Ibus (N in S16), the corresponding control unit 16 compares the temperature Temp of its own battery pack 12 with the threshold value (upper limit temperature value) of the temperature Temp of the battery pack 12 (S17). If the temperature Temp of its own battery pack 12 is equal to or less than the threshold value of the temperature Temp of the battery pack 12 (N in S17), the process proceeds to step S21, which will be described later.

[0040] If the discharge current Ibat of the battery pack 12 included in its own battery unit 10 exceeds the discharge current Ibat threshold (Y in S15), if the output current Ibus of its own DC / DC converter 15 exceeds the output current Ibus threshold (Y in S16), or if the temperature Temp of its own battery pack 12 exceeds the temperature Temp threshold of the battery pack 12 (Y in S17), the corresponding control unit 16 compares the target value of the output voltage Vbus of its own DC / DC converter 15 with the threshold of the voltage Vbus of the DC bus 2 (S18).

[0041] If the target value of the output voltage Vbus is higher than the threshold value of the voltage Vbus of the DC bus 2 (Y in S18), the control unit 16 updates the target value of the output voltage Vbus of its own DC / DC converter 15 to a set value (S19). Specifically, the control unit updates the target value of the output voltage Vbus to a value that is lower than the current value by a predetermined value. If the target value of the output voltage Vbus is equal to or lower than the threshold value of the voltage Vbus of the DC bus 2 (N in S18), the control unit 16 updates the threshold value of the voltage Vbus of the DC bus 2 to a set value if the lower limit of the voltage Vbus of the DC bus 2 has changed due to load fluctuations or the like (S20). Note that the threshold value of the voltage Vbus of the DC bus 2 may be a fixed value. In this case, specifically, if the target value of the output voltage Vbus is equal to or lower than the threshold value of the voltage Vbus of the DC bus 2 (N in S18), instead of the processing in step S20, the control unit 16 changes the target value of the output voltage Vbus to the same value as the threshold value of the voltage Vbus of the DC bus 2, which is set as a fixed value. The processing of steps S10-S20 is continuously executed (N in S21) until the discharge from the battery system 1 is completed (Y in S21).

[0042] In the flowchart shown in Figure 2, when the target value of the output voltage Vbus of a certain battery unit 10 decreases, the output power of that battery unit 10 decreases, and therefore upward pressure is placed on the output current Ibus of the other battery units 10. Due to this upward pressure, in a battery unit 10 whose output current Ibus or discharge current Ibat has reached its upper limit, the target value of the output voltage Vbus is lowered, thereby reducing the output power. Due to this decrease in output power, upward pressure is placed on the output current Ibus of the other battery units 10. Due to this upward pressure, in a battery unit 10 whose output current Ibus or discharge current Ibat has reached its upper limit, the target value of the output voltage Vbus is lowered, thereby reducing the output power. Hereinafter, the control shown in the flowchart of Figure 2 will be referred to as CCV (Closed Circuit Voltage) control.

[0043] 3(a) and 3(b) are diagrams showing specific examples of the discharge voltage Vbat, discharge current Ibat, output voltage Vbus, and output current Ibus when there is characteristic variation in the converter module 14. FIG. 3(a) shows specific examples of the discharge voltage Vbat, discharge current Ibat, output voltage Vbus, and output current Ibus for current sharing control (comparative example) and CCV control (embodiment) at the start of discharge. FIG. 3(b) shows specific examples of the discharge voltage Vbat, discharge current Ibat, output voltage Vbus, and output current Ibus for current sharing control (comparative example) and CCV control (embodiment) during discharge. In the examples shown in FIGS. 3(a) and 3(b), the first battery module 11a to the eighth battery module 11h have the same characteristics.

[0044] In this specific example, in current share control (comparison example), the target value of the output voltage Vbus is set to 50.0 V. In CCV control (example), the initial target value of the output voltage Vbus is set to 50.0 V, but the target value of the output voltage Vbus is changed depending on the value of the discharge voltage Vbat of each battery unit.

[0045] The example of the variation in accuracy of the output current Ibus shown in Figure 3(a) shows an example in which the accuracy of the output current Ibus of the first cell unit 10a to the seventh cell unit 10g is the same, but the output current Ibus of the eighth cell unit 10h is measured lower than it actually is. As a result, the output power of the eighth cell unit 10h increases, and the actual current value of the output current Ibus of the eighth cell unit 10h is 50.5 A.

[0046] Because the output power of the eighth battery unit 10h is higher than the output power of the first battery unit 10a to the seventh battery unit 10g, the discharge current Ibat (57.4 A) at the start of discharge of the eighth battery unit 10h is higher than the discharge current Ibat (56.8 A) at the start of discharge of the first battery unit 10a to the seventh battery unit 10g.

[0047] When the discharge current Ibat of the eighth cell unit 10h is higher than the discharge current Ibat of the first cell unit 10a through the seventh cell unit 10g, the output power of the eighth cell unit 10h remains higher than the output power of the first cell unit 10a through the seventh cell unit 10g under current sharing control (comparison example). This causes the discharge voltage Vbat of the eighth cell unit 10h to decrease faster than the discharge voltages Vbat of the first cell unit 10a through the seventh cell unit 10g. Accordingly, the discharge current Ibat of the eighth cell unit 10h increases faster than the discharge current Ibat of the first cell unit 10a through the seventh cell unit 10g.

[0048] 3(b), the eighth cell unit 10h has a discharge voltage Vbat of 37.0 V and a discharge current Ibat of 68.2 A, while the first cell unit 10a to the seventh cell unit 10g have a discharge voltage Vbat of 40.0 V and a discharge current Ibat of 62.5 A. Compared to the start of discharge, it can be seen that the discharge voltage Vbat of the eighth cell unit 10h has decreased significantly compared to the first cell unit 10a to the seventh cell unit 10g, and the discharge current Ibat of the eighth cell unit 10h has increased significantly compared to the first cell unit 10a to the seventh cell unit 10g.

[0049] In contrast, under CCV control (embodiment), if the discharge voltage Vbat of the eighth cell unit 10h falls below the discharge voltages Vbat of the first through seventh cell units 10a through 10g, the target value of the output voltage Vbus of the eighth cell unit 10h falls, causing a decrease in the output power of the eighth cell unit 10h, which in turn reduces the input power of the DC / DC converter 15h of the eighth cell unit 10h.

[0050] By reducing the input power of the DC / DC converter 15h of the eighth battery unit 10h, the discharge voltages Vbat of the first battery unit 10a to the eighth battery unit 10h can be uniformly reduced, and the discharge currents Ibat of the first battery unit 10a to the eighth battery unit 10h can be uniformly increased.

[0051] The example of variation in conversion efficiency of the DC / DC converters 15 shown in Figure 3(a) shows an example in which the DC / DC converters 15a-15g of the first cell unit 10a through the seventh cell unit 10g have the same conversion efficiency, but the DC / DC converter 15h of the eighth cell unit 10h has a lower conversion efficiency. Because the DC / DC converter 15h of the eighth cell unit 10h has a lower conversion efficiency, the input power of the DC / DC converter 15h of the eighth cell unit 10h is higher than the input power of the DC / DC converters 15a-15g of the first cell unit 10a through the seventh cell unit 10g. As a result, the discharge current Ibat (59.8 A) at the start of discharge of the eighth cell unit 10h is higher than the discharge current Ibat (56.8 A) at the start of discharge of the first cell unit 10a through the seventh cell unit 10g.

[0052] When the discharge current Ibat of the eighth battery unit 10h is higher than the discharge current Ibat of the first through seventh battery units 10a through 10g, the input power of the DC / DC converter 15h of the eighth battery unit 10h remains higher than the input power of the DC / DC converters 15a through 15g of the first through seventh battery units 10a through 10g under current sharing control (comparative example). This causes the discharge voltage Vbat of the eighth battery unit 10h to decrease faster than the discharge voltages Vbat of the first through seventh battery units 10a through 10g. Accordingly, the discharge current Ibat of the eighth battery unit 10h increases faster than the discharge current Ibat of the first through seventh battery units 10a through 10g.

[0053] 3(b), the eighth cell unit 10h has a discharge voltage Vbat of 37.0 V and a discharge current Ibat of 71.1 A, while the first cell unit 10a to the seventh cell unit 10g have a discharge voltage Vbat of 40.0 V and a discharge current Ibat of 62.5 A. Compared to the start of discharge, it can be seen that the discharge voltage Vbat of the eighth cell unit 10h has decreased significantly compared to the first cell unit 10a to the seventh cell unit 10g, and the discharge current Ibat of the eighth cell unit 10h has increased significantly compared to the first cell unit 10a to the seventh cell unit 10g.

[0054] In contrast, under CCV control (embodiment), if the discharge voltage Vbat of the eighth cell unit 10h falls below the discharge voltages Vbat of the first through seventh cell units 10a through 10g, the target value of the output voltage Vbus of the eighth cell unit 10h falls, causing a decrease in the output power of the eighth cell unit 10h, which in turn reduces the input power of the DC / DC converter 15h of the eighth cell unit 10h.

[0055] By reducing the input power of the DC / DC converter 15h of the eighth battery unit 10h, the discharge voltages Vbat of the first cell unit 10a to the eighth cell unit 10h can be uniformly reduced, and the discharge currents Ibat of the first cell unit 10a to the eighth cell unit 10h can be uniformly increased. Note that although the output current Ibus of the DC / DC converter 15h of the eighth battery unit 10h will be lower than the output current Ibus of the first cell unit 10a to the eighth cell unit 10h, this is not a problem as long as the output current of the entire battery system 1 meets the requirements of the load 3.

[0056] 4(a) and 4(b) are diagrams showing specific examples of the discharge voltage Vbat, discharge current Ibat, output voltage Vbus, and output current Ibus when there is variation in the characteristics of the battery modules 11. FIG. 4(a) shows specific examples of the discharge voltage Vbat, discharge current Ibat, output voltage Vbus, and output current Ibus for current sharing control (comparison example) and CCV control (example) at the start of discharge. FIG. 4(b) shows specific examples of current sharing control (comparison example) and CCV control (example) during discharge. In the examples shown in FIGS. 4(a) and 4(b), the first to eighth converter modules 14a to 14h have the same characteristics.

[0057] 4(a) shows an example of capacity variation in which the battery packs 12a-12g of the first battery unit 10a to the seventh battery unit 10g have the same capacity, but the battery pack 12h of the eighth battery unit 10h has a lower capacity (higher degree of deterioration). In this example, the battery packs 12a-12g of the first battery unit 10a to the eighth battery unit 10h have the same voltage, but the battery pack 12h of the eighth battery unit 10h has a lower capacity than the battery packs 12a-12g of the first battery unit 10a to the seventh battery unit 10g.

[0058] In current sharing control (comparison example), the battery packs 12a-12h of the first through eighth battery units 10a-10h continue to request the same input power from the DC / DC converters 15a-15g. Because the capacity of the battery pack 12h of the eighth battery unit 10h is smaller than that of the battery packs 12a-12g of the first through seventh battery units 10a-10g, after discharge begins, the discharge voltage Vbat of the eighth battery unit 10h decreases faster than the discharge voltage Vbat of the first through seventh battery units 10a-10g. Accordingly, the discharge current Ibat of the eighth battery unit 10h increases faster than the discharge current Ibat of the first through seventh battery units 10a-10g.

[0059] In the state shown in Figure 4(b), the eighth cell unit 10h has a discharge voltage Vbat of 37.0 V and a discharge current Ibat of 67.6 A, while the first cell unit 10a to the seventh cell unit 10g have a discharge voltage Vbat of 40.0 V and a discharge current Ibat of 62.5 A. Compared to the start of discharge, it can be seen that the discharge voltage Vbat of the eighth cell unit 10h has decreased significantly compared to the first cell unit 10a to the seventh cell unit 10g, and the discharge current Ibat of the eighth cell unit 10h has increased significantly compared to the first cell unit 10a to the seventh cell unit 10g.

[0060] The above phenomenon also occurs when the internal resistance IR of the battery pack 12h of the eighth battery unit 10h is higher than the internal resistance IR of the battery packs 12a to 12g of the first to seventh battery units 10a to 10g.

[0061] In contrast, under CCV control (embodiment), if the discharge voltage Vbat of the eighth cell unit 10h falls below the discharge voltages Vbat of the first through seventh cell units 10a through 10g, the target value of the output voltage Vbus of the eighth cell unit 10h falls, causing a decrease in the output power of the eighth cell unit 10h, which in turn reduces the input power of the DC / DC converter 15h of the eighth cell unit 10h.

[0062] By reducing the input power of the DC / DC converter 15h of the eighth battery unit 10h, the discharge voltages Vbat of the first to eighth battery units 10a to 10h can be uniformly reduced. The discharge currents Ibat of the first to seventh battery units 10a to 10g increase, while the discharge current Ibat of the eighth battery unit 10h decreases. Note that the output current Ibus of the DC / DC converter 15h of the eighth battery unit 10h will be lower than the output current Ibus of the first to eighth battery units 10a to 10h, but this is not a problem as long as the output current of the entire battery system 1 meets the requirements of the load 3.

[0063] 4(a) shows an example of SOC variation in which the battery packs 12a to 12g of the first to seventh battery units 10a to 10g have the same SOC, but the battery pack 12h of the eighth battery unit 10h has a lower SOC (lower voltage). Because the voltage of the battery pack 12h of the eighth battery unit 10h is lower than the voltage of the battery packs 12a to 12g of the first to seventh battery units 10a to 10g, the discharge current Ibat (61.0 A) at the start of discharge of the eighth battery unit 10h is higher than the discharge current Ibat (56.8 A) at the start of discharge of the first to seventh battery units 10a to 10g.

[0064] In current sharing control (comparison example), the battery packs 12a-12h of the first cell unit 10a-eighth cell unit 10h continue to request the same input power from the DC / DC converters 15a-15g. Because the discharge current Ibat of the eighth cell unit 10h is higher than the discharge current Ibat of the first cell unit 10a-seventh cell unit 10g, the discharge voltage Vbat of the eighth cell unit 10h decreases faster than the discharge voltage Vbat of the first cell unit 10a-seventh cell unit 10g. Accordingly, the discharge current Ibat of the eighth cell unit 10h increases faster than the discharge current Ibat of the first cell unit 10a-seventh cell unit 10g.

[0065] In the state shown in Figure 4(b), the discharge voltage Vbat of the eighth cell unit 10h is 35.0 V and the discharge current Ibat is 71.4 A, while the discharge voltage Vbat of the first cell unit 10a to the seventh cell unit 10g is 40.0 V and the discharge current Ibat is 59.5 A. Compared to the start of discharge, it can be seen that the discharge voltage Vbat of the eighth cell unit 10h has decreased significantly compared to the first cell unit 10a to the seventh cell unit 10g, and the discharge current Ibat of the eighth cell unit 10h has increased significantly compared to the first cell unit 10a to the seventh cell unit 10g.

[0066] In contrast, under CCV control (embodiment), if the discharge voltage Vbat of the eighth cell unit 10h falls below the discharge voltages Vbat of the first through seventh cell units 10a through 10g, the target value of the output voltage Vbus of the eighth cell unit 10h falls, causing a decrease in the output power of the eighth cell unit 10h, which in turn reduces the input power of the DC / DC converter 15h of the eighth cell unit 10h.

[0067] By reducing the input power of the DC / DC converter 15h of the eighth battery unit 10h, the discharge voltages Vbat of the first to eighth battery units 10a to 10h can be uniformly reduced. The discharge currents Ibat of the first to seventh battery units 10a to 10g increase, while the discharge current Ibat of the eighth battery unit 10h decreases. Note that the output current Ibus of the DC / DC converter 15h of the eighth battery unit 10h will be lower than the output current Ibus of the first to eighth battery units 10a to 10h, but this is not a problem as long as the output current of the entire battery system 1 meets the requirements of the load 3.

[0068] As described above, according to this embodiment, the SOCs of the multiple battery packs 12 during discharge can be made closer to uniform by adjusting the output voltage Vbus of each DC / DC converter 15 so that the discharge voltage Vbat of each battery pack 12 is uniform. Specifically, SOC variations between the battery packs 12 due to output current accuracy errors between the battery units 10, deterioration or internal resistance variations between the battery packs 12, temperature variations, module replacement, etc. can be suppressed. If the SOCs of the battery packs 12 are uniform during discharge, this state can be maintained, and if variations in the SOCs of the battery packs 12 occur, the SOC variations can be reduced.

[0069] This makes it possible to prevent some of the battery packs 12 from prematurely completing discharge. If some of the battery packs 12 are disconnected from the parallel system, the current burden on the remaining battery packs 12 increases, leading to the shutdown of the entire battery system 1. Preventing some of the battery packs 12 from prematurely completing discharge helps ensure backup time for the battery system 1 in the event of a power outage.

[0070] Furthermore, in this embodiment, the output of DC / DC converter 15 is controlled based on the instantaneous value of discharge voltage Vbat, so that it is possible to instantly respond to environmental changes such as temperature changes. In the method of controlling output based on battery deterioration as disclosed in Patent Document 1, if some battery packs deteriorate extremely, the deteriorated battery packs will hardly supply any current, which will increase the current burden on the other battery packs and lead to the shutdown of the battery system.

[0071] In contrast to this, in the present embodiment, a lower limit is set for the output voltage Vbus of the DC / DC converter 15, an upper limit is set for the output current Ibus of the DC / DC converter 15, and an upper limit is set for the discharge current Ibat of the battery pack 12. Therefore, even if some of the battery packs are extremely deteriorated, it is possible to avoid a situation in which almost no current is supplied from the deteriorated battery packs.

[0072] The present disclosure has been described above based on the embodiments. The embodiments are merely examples, and it will be understood by those skilled in the art that various modifications are possible in the combination of the respective components and processing steps, and that such modifications are also within the scope of the present disclosure.

[0073] For example, the battery module 11 and converter module 14 in the battery unit 10 may be integrated, and the microcontroller of the battery control unit 13 and the microcontroller of the control unit 16 may be integrated.

[0074] In the above embodiment, an example has been described in which the battery system 1 is used for backup purposes in a data center. In this regard, the battery system 1 can also be used for backup purposes in factory automation, base stations, commercial facilities, offices, apartment buildings, and detached houses. In addition to backup purposes, the battery system 1 can also be used for peak shifting and ancillary purposes. It can also be used for in-vehicle purposes.

[0075] In the above embodiment, an example has been described in which the battery pack 12 including lithium ion battery cells is used. In this regard, a capacitor pack including electric double layer capacitor cells, lithium ion capacitor cells, etc. may also be used. In this specification, the battery pack and the capacitor pack are collectively referred to as a power storage pack.

[0076] The embodiment may be specified by the following items.

[0077] [Item 1] An electricity storage system (1) comprising: a plurality of electricity storage units (10a-10h) connected in parallel to a DC bus (2) to which a load (3) is connected; each of the plurality of electricity storage units (10) including: an electricity storage pack (12); a DC / DC converter (15) for charging and discharging the electricity storage pack (12); and a control unit (16) for controlling the DC / DC converter (15), wherein the control unit (16) acquires, via communication, discharge voltages of the electricity storage packs (12) included in the other electricity storage units (10) from the other control units (16), and adaptively controls the output of the DC / DC converter (15) included in its own electricity storage unit (10) (among the plurality of electricity storage units that includes the control unit) so that discharge voltages of the plurality of electricity storage packs (12a-12h) included in the plurality of electricity storage units (10a-10h) approach each other.

[0078] That is, the plurality of power storage units (10) include a plurality of power storage packs (12), a plurality of DC / DC converters (15) for charging and discharging the plurality of power storage packs (12), and a plurality of control units (16) for controlling the plurality of DC / DC converters (15).

[0079] Each of the plurality of storage units (10) includes: a corresponding one of a plurality of storage packs (12); a corresponding one of a plurality of DC / DC converters (15) for charging and discharging the corresponding one of the storage packs (12); and a corresponding one of a plurality of control units (16) for controlling the corresponding one of the DC / DC converters (15).

[0080] Another of the plurality of power storage units (10) includes: another corresponding one of the plurality of power storage packs (12); another corresponding one of the plurality of DC / DC converters (15) for charging and discharging the other corresponding one of the power storage packs (12); and another corresponding one of the plurality of control units (16) for controlling the other corresponding one of the DC / DC converters (15).

[0081] The one corresponding control unit (16) acquires the discharge voltage of the other corresponding electricity storage pack (12) from the other corresponding control unit (16) through communication, and adaptively controls the output of the one corresponding DC / DC converter (15) so that the discharge voltages of the plurality of electricity storage packs (12a-12h) included in the plurality of electricity storage units (10a-10h) approach each other.

[0082] This makes it possible to suppress variations in SOC during discharge among the plurality of parallel-connected power storage units (10a-10h).

[0083] [Item 2] The control unit (16) controls the output voltage of the DC / DC converter (15) included in its own power storage unit (10) (including the control unit) so as to maintain a target value, and when a discharge voltage of the power storage pack (12) included in its own power storage unit (10) (including the control unit) is lower than the discharge voltages of all other power storage packs (12) obtained from other control units (16), reduces the target value of the output voltage of the DC / DC converter (15) by a predetermined value.

[0084] That is, the one corresponding control unit (16) controls the output voltage of the one corresponding DC / DC converter (15) so as to maintain the target value, and when the discharge voltage of the one corresponding electricity storage pack (12) is lower than the discharge voltages of all of the other one or more electricity storage packs (12) of the one corresponding electricity storage pack (12) of the plurality of electricity storage packs (12) obtained from the other one or more control units (16) of each of the plurality of control units (16), reduces the target value of the output voltage of the one corresponding DC / DC converter (15) by a predetermined value.

[0085] This makes it possible to prevent a large increase in the discharge current of the electricity storage pack (12) having a lower discharge voltage than the others.

[0086] [Item 3] The power storage system (1) according to Item 1, wherein the control unit (16) controls the output voltage of the DC / DC converter (15) included in its own power storage unit (10) (including the control unit) so as to maintain a target value, and when the output current of the DC / DC converter (15) included in its own power storage unit (10) (including the control unit) exceeds an upper limit value, reduces the target value of the output voltage of the DC / DC converter (15) by a predetermined value.

[0087] That is, the corresponding one control unit (16) controls the output voltage of the corresponding one DC / DC converter (15) so as to maintain the target value, and when the output current of the corresponding one DC / DC converter (15) exceeds an upper limit value, reduces the target value of the output voltage of the corresponding one DC / DC converter (15) by a predetermined value.

[0088] This makes it possible to prevent output from being stopped due to overcurrent protection.

[0089] [Item 4] The power storage system (1) according to Item 1, wherein the control unit (16) controls the output voltage of the DC / DC converter (15) included in its own power storage unit (10) (including the control unit) so as to maintain a target value, and when a discharge current of the power storage pack (12) included in its own power storage unit (10) (including the control unit) exceeds an upper limit value, the control unit (16) reduces the target value of the output voltage of the DC / DC converter (15) by a predetermined value.

[0090] That is, the corresponding one control unit (16) controls the output voltage of the corresponding one DC / DC converter (15) so as to maintain the target value, and when the discharge current of the corresponding one storage pack (12) exceeds an upper limit value, reduces the target value of the output voltage of the corresponding one DC / DC converter (15) by a predetermined value.

[0091] This makes it possible to prevent output from being stopped due to overcurrent protection.

[0092] [Item 5] The power storage system (1) according to any one of Items 2 to 4, wherein the control unit (16) reduces the target value of the output voltage of the DC / DC converter (15) in a stepwise manner.

[0093] That is, the corresponding one control unit (16) reduces the target value of the output voltage of the corresponding one DC / DC converter (15) in a stepwise manner.

[0094] This makes it possible to prevent sudden fluctuations in the discharge current and suppress variations in the SOC.

[0095] [Item 6] An energy storage unit used in parallel connection with another energy storage unit to a DC bus (2) to which a load (3) is connected, the energy storage unit including: an energy storage pack (12); a DC / DC converter (15) for charging and discharging the energy storage pack (12); and a control unit (16) for controlling the DC / DC converter (15), wherein the other energy storage unit includes: the other energy storage pack (12); another DC / DC converter (15) for charging and discharging the other energy storage pack (12); and another control unit (16) for controlling the other DC / DC converter (15), wherein the control unit (16) acquires a discharge voltage of the other energy storage pack (12) included in the other energy storage unit (10) from the other control unit (16) through communication; and adaptively controls an output of the DC / DC converter (15) so that discharge voltages of a plurality of energy storage packs (12) included in a plurality of energy storage units (10a-10h) connected in parallel approach each other. A power storage unit (10).

[0096] That is, the power storage unit (10a) is used by being connected in parallel with one or more other power storage units (10b-10h) to the DC bus (2) to which the load (3) is connected.

[0097] The electricity storage unit (10a) includes an electricity storage pack (12), a DC / DC converter (15) for charging and discharging the electricity storage pack (12), and a control unit (16) for controlling the DC / DC converter (15).

[0098] Each of the one or more other power storage units (10b-10h) includes another power storage pack (12), another DC / DC converter (15) for charging and discharging the other power storage pack (12), and another control unit (16) for controlling the other DC / DC converter (15).

[0099] The control unit (16) acquires the discharge voltage of the other electricity storage pack (12) from the other control unit (16) through communication, and adaptively controls the output of the DC / DC converter (15) so that the discharge voltages of the electricity storage pack (12) and the other electricity storage pack (12) approach each other.

[0100] This makes it possible to suppress variations in SOC during discharge among the plurality of parallel-connected power storage units (10a-10h).

[0101] REFERENCE SIGNS LIST 1 Battery system 2 DC bus 3 Load 10 Battery unit 11 Battery module 12 Battery pack 13 Battery control unit 14 Converter module 15 DC / DC converter 16 Control unit 20 Communication line

Claims

1. A power storage system comprising: a plurality of power storage units connected in parallel to a DC bus to which a load is connected, each of the plurality of power storage units including a power storage pack, a DC / DC converter for charging and discharging the power storage pack, and a control unit for controlling the DC / DC converter, wherein the control unit acquires, via communication, a discharge voltage of the power storage pack included in another power storage unit from another control unit, and adaptively controls an output of the DC / DC converter included in a power storage unit including the control unit among the plurality of power storage units so that the discharge voltages of the multiple power storage packs included in the plurality of power storage units approach each other.

2. The energy storage system according to claim 1, wherein the control unit controls the output voltage of the DC / DC converter included in the energy storage unit including the control unit so as to maintain a target value, and when the discharge voltage of the energy storage pack included in the energy storage unit including the control unit is lower than the discharge voltages of all other energy storage packs obtained from other control units, reduces the target value of the output voltage of the DC / DC converter by a predetermined value.

3. The energy storage system according to claim 1, wherein the control unit controls the output voltage of the DC / DC converter included in the energy storage unit including the control unit so as to maintain a target value, and when the output current of the DC / DC converter included in the energy storage unit including the control unit exceeds an upper limit value, the target value of the output voltage of the DC / DC converter is reduced by a predetermined value.

4. The energy storage system according to claim 1, wherein the control unit controls the output voltage of the DC / DC converter included in the energy storage unit including the control unit so as to maintain a target value, and when a discharge current of a storage pack included in the energy storage unit including the control unit exceeds an upper limit value, the target value of the output voltage of the DC / DC converter is reduced by a predetermined value.

5. The power storage system according to claim 2, wherein the control unit gradually reduces a target value of the output voltage of the DC / DC converter.

6. A power storage unit used in parallel connection together with another power storage unit to a DC bus to which a load is connected, comprising: a power storage pack; a DC / DC converter for charging and discharging the power storage pack; and a control unit for controlling the DC / DC converter, wherein the other power storage unit comprises: another power storage pack; another DC / DC converter for charging and discharging the other power storage pack; and another control unit for controlling the other DC / DC converter, wherein the control unit acquires a discharge voltage of the other power storage pack included in the other power storage unit from the other control unit through communication, and adaptively controls an output of the DC / DC converter so that the discharge voltages of a plurality of power storage packs included in a plurality of power storage units connected in parallel approach each other.

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