Battery pack and energy storage system

Abstract

The present application belongs to the technical field of energy storage, and provides a battery pack and an energy storage system. The battery pack provided in the present application is provided with a DC / DC conversion circuit, and the DC / DC conversion circuit can provide a first excitation current for a plurality of battery cells in the battery pack. A BMS in the battery pack can detect, by means of a current sampling circuit, a second excitation current flowing through the plurality of battery cells, and measure the voltages generated by the second excitation current at two ends of each battery cell to then determine the impedance of each battery cell on the basis of the second excitation current and the voltages. Since the DC / DC conversion circuit is disposed independent of the BMS and is not integrated into the BMS, the heat dissipation performance of the BMS is not affected when the DC / DC conversion circuit provides the first excitation current. Correspondingly, the first excitation current provided by the DC / DC conversion circuit can be large, thereby ensuring that the voltages sampled by the BMS are large, so as to accurately measure the impedances of the battery cells.

Description

Battery pack and energy storage system

[0001] The present application claims priority to the Chinese patent application No. 202410877237.1, filed on July 01, 2024, and entitled "Battery pack and energy storage system", the entire content of which is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application relates to the technical field of energy storage, and in particular, to a battery pack and an energy storage system. BACKGROUND

[0003] An energy storage system generally includes a plurality of battery packs, and each battery pack can include a battery management system (BMS) and a plurality of battery cells (also referred to as battery monomers). In order to improve the reliability of the energy storage system, the state of the battery cells needs to be periodically detected. Generally, the detection of the state of the battery cells can be achieved by detecting the impedance of the battery cells, for example, by detecting the electrochemical impedance spectroscopy (EIS) of the battery cells.

[0004] When detecting the impedance of the battery cells, the BMS in the battery pack can provide an excitation current for the plurality of battery cells, and the BMS can sample the voltage generated by the excitation current, thereby achieving the detection of the impedance of the battery cells. However, since the BMS is an integrated chip, in order to avoid excessive heat, the BMS can only provide a small excitation current. As a result, the voltage sampled by the BMS is small, and thus the accuracy of the impedance detection is low. SUMMARY

[0005] The present application provides a battery pack and an energy storage system, which can solve the technical problem of low impedance detection accuracy caused by the small excitation current provided by the BMS.

[0006] In a first aspect, a battery pack is provided, which includes a plurality of battery cells, a direct current / direct current (DC / DC) conversion circuit, a current sampling circuit, and a BMS. The plurality of battery cells are connected in series between the positive electrode and the negative electrode of the battery pack. The first end of the DC / DC conversion circuit is connected to the positive electrode and the negative electrode of the battery pack, and is configured to provide a first excitation current to the plurality of battery cells. The BMS is configured to detect a second excitation current flowing through the plurality of battery cells through the current sampling circuit, the second excitation current being the excitation current flowing through the plurality of battery cells when the DC / DC conversion circuit provides the first excitation current to the plurality of battery cells. The BMS is further configured to detect the voltage generated across each battery cell in the plurality of battery cells by the second excitation current. The BMS is further configured to determine the impedance of each battery cell in the plurality of battery cells based on the second excitation current and the voltage.

[0007] The battery pack provided by the application is provided with a DC / DC conversion circuit, which can provide a first excitation current for a plurality of battery cells. Since the DC / DC conversion circuit is independent of the BMS and is not integrated in the BMS, the DC / DC conversion circuit will not affect the heat dissipation performance of the BMS when providing the first excitation current. Correspondingly, the first excitation current provided by the DC / DC conversion circuit can be larger, thereby ensuring that the voltage sampled by the BMS is larger, so as to realize accurate detection of the impedance of the battery cell.

[0008] In addition, after the DC / DC conversion circuit provides the first excitation current, the BMS can simultaneously detect the voltages of a plurality of battery cells, and further detect the impedances of the plurality of battery cells, and the detection efficiency is higher. Since the process of the DC / DC conversion circuit providing the first excitation current, and the process of the BMS detecting the second excitation current and the voltage, and determining the impedance, will not affect the normal operation (such as normal charging and discharging) of the battery pack, the online detection of the impedance of the battery cell can be realized during the operation of the battery pack. In addition, the topology of the battery pack provided by the application is relatively simple, and the volume and cost of the battery pack can be avoided.

[0009] Optionally, the battery pack can further include a capacitor. One end of the DC / DC conversion circuit is connected with the positive electrode and the negative electrode of the battery pack, and the other end of the DC / DC conversion circuit is connected with the capacitor in parallel, and the DC / DC conversion circuit is used for alternately charging and discharging the capacitor to provide the first excitation current for the plurality of battery cells. Wherein, when the DC / DC conversion circuit charges the capacitor, the first excitation current flows from the negative electrode to the positive electrode of the battery pack; when the DC / DC conversion circuit discharges the capacitor, the first excitation current flows from the positive electrode to the negative electrode of the battery pack.

[0010] The battery pack provided by the application can alternately charge and discharge the capacitor through the DC / DC conversion circuit to provide the first excitation current of the sine wave. Since the DC / DC conversion circuit can provide the first excitation current when charging and discharging the capacitor, the energy loss can be effectively reduced. In addition, the DC / DC conversion circuit can be CLLC topology, LLC topology, LLLC topology, dual active bridge topology or bidirectional flyback topology. Wherein, C refers to capacitor, and L refers to inductor. The DC / DC conversion circuit can adopt a plurality of different types of topologies, and the application flexibility is higher.

[0011] Optionally, the DC / DC conversion circuit can also be used for outputting power to other battery packs, and receiving power input by other battery packs, to provide the first excitation current for the plurality of battery cells. Wherein, the other battery packs are other battery packs in the same energy storage system except the battery pack.

[0012] Wherein, when the DC / DC conversion circuit outputs power, the first excitation current flows from the negative electrode to the positive electrode of the battery pack; when the DC / DC conversion circuit receives power input by other battery packs, the first excitation current flows from the positive electrode to the negative electrode of the battery pack. In this way, on the one hand, the DC / DC conversion circuit can provide a larger first excitation current to the plurality of battery cells, and on the other hand, the energy loss of the energy storage system during the process of providing the first excitation current can be reduced.

[0013] Optionally, the battery pack can further include a control chip. The control chip is configured to send a synchronization signal to the BMS, and the synchronization signal is a digital signal. The frequency of the synchronization signal is the same as the frequency of the first excitation current, and the phase of the synchronization signal is the same as the phase of the first excitation current. The BMS can be configured to calibrate the phase of the second excitation current based on the phase of the synchronization signal, and determine the impedance of each battery cell at the frequency of the synchronization signal based on the calibrated second excitation current and the voltage of each battery cell.

[0014] It can be understood that the frequency of the first excitation current can vary within a preset frequency range, and the BMS can detect the impedance of each battery cell at different frequencies within the frequency range. Since the frequency of the second excitation current detected by the BMS through the current sampling circuit can be superimposed with the frequency of the interference signal, and the synchronization signal is a digital signal and the transmission path of the synchronization signal is shorter than that of the second excitation current, the synchronization signal is less affected by interference. Accordingly, the BMS can more accurately determine the frequency of the first excitation current through the synchronization signal provided by the control chip, and thus can accurately detect the impedance of each battery cell at different frequencies.

[0015] It can also be understood that the phase of the second excitation current has a certain offset compared to the actual phase of the first excitation current, and the synchronization signal is less affected by interference. Therefore, the BMS can determine the phase of the first excitation current based on the phase of the synchronization signal, and calibrate the phase of the second excitation current based on the phase. Then, the BMS detects the impedance of the battery cell based on the phase-calibrated second excitation current and the voltage, to ensure that the accuracy of impedance detection is high.

[0016] Optionally, the DC / DC conversion circuit and the current sampling circuit can be integrated on the same circuit board. In this way, the integration of the battery pack can be effectively improved, and the volume of the battery pack can be avoided.

[0017] Optionally, the current sampling circuit can include a current sensor and an operational amplifier. The current sensor is connected between the first end of the DC / DC conversion circuit and the positive electrode or the negative electrode of the battery pack, and is configured to convert a second excitation current flowing through the current sensor into a voltage signal. The input end of the operational amplifier is connected with the current sensor, and the output end of the operational amplifier is connected with the BMS. The operational amplifier is configured to amplify the voltage signal and transmit the amplified voltage signal to the BMS.

[0018] Since the operational amplifier can amplify the voltage signal sampled by the current sensor and transmit the amplified voltage signal to the BMS, it can ensure that the voltage signal received by the BMS for characterizing the second excitation current has a large amplitude, thereby ensuring that the detection accuracy of the BMS based on the second excitation current is high. It can be understood that the total current flowing through the plurality of battery cells includes not only the second excitation current but also the charging current or discharging current of the battery pack itself. Since the current sensor is connected between the first end of the DC / DC conversion circuit and the positive electrode or the negative electrode of the battery pack, it can directly detect the second excitation current flowing through the plurality of battery cells. Accordingly, the BMS can directly determine the impedance based on the second excitation current without extracting the second excitation current from the total current flowing through the plurality of battery cells. Thus, the processing logic of the BMS is effectively simplified, and the calculation complexity of the BMS in determining the impedance is reduced.

[0019] Optionally, the current sampling circuit can include a current sensor and an operational amplifier. The current sensor is connected in series with the plurality of battery cells, and is configured to convert a total current flowing through the current sensor into a voltage signal. The input end of the operational amplifier is connected with the current sensor, and the output end of the operational amplifier is connected with the BMS. The operational amplifier is configured to amplify the voltage signal and transmit the amplified voltage signal to the BMS. The BMS is further configured to determine the second excitation current flowing through the plurality of battery cells based on the total current.

[0020] Since the current sensor is connected in series with the plurality of battery cells, the total current detected by the current sensor includes not only the second excitation current but also the charging current or discharging current of the battery pack itself. In order to accurately detect the impedance of the battery cells, the BMS can extract the second excitation current from the total current and then determine the impedance of the battery cells.

[0021] Optionally, the size of the first excitation current is positively correlated with the capacity of any battery cell in the plurality of battery cells. It can be understood that the capacity of the battery cell is negatively correlated with the internal resistance, i.e., the larger the capacity of the battery cell, the smaller the internal resistance. In the scheme provided in the present application, if the capacity of the battery cell is large, i.e., the internal resistance of the battery cell is small, the first excitation current can be large to ensure that the second excitation current and the voltage detected by the BMS are large, thereby ensuring that the impedance detection accuracy is high.

[0022] Optionally, the first excitation current can have a magnitude ranging from 2 amperes (A) to 30 A. The first excitation current in this range can ensure accurate detection of the impedance of the battery cell.

[0023] Optionally, the first excitation current can have a frequency ranging from 0.01 hertz (Hz) to 8 kilohertz (kHz) to enable impedance detection at different frequencies in this frequency range.

[0024] Optionally, the first excitation current can be composed of at least two sinusoidal signals of different frequencies. In this way, impedance detection at different frequencies can be achieved based on one excitation current, improving detection efficiency.

[0025] Optionally, the BMS can also be configured to report the impedance of each battery cell in the plurality of battery cells to a cloud server for the cloud server to detect the health of each battery cell in the plurality of battery cells and identify potential safety risks of the battery pack in a timely manner.

[0026] In a second aspect, a power storage system is provided, which includes a housing and a plurality of battery packs according to the first aspect, and the plurality of battery packs can be arranged in the housing.

[0027] Optionally, the second ends of the DC / DC conversion circuits in the plurality of battery packs can be connected in parallel, and the DC / DC conversion circuit in the first battery pack can be configured to output power to the second battery pack and receive power input from the second battery pack to provide the first excitation current to the plurality of battery cells in the first battery pack. When the DC / DC conversion circuit in the first battery pack outputs power, the first excitation current flows from the negative electrode to the positive electrode of the first battery pack; when the DC / DC conversion circuit in the first battery pack receives power input from the second battery pack, the first excitation current flows from the positive electrode to the negative electrode of the first battery pack. In this way, on the one hand, the DC / DC conversion circuit can provide a larger first excitation current to the battery cells in the first battery pack, and on the other hand, the energy loss of the power storage system can be reduced during the provision of the first excitation current.

[0028] Optionally, the power storage system can further include a cloud server configured to receive the impedance of the plurality of battery cells reported by each battery pack and detect the health of each battery cell in the plurality of battery cells based on the impedance of the plurality of battery cells. In this way, the cloud server can achieve online monitoring of the impedance of the battery cells to identify potential safety risks of the plurality of battery packs in a timely manner.

[0029] In summary, the application provides a battery pack and an energy storage system. The battery pack provided by the application is provided with a DC / DC conversion circuit, which can provide a first excitation current for a plurality of battery cells of the battery pack. The BMS in the battery pack can detect a second excitation current flowing through the plurality of battery cells through a current sampling circuit, and detect the voltage of each battery cell, and then determine the impedance of each battery cell based on the second excitation current and the voltage. Since the DC / DC conversion circuit is provided independently of the BMS and is not integrated in the BMS, the DC / DC conversion circuit will not affect the heat dissipation performance of the BMS when providing the first excitation current. Correspondingly, the first excitation current provided by the DC / DC conversion circuit can be larger, and thus the voltage sampled by the BMS can be larger, so as to realize accurate detection of the impedance of the battery cell.

[0030] In addition, after the DC / DC conversion circuit provides the first excitation current, the BMS can simultaneously detect the voltage of the plurality of battery cells, and then detect the impedance of the plurality of battery cells, and the detection efficiency is higher. Since the process of providing the first excitation current by the DC / DC conversion circuit, and the process of detecting the current and voltage by the BMS and determining the impedance will not affect the normal operation (such as normal charging and discharging) of the battery pack, the online detection of the impedance of the battery cell can be realized during the operation of the battery pack. In addition, the topology of the battery pack provided by the application is simple, and the volume and cost of the battery pack can be avoided. BRIEF DESCRIPTION OF DRAWINGS

[0031] FIG. 1 is a structural schematic diagram of an application scenario of a battery pack provided by an embodiment of the application;

[0032] FIG. 2 is a structural schematic diagram of a battery pack provided by an embodiment of the application;

[0033] FIG. 3 is a structural schematic diagram of another battery pack provided by an embodiment of the application;

[0034] FIG. 4 is a structural schematic diagram of another battery pack provided by an embodiment of the application;

[0035] FIG. 5 is a structural schematic diagram of another battery pack provided by an embodiment of the application;

[0036] FIG. 6 is a structural schematic diagram of another battery pack provided by an embodiment of the application;

[0037] FIG. 7 is a structural schematic diagram of another battery pack provided by an embodiment of the application;

[0038] FIG. 8 is a structural schematic diagram of a DC / DC conversion circuit provided by an embodiment of the application;

[0039] FIG. 9 is a waveform schematic diagram of a first excitation current and a synchronization signal provided by an embodiment of the application;

[0040] FIG. 10 is a structural schematic diagram of a current sampling circuit according to an embodiment of the present application;

[0041] FIG. 11 is a structural schematic diagram of an energy storage system according to an embodiment of the present application;

[0042] FIG. 12 is a structural schematic diagram of another energy storage system according to an embodiment of the present application;

[0043] FIG. 13 is a structural schematic diagram of yet another energy storage system according to an embodiment of the present application;

[0044] FIG. 14 is a structural schematic diagram of still another energy storage system according to an embodiment of the present application;

[0045] FIG. 15 is a structural schematic diagram of still another energy storage system according to an embodiment of the present application;

[0046] FIG. 16 is a structural schematic diagram of still another energy storage system according to an embodiment of the present application;

[0047] FIG. 17 is a structural schematic diagram of still another energy storage system according to an embodiment of the present application. DETAILED DESCRIPTION

[0048] The battery pack and the energy storage system according to the embodiments of the present application will be described in detail below with reference to the accompanying drawings. First, key terms related to the embodiments of the present application will be introduced.

[0049] In order to maintain the stability of a new energy power generation system (for example, a photovoltaic power generation system), it is often necessary to configure an energy storage system with a certain capacity for the new energy power generation system. Moreover, with the rapid development of new energy power generation technology, the capacity of the energy storage system to be configured also gradually becomes larger. Correspondingly, the application of large-capacity lithium ion cells in the energy storage system is becoming more and more common, and the measurability of the state of the cells has become a key factor for improving the reliability of the energy storage system.

[0050] FIG. 1 is a structural schematic diagram of an application scenario of a battery pack according to an embodiment of the present application. As shown in FIG. 1, the application scenario can include a plurality of battery packs 10 and a power conversion system (PCS) 20. The plurality of battery packs 10 are connected in series and connected to the direct current end of the power conversion system 20, and the alternating current end of the power conversion system 20 can be connected to a power grid 30 and / or a load 40. The power conversion system 20 can convert the direct current provided by the plurality of battery packs 10 into alternating current and output to the power grid 30 and / or the load 40. Moreover, the power conversion system 20 can also convert the alternating current provided by the power grid 20 into direct current and output to the plurality of battery packs 10, thereby charging the plurality of battery packs 10.

[0051] Each battery pack 10 can include a plurality of battery cells 11 connected in series, which can be lithium ion battery cells, such as lithium iron phosphate battery cells or ternary lithium battery cells, etc.

[0052] In some embodiments, the BMS in the battery pack can provide an excitation current for the plurality of battery cells, and the BMS can sample the voltage generated by the excitation current to achieve impedance detection of the battery cells. However, since the BMS is an integrated chip, to avoid excessive heat of the chip, the excitation current that the BMS can provide is small. As a result, the voltage sampled by the BMS is small, which leads to low accuracy of impedance detection.

[0053] In other embodiments, the battery pack includes a BMS and a current induction circuit. The DC / DC in the BMS can output a direct current, and the current induction circuit can convert the direct current into an alternating excitation current and provide it to a single battery cell. In this way, the direct current provided by the BMS is small, which leads to a small excitation current provided by the current induction circuit. In addition, in this way, a current induction circuit needs to be configured for each battery cell, which has a complex structure and high cost.

[0054] Embodiments of the present application provide a battery pack, which can be applied to an application scenario such as shown in FIG. 1. As shown in FIG. 2, the battery pack 10 includes a plurality of battery cells 11, a DC / DC conversion circuit 12, a current sampling circuit 13, and a BMS 14.

[0055] The plurality of battery cells 11 are connected in series between the positive electrode BAT+ and the negative electrode BAT- of the battery pack 10. The first end of the DC / DC conversion circuit 12 is connected to the positive electrode BAT+ and the negative electrode BAT- of the battery pack 10, and the DC / DC conversion circuit 12 is configured to provide a first excitation current to the plurality of battery cells 11. The BMS 14 is configured to detect a second excitation current flowing through the plurality of battery cells 11 through the current sampling circuit 13, the second excitation current being the excitation current flowing through the plurality of battery cells 11 when the DC / DC conversion circuit 12 provides the first excitation current to the plurality of battery cells 11. It can be understood that the phase of the second excitation current can be offset compared to the first excitation current, and the frequency of the second excitation current can be superimposed with the frequency of an interference signal. The BMS 14 is further configured to detect a voltage generated across each battery cell 11 of the plurality of battery cells 11 by the second excitation current. The BMS 14 is further configured to determine an impedance of each battery cell 11 of the plurality of battery cells 11 based on the second excitation current and the voltage of the plurality of battery cells 11. The BMS 14 includes a battery management integrated circuit (BMIC), and the BMIC is disposed on a BMS single board.

[0056] For example, for each battery cell 11 in the battery pack 10, the BMS 14 can determine the EIS of the battery cell 11 based on the detected second excitation current and the voltage across the battery cell 11, i.e., the BMS 14 can perform online EIS measurement for each battery cell 11 in the plurality of battery cells 11. The measurement result of the EIS can include a real part and an imaginary part, wherein the real part is the impedance of the battery cell 11.

[0057] In summary, the embodiments of the present application provide a battery pack, and the DC / DC conversion circuit in the battery pack can provide a first excitation current for a plurality of battery cells. Since the DC / DC conversion circuit is independent of the BMS and is not integrated in the BMS, the DC / DC conversion circuit will not affect the heat dissipation performance of the BMS when providing the first excitation current. Accordingly, the first excitation current provided by the DC / DC conversion circuit can be larger, and thus the voltage sampled by the BMS can be larger to ensure accurate detection of the impedance of the battery cells.

[0058] In addition, after the DC / DC conversion circuit provides the first excitation current, the BMS can simultaneously detect the voltages of the plurality of battery cells and further detect the impedances of the plurality of battery cells, and the detection efficiency is higher. Since the process of providing the first excitation current by the DC / DC conversion circuit and the process of detecting the current and voltage by the BMS and determining the impedance will not affect the normal operation (such as normal charging and discharging) of the battery pack, the online detection of the impedance of the battery cells can be realized during the operation of the battery pack. In addition, the topology of the battery pack provided by the embodiments of the present application is simple, and the volume and cost of the battery pack can be avoided.

[0059] As an optional implementation manner, as shown in FIG. 3, the battery pack 10 can further include a capacitor C0. The second end of the DC / DC conversion circuit 12 is connected in parallel with the capacitor C0, and the DC / DC conversion circuit 12 is configured to alternately charge and discharge the capacitor C0 to provide a first excitation current for the plurality of battery cells 11 in the battery pack 10.

[0060] Wherein, when the DC / DC conversion circuit 12 charges the capacitor C0, the first excitation current flows from the negative electrode BAT- to the positive electrode BAT+ of the battery pack 10; and when the DC / DC conversion circuit 12 discharges the capacitor C0, the first excitation current flows from the positive electrode BAT+ to the negative electrode BAT- of the battery pack 10. Since the first excitation current can be provided when charging and discharging the capacitor C0, the energy loss in the impedance detection process can be effectively reduced.

[0061] As another optional implementation manner, as shown in FIG. 12, the DC / DC conversion circuit 12 can also be configured to output power to other battery packs and receive power input from other battery packs to provide a first excitation current for the plurality of battery cells 11. Wherein, the other battery packs are other battery packs in the same energy storage system except the battery pack.

[0062] Wherein, when the DC / DC conversion circuit 12 outputs power, the first excitation current flows from the negative pole BAT- to the positive pole BAT+ of the battery pack 10; when the DC / DC conversion circuit 12 receives power input by other battery packs, the first excitation current flows from the positive pole BAT+ to the negative pole BAT- of the battery pack 10. In this way, on the one hand, the DC / DC conversion circuit 12 can ensure that the first excitation current provided to the plurality of battery cells 11 is large, and on the other hand, the energy loss of the energy storage system during the process of providing the first excitation current can be small.

[0063] In the embodiments of the present application, the DC / DC conversion circuit 12 can be a CLLC topology, an LLC topology, an LLLC topology, a dual active bridge topology or a bidirectional flyback topology. C in the above topologies refers to a capacitor, and L refers to an inductor. Since the DC / DC conversion circuit 12 can adopt a plurality of different types of topologies, it has high application flexibility.

[0064] For example, the DC / DC conversion circuit 12 can be a CLLC topology as shown in FIG. 3, or can be a variant topology of the CLLC topology.

[0065] Alternatively, the DC / DC conversion circuit 12 can be an LLC topology as shown in FIG. 4, or can be a variant topology of the LLC topology.

[0066] Alternatively, the DC / DC conversion circuit 12 can be an LLLC topology as shown in FIG. 5, or can be a variant topology of the LLLC topology.

[0067] Alternatively, the DC / DC conversion circuit 12 can be a dual active bridge (DAB) topology as shown in FIG. 6, or can be a variant topology of the DAB topology.

[0068] Alternatively, the DC / DC conversion circuit 12 can be a bidirectional flyback topology as shown in FIG. 7, or can be a variant topology of the bidirectional flyback topology.

[0069] As shown in FIG. 8, and as can be seen with reference to FIGS. 3-6, the DC / DC conversion circuit 12 can include a first bridge circuit 121, a transformer T0 and a second bridge circuit 122. The primary side of the transformer T0 is connected with the first bridge circuit 121, and the secondary side of the transformer T0 is connected with the second bridge circuit 122.

[0070] The first bridge circuit 121 can be a full-bridge resonant circuit, which can include a first bridge arm 1a, a second bridge arm 1b, and a resonant device 1c. The first bridge arm 1a and the second bridge arm 1b are connected in parallel, and each of the first bridge arm 1a and the second bridge arm 1b includes two switch tubes connected in series. One end of the primary side of the transformer T0 is connected to the bridge arm midpoint of the first bridge arm 1a through the resonant device 1c, and the other end of the primary side of the transformer T0 is connected to the bridge arm midpoint of the second bridge arm 1b through the resonant device 1c. The bridge arm midpoint of the bridge arm can refer to the series node between the two switch tubes in the bridge arm. The resonant device 1c can include at least one inductor and at least one capacitor.

[0071] As a possible example, as shown in FIGS. 4-6 and FIG. 8, the second bridge circuit 122 can be a full-bridge circuit, which can include a third bridge arm 2a and a fourth bridge arm 2b connected in parallel. Each of the third bridge arm 2a and the fourth bridge arm 2b includes two switch tubes connected in series. One end of the secondary side of the transformer T0 is connected to the bridge arm midpoint of the third bridge arm 2a, and the other end of the secondary side of the transformer T0 is connected to the bridge arm midpoint of the fourth bridge arm 2b.

[0072] As another possible example, as shown in FIG. 3, the second bridge circuit 122 can be a full-bridge resonant circuit, i.e., the second bridge circuit 122 can include a resonant device in addition to the third bridge arm 2a and the fourth bridge arm 2b connected in parallel. For example, the resonant device can include an inductor and a capacitor, one end of the secondary side of the transformer T0 is connected to the bridge arm midpoint of the third bridge arm 2a through the capacitor, and the other end of the secondary side of the transformer T0 is connected to the bridge arm midpoint of the fourth bridge arm 2b through the inductor.

[0073] Since at least one of the first bridge circuit 121 and the second bridge circuit 122 in the DC / DC conversion circuit 12 shown in FIGS. 3-6 above is a resonant circuit, the DC / DC conversion circuit 12 above can also be referred to as a resonant converter.

[0074] As shown in FIG. 7, for the scenario where the DC / DC conversion circuit 12 adopts a bidirectional flyback topology, the DC / DC conversion circuit 12 can include a transformer T0 and two switch tubes. The primary side and the secondary side of the transformer T0 are respectively connected to one switch tube. Optionally, the DC / DC conversion circuit 12 can also include two inductors, and the primary side and the secondary side of the transformer T0 are respectively connected to one inductor.

[0075] Optionally, as shown in FIGS. 3-7, the battery pack 10 further comprises a control chip 15 disposed on the circuit board. The control chip 15 can be configured to send a synchronization signal to the BMS 14, which can be a digital signal. As shown in FIG. 9, the synchronization signal can have the same frequency as the first excitation current, and the synchronization signal can have the same phase as the first excitation current.

[0076] The BMS 14 can be further configured to calibrate the phase of the second excitation current based on the phase of the synchronization signal, and determine the impedance of each of the plurality of battery cells 11 at the frequency of the synchronization signal based on the calibrated second excitation current and the voltage of each of the plurality of battery cells 11.

[0077] It can be understood that the frequency of the first excitation current provided by the DC / DC conversion circuit 12 can vary within a preset frequency range, and the BMS 14 can detect the impedance of each of the plurality of battery cells 11 at different frequencies within the frequency range, for example, the EIS of each of the plurality of battery cells 11 at different frequencies. Since the frequency of the second excitation current detected by the BMS 14 through the current sampling circuit 13 can be superimposed with the frequency of the interference signal, and the synchronization signal is a digital signal and the transmission path of the synchronization signal is shorter than that of the second excitation current, the synchronization signal is less disturbed. Therefore, the BMS 14 can more accurately determine the frequency of the first excitation current through the synchronization signal provided by the control chip 15, and thus can accurately detect the impedance of each of the plurality of battery cells 11 at different frequencies.

[0078] It can also be understood that the phase of the second excitation current has a certain offset compared to the actual phase of the first excitation current. Since the synchronization signal is a digital signal and the transmission path of the synchronization signal is shorter than that of the second excitation current, the synchronization signal is less disturbed. Accordingly, the BMS 14 can determine the phase of the first excitation current based on the phase of the synchronization signal, and then calibrate the phase of the second excitation current based on the phase, so that the phase of the second excitation current is aligned with the phase of the voltage sampled by the BMS 14. Then, the BMS 14 detects the impedance of the battery cell 11 based on the calibrated second excitation current and the voltage, to ensure that the impedance detection is more accurate.

[0079] Optionally, the DC / DC conversion circuit 12, the current sampling circuit 13, and the control chip 15 can be disposed on the same circuit board, i.e., the DC / DC conversion circuit 12, the current sampling circuit 13, and the control chip 15 can be integrated. For example, as shown in FIG. 2, the DC / DC conversion circuit 12, the current sampling circuit 13, and the control chip 15 can be integrated in a DC / DC converter (also referred to as a DC / DC module). Thus, the integration of the battery pack 10 can be effectively improved, and the volume and structural complexity of the battery pack 10 can be avoided.

[0080] Optionally, the control chip 15 can be a micro-controller unit (MCU). Moreover, the control chip 15 can be a control device of the DC / DC conversion circuit 12, i.e., the control chip 15 is also capable of controlling the working state of each switch tube in the DC / DC conversion circuit 12, so as to enable the DC / DC conversion circuit 12 to output the first excitation current. Since the control chip 15 is used to control the DC / DC conversion circuit 12 to output the first excitation current, the control chip 15 is capable of outputting a synchronization signal with the same frequency and phase as the first excitation current.

[0081] FIG. 10 is a structural schematic diagram of a current sampling circuit according to an embodiment of the present application. As shown in FIG. 10, the current sampling circuit 13 can include a current sensor 131 and an operational amplifier 132.

[0082] As an optional implementation, the current sensor 131 can be connected between the first end of the DC / DC conversion circuit 12 and the positive electrode BAT+ or the negative electrode BAT- of the battery pack 10, as shown in FIG. 2. FIG. 2 takes the example of the current sensor 131 being connected between the first end of the DC / DC conversion circuit 12 and the negative electrode BAT- of the battery pack 10. The current sensor 131 is used to convert the second excitation current flowing through the current sensor 131 into a voltage signal. The input end of the operational amplifier 132 is connected with the current sensor 131, and the output end of the operational amplifier 132 is connected with the BMS 14. The operational amplifier 132 is used to amplify the voltage signal and transmit the amplified voltage signal to the BMS 14.

[0083] It can be understood that the total current flowing through the plurality of battery cells 11 includes not only the second excitation current but also the charging current or discharging current of the battery pack 10 itself. Since the current sensor 131 is connected between the first end of the DC / DC conversion circuit 12 and the positive electrode BAT+ or the negative electrode BAT- of the battery pack 10, the second excitation current flowing through the plurality of battery cells 11 can be directly detected. Accordingly, the BMS 14 is capable of directly determining the impedance based on the second excitation current, without the need to extract the second excitation current from the total current flowing through the plurality of battery cells 11. In this way, the processing logic of the BMS 14 is effectively simplified, and the calculation complexity of the BMS 14 in determining the impedance is reduced.

[0084] Optionally, the current sensor 131 can be a shunt, or a Hall current sensor, etc., and the type of the current sensor 131 is not limited in the embodiments of the present application. As shown in FIG. 10, the input end of the first operational amplifier 132 includes a non-inverting input end (+) and an inverting input end (-). The non-inverting input end (+) is connected with the current sensor 131 through a resistor R1, and the inverting input end (-) is connected with the current sensor 131 through a resistor R2. In addition, the non-inverting input end (+) is also connected with the ground GND through a resistor R3, and the inverting input end (-) is also connected with the output end of the first operational amplifier 132 through a resistor R4. Referring back to FIG. 10, the first operational amplifier 132 also has a power supply positive pole V+ and a power supply negative pole V-, the power supply negative pole V- is connected with the ground GND, and the power supply positive pole V+ can be connected with a power supply end of 3.3V.

[0085] As another optional implementation, as shown in FIG. 7, the current sensor 131 can be connected in series with the plurality of battery cells 11, and the current sensor 131 is configured to convert the total current flowing through the current sensor 131 (i.e., the total current flowing through the plurality of battery cells 11) into a voltage signal. It can be understood that in the scheme shown in FIG. 7, the total current flowing through the current sensor 131 is the current obtained by superimposing the second excitation current and the charging current or discharging current of the battery pack 10 itself.

[0086] The input end of the operational amplifier 132 is connected with the current sensor 131, and the output end of the operational amplifier 132 is connected with the BMS 14. The operational amplifier 132 is configured to amplify the voltage signal and transmit the amplified voltage signal to the BMS 14.

[0087] The BMS 14 is further configured to determine the second excitation current flowing through the plurality of battery cells 11 according to the total current. For example, the control chip 133 can calculate the average value of the total current in a period of time, and determine the difference between the total current in the period of time and the average value as the second excitation current in the period of time.

[0088] It can be understood that the amplitude of the second excitation current is usually much smaller than the amplitude of the charging current or discharging current of the battery pack 10 itself, so if the BMS 14 directly calculates the impedance of the battery cell 11 based on the total current, the second excitation current will be overwhelmed by the charging current or discharging current of the battery pack 10 itself. In the embodiments of the present application, since the BMS 14 can first extract the second excitation current from the total current, and then calculate the impedance of the battery cell 11 based on the second excitation current, the accuracy of impedance detection can be effectively ensured.

[0089] It can also be understood that, since the second excitation current is a sine signal, the voltage change of the battery cell 11 caused by the second excitation current is also a sine signal, that is, the voltage generated by the second excitation current is a sine wave voltage, also called ripple voltage. Since the amplitude of the ripple voltage is smaller than the amplitude of the direct current voltage of the battery cell 11 itself, if the impedance of the battery cell 11 is directly detected based on the total voltage across the battery cell 11, the ripple voltage will be submerged by the direct current voltage of the battery cell 11 itself. In the embodiment of the present application, after the BMS 14 detects the total voltage across the battery cell 11, the ripple voltage generated by the second excitation current can be calculated by a software algorithm. For example, the BMS 14 can calculate the average value of the total voltage in a period of time, and determine the difference between the total voltage in the period of time and the average value as the ripple voltage generated by the second excitation current in the period of time. Thus, the ripple voltage generated by the second excitation current can be accurately sampled, and the impedance of the battery cell 11 can be accurately detected.

[0090] In the embodiment of the present application, the capacities of the plurality of battery cells 11 in the battery pack 10 can be the same, which can refer to the theoretical capacity, i.e. the nominal capacity, of the battery cell 11. It can be understood that, during the operation of the battery pack 10, if an abnormality occurs in a certain battery cell 11, the actual capacity of the battery cell 11 will be smaller than the theoretical capacity, i.e. the actual internal resistance of the battery cell 11 will be greater than the theoretical internal resistance. Correspondingly, the impedance of the battery cell 11 detected by the impedance detection circuit (i.e. the actual impedance) will be greater than the theoretical impedance of the battery cell 11, thereby identifying that the battery cell 11 has an abnormality.

[0091] Alternatively, the size of the first excitation current provided by the DC / DC conversion circuit 12 can be positively correlated with the capacity (i.e. the theoretical capacity, and hereinafter the capacity refers to the theoretical capacity of the battery cell 11 unless otherwise specified) of any battery cell 11 in the battery pack 10. That is, the greater the capacity of the battery cell 11, the greater the first excitation current can be.

[0092] It can be understood that, in the case of a certain internal resistance of the battery cell 11, the greater the first excitation current, the greater the amplitude of the second excitation current and the voltage of the battery cell 11 detected by the BMS 14. In the case of a certain detection resolution (also called sampling resolution) of the BMS 14, the greater the amplitude of the current and voltage detected, the higher the detection accuracy. It can also be understood that the capacity of the battery cell 11 is negatively correlated with the internal resistance, i.e. the greater the capacity of the battery cell 11, the smaller the internal resistance. Therefore, if the capacity of the battery cell 11 is large, the internal resistance of the battery cell 11 is small. At this time, the first excitation current provided by the DC / DC conversion circuit 12 can be large, so as to ensure that the amplitudes of the current and voltage detected by the BMS 14 are large, thereby ensuring that the detection accuracy of the BMS 14 is high.

[0093] For example, assuming that the detection resolution of the BMS 14 is 5 millivolts (mV), in order to ensure the voltage detection accuracy, the ripple voltage generated by the second excitation current should be greater than 5 mV. If each battery cell 11 in the battery pack 10 is a lithium iron phosphate battery cell with a capacity of 280 ampere-hours (AH) and an internal resistance of 0.25 milliohms (mohm), the effective value I of the first excitation current provided by the DC / DC conversion circuit 12 can satisfy: I > (5 / 0.25) = 20 A.

[0094] It can also be understood that the detection resolution of the BMS 14 can refer to the resolution of an analog to digital converter (ADC) in the BMS 14. Generally, the higher the resolution of the ADC in the BMS 14, the higher the hardware cost of the BMS 14. In the embodiment of the present application, since the DC / DC conversion circuit 12 can provide a first excitation current with a suitable size based on the capacity of the battery cell 11, the accuracy of impedance detection can be effectively improved without improving the resolution of the ADC in the BMS 14, i.e., without increasing the hardware cost of the BMS 14.

[0095] Optionally, in the embodiment of the present application, the size of the first excitation current can be 2 A to 30 A. The first excitation current in this range can ensure accurate detection of the impedance of the battery cell.

[0096] Optionally, the first excitation current provided by the DC / DC conversion circuit 12 can be a sinusoidal signal, i.e., the first excitation current is a sinusoidal current, and the frequency range can be 0.01 Hz to 8 kHz, for example, the frequency range can be 0.1 Hz to 2 kHz. It can be understood that the frequency of the first excitation current provided by the DC / DC conversion circuit 12 can vary within the frequency range, and the BMS 14 can detect the impedance of each battery cell 11 in the battery pack 10 at different frequencies within the frequency range, i.e., detect the change of the impedance of each battery cell 11 with the frequency of the first excitation current.

[0097] As a possible example, the first excitation current can be a sinusoidal signal with a single frequency. Moreover, the DC / DC conversion circuit 12 can adjust the frequency of the sinusoidal signal within the above-mentioned frequency range.

[0098] As another possible example, the first excitation current can also be composed of at least two sinusoidal signals with different frequencies, i.e., the first excitation current can include at least two frequencies. The at least two frequencies both belong to the above-mentioned frequency range, and the DC / DC conversion circuit 12 can adjust the at least two frequencies within the above-mentioned frequency range.

[0099] In an example, the first excitation current can be a resultant signal composed of a basic sinusoidal signal. The basic sinusoidal signal has a first frequency and a first amplitude, and the resultant signal can be a sinusoidal signal or a square wave signal, which has a second frequency and a second amplitude. The first frequency is higher than the second frequency, and the first amplitude is smaller than the second amplitude. Alternatively, the basic sinusoidal signal can be understood as fluctuating at the second frequency within the second amplitude range, thereby forming the resultant signal.

[0100] For the scenario that the first excitation current is composed of at least two sinusoidal signals with different frequencies, the BMS 14 can detect the impedance of each battery cell 11 at the at least two different frequencies simultaneously after the DC / DC conversion circuit 12 provides a first excitation current, thereby effectively improving the efficiency of impedance detection.

[0101] Alternatively, the BMS 14 can further be configured to report the impedance (e.g., EIS) of each battery cell 11 in the plurality of battery cells 11 to a cloud server, so that the cloud server detects the health degree of each battery cell 11 in the plurality of battery cells 11. In this way, the cloud server can realize online monitoring of the health degree of each battery cell 11 in the battery pack 10, thereby identifying potential safety risks of the battery pack 10 in a timely manner.

[0102] Alternatively, the BMS 14 can further perform outlier analysis on the impedance of the plurality of battery cells 11 by using an outlier algorithm to detect whether there is an abnormal battery cell 11 in the plurality of battery cells 11. Moreover, the BMS 14 can upload the result of the outlier analysis to the cloud server for rechecking.

[0103] In summary, the embodiments of the present application provide a battery pack, and the DC / DC conversion circuit in the battery pack can provide a first excitation current for a plurality of battery cells. Since the DC / DC conversion circuit is independent of the BMS and is not integrated in the BMS, the DC / DC conversion circuit will not affect the heat dissipation performance of the BMS when providing the first excitation current. Accordingly, the first excitation current provided by the DC / DC conversion circuit can be larger, thereby ensuring that the voltage sampled by the BMS is larger, so as to realize accurate detection of the impedance of the battery cells.

[0104] Moreover, after the DC / DC conversion circuit provides the first excitation current, the BMS can simultaneously detect the voltage of the plurality of battery cells, and further detect the impedance of the plurality of battery cells, thereby improving the detection efficiency. Since the process of providing the first excitation current by the DC / DC conversion circuit, and the process of detecting the current and voltage by the BMS and determining the impedance will not affect the normal operation (e.g., normal charging and discharging) of the battery pack, the online detection of the impedance of the battery cells can be realized during the operation of the battery pack. In addition, the topology of the battery pack provided by the embodiments of the present application is relatively simple, and the volume and cost of the battery pack can be avoided.

[0105] The embodiment of the present application also provides a storage system, as shown in FIG. 11, which can include a shell 00 and a plurality of battery packs 10 provided by the above-mentioned embodiment. The plurality of battery packs 10 can be arranged in the shell 00.

[0106] Optionally, the plurality of battery packs 10 can be connected in series in sequence. As shown in FIG. 12, the second ends of the DC / DC conversion circuits 12 in the plurality of battery packs 10 can be connected in parallel. The plurality of battery packs 10 include a first battery pack and a second battery pack, the DC / DC conversion circuit 12 in the first battery pack is configured to output power to the second battery pack and receive power input by the second battery pack to provide a first excitation current to the plurality of battery cells 11 in the first battery pack.

[0107] When the DC / DC conversion circuit 12 in the first battery pack outputs power to the second battery pack, the first excitation current provided by the DC / DC conversion circuit 12 in the first battery pack to the plurality of battery cells 11 in the first battery pack can flow from the negative electrode BAT- of the first battery pack 10 to the positive electrode BAT+ of the battery pack 10. When the DC / DC conversion circuit 12 in the first battery pack receives power input by the second battery pack, the first excitation current provided by the DC / DC conversion circuit 12 in the first battery pack to the plurality of battery cells 11 in the first battery pack can flow from the positive electrode BAT+ of the first battery pack 10 to the negative electrode BAT- of the battery pack 10.

[0108] Based on the above-mentioned manner, on the one hand, the first excitation current provided by the DC / DC conversion circuit 12 to the plurality of battery cells 11 in the battery pack 10 can be large, and on the other hand, the energy loss of the storage system in the process of providing the first excitation current can be small.

[0109] It can be understood that the first battery pack and the second battery pack can be any two battery packs 10 in the plurality of battery packs 10, that is, the DC / DC conversion circuits 12 in any two battery packs 10 can interact power to provide the first excitation current to the plurality of battery cells 11 in the respective battery pack 10.

[0110] As shown in FIG. 12, the power conversion topology adopted by the DC / DC conversion circuit 12 of each battery pack 10 in the energy storage system can be a CLLC topology and its variant topologies. Alternatively, as shown in FIG. 13, the power conversion topology adopted by the DC / DC conversion circuit 12 of each battery pack 10 in the energy storage system can be an LLC topology and its variant topologies. Alternatively, as shown in FIG. 14, the power conversion topology adopted by the DC / DC conversion circuit 12 of each battery pack 10 in the energy storage system can be an LLLC topology and its variant topologies. Alternatively, as shown in FIG. 15, the power conversion topology adopted by the DC / DC conversion circuit 12 of each battery pack 10 in the energy storage system can be a DAB topology and its variant topologies. Alternatively, as shown in FIG. 16, the power conversion topology adopted by the DC / DC conversion circuit 12 of each battery pack 10 in the energy storage system can be a bidirectional flyback topology and its variant topologies.

[0111] It can also be understood that, for the scenario in which the DC / DC conversion circuits 12 in two battery packs 10 interact with each other to provide the first excitation current, the capacitance value of the capacitor C0 in the two battery packs 10 can be small, or, as shown in FIGS. 12-16, the capacitor C0 can not be provided in the two battery packs 10.

[0112] Since the plurality of battery packs 10 in the energy storage system are connected in series, the plurality of battery packs 10 are simultaneously charged and simultaneously discharged. Ideally, the plurality of battery packs 10 can be simultaneously fully charged and can be simultaneously fully discharged. However, due to factors such as manufacturing process, use environment, and use duration, the state of charge (SOC) of different battery packs 10 will be different. As a result, when the plurality of battery packs 10 are simultaneously charged, some battery packs 10 can be fully charged while other battery packs 10 are not fully charged. At this time, in order to avoid the risk of continuing to charge the battery packs 10 that are already fully charged, the energy storage system will stop charging the plurality of battery packs 10. Similarly, when some battery packs 10 are fully discharged while other battery packs 10 still have residual power, in order to avoid the risk of continuing to discharge the battery packs 10 that are already fully discharged, the energy storage system will stop discharging the plurality of battery packs 10.

[0113] Based on the above analysis, in the scenario in which the plurality of battery packs 10 are connected in series, if the SOC of different battery packs 10 is different, some battery packs 10 cannot be fully charged or cannot be fully discharged, which seriously affects the operation efficiency of the energy storage system. In the embodiments of the present application, the DC / DC conversion circuit 12 in each battery pack 10 can also have the function of actively balancing the SOC difference between different battery packs 10, i.e., the DC / DC conversion circuit 12 can also balance the SOC difference between different battery packs 10.

[0114] For example, if the SOC of the first battery pack 10 is lower than the SOC of the second battery pack 10, the DC / DC conversion circuit 12 in the first battery pack 10 can receive the discharge power output by the second battery pack 10 and input the charging power to the plurality of battery cells 11 in the first battery pack 10. If the SOC of the first battery pack 10 is higher than the SOC of the second battery pack 10, the DC / DC conversion circuit 12 in the first battery pack 10 can receive the discharge power output by the plurality of battery cells 11 in the first battery pack 10 and output the charging power to the second battery pack 10.

[0115] Since the DC / DC conversion circuit 12 in the battery pack 10 provided in the embodiments of the present application can not only provide the first excitation current required for impedance detection, but also has the function of active balancing, the active balancing circuit does not need to be additionally configured in the battery pack 10, thereby effectively reducing the cost and structural complexity of the battery pack 10.

[0116] Optionally, referring to FIG. 1, the energy storage system can further include an energy storage converter 20, and the plurality of battery packs 10 connected in series can be connected to a direct-current end of the energy storage converter 20, and an alternating-current end of the energy storage converter 20 can be connected to the power grid 30 and / or the load 40.

[0117] Optionally, as shown in FIG. 17, the energy storage system can further include a cloud server 50, which can receive the impedance (for example, the EIS) of the plurality of battery cells 11 reported by the BMS 14 in each battery pack 10, and detect the health degree of each battery cell 11 based on the received impedance of the plurality of battery cells 11. For example, the cloud server 50 can perform outlier analysis on the impedance of the plurality of battery cells 11 by using an outlier algorithm to detect the health degree of each battery cell 11. The cloud server 50 can timely identify the potential safety risk of the battery pack 10 based on the health degree of the battery cell 11.

[0118] Optionally, the cloud server 50 can further establish a communication connection with a user terminal and can send the impedance (for example, the EIS) of the battery cell 11 to the user terminal for display in the user interface of the user terminal. In this way, the user can monitor the health status of the battery pack 10 in real time and timely identify the potential safety risk of the battery pack 10.

[0119] In the embodiments of the present application, the terms "first", "second" and "third" are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance. The term "at least one" means one or more, and "a plurality of" means two or more.

[0120] The term "and / or" in the embodiments of the present application is only used to describe the association relationship of the associated objects, and indicates that there can be three relationships, for example, A and / or B can represent three cases of A existing alone, A and B existing simultaneously, and B existing alone. In addition, the character " / " in this paper generally represents that the front and rear associated objects have an "or" relationship.

[0121] The above is only an optional embodiment of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art can easily think of various equivalent modifications or replacements within the technical range disclosed by the present application, and these modifications or replacements should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A battery pack, characterized by, The battery pack comprises a plurality of battery cells, a direct current / direct current conversion circuit, a current sampling circuit and a battery management system; The plurality of battery cells are connected in series between the positive electrode and the negative electrode of the battery pack; The first end of the direct current / direct current conversion circuit is connected to the positive electrode and the negative electrode of the battery pack, and the direct current / direct current conversion circuit is configured to provide a first excitation current to the plurality of battery cells; The battery management system is configured to detect a second excitation current flowing through the plurality of battery cells by the current sampling circuit, the second excitation current being the excitation current flowing through the plurality of battery cells when the direct current / direct current conversion circuit provides the first excitation current to the plurality of battery cells; The battery management system is further configured to detect a voltage generated across each battery cell in the plurality of battery cells by the second excitation current, and determine the impedance of each battery cell in the plurality of battery cells based on the second excitation current and the voltage.

2. The battery pack of claim 1, wherein, The battery pack further comprises a capacitor; The second end of the direct current / direct current conversion circuit is connected in parallel to the capacitor, and the direct current / direct current conversion circuit is configured to alternately charge and discharge the capacitor to provide the first excitation current to the plurality of battery cells; Wherein, when the direct current / direct current conversion circuit charges the capacitor, the first excitation current flows from the negative electrode to the positive electrode of the battery pack; and when the direct current / direct current conversion circuit discharges the capacitor, the first excitation current flows from the positive electrode to the negative electrode of the battery pack.

3. The battery pack of claim 1, wherein The direct current / direct current conversion circuit is configured to output power to other battery packs and receive power input from the other battery packs to provide the first excitation current to the plurality of battery cells, the other battery packs being other battery packs in the same energy storage system except the battery pack.

4. The battery pack of any one of claims 1 to 3, wherein, The battery pack further comprises a control chip; The control chip is configured to send a synchronization signal to the battery management system, the synchronization signal being a digital signal, and the frequency of the synchronization signal being the same as the frequency of the first excitation current, and the phase of the synchronization signal being the same as the phase of the first excitation current; The battery management system is configured to calibrate the phase of the second excitation current based on the phase of the synchronization signal, and determine the impedance of each battery cell in the plurality of battery cells at the frequency of the synchronization signal based on the calibrated second excitation current and the voltage of each battery cell in the plurality of battery cells.

5. The battery pack of claim 4, wherein, The direct current / direct current conversion circuit, the current sampling circuit and the control chip are arranged on the same circuit board.

6. The battery pack of any one of claims 1 to 5, wherein, The current sampling circuit comprises a current sensor and an operational amplifier; The current sensor is connected between the first end of the direct current / direct current conversion circuit and the positive electrode or the negative electrode of the battery pack, and the current sensor is configured to convert the second excitation current flowing through the current sensor into a voltage signal; The input end of the operational amplifier is connected to the current sensor, and the output end of the operational amplifier is connected to the battery management system, and the operational amplifier is configured to amplify the voltage signal and transmit the amplified voltage signal to the battery management system.

7. The battery pack of any one of claims 1 to 5, wherein, The current sampling circuit comprises a current sensor and an operational amplifier; The current sensor is connected in series with the plurality of battery cells, and is configured to convert total current flowing through the current sensor into a voltage signal; The input end of the operational amplifier is connected with the current sensor, the output end of the operational amplifier is connected with the battery management system, and the operational amplifier is configured to amplify the voltage signal and transmit the amplified voltage signal to the battery management system; The battery management system is further configured to determine a second excitation current flowing through the plurality of battery cells according to the total current.

8. The battery pack of any one of claims 1 to 7, wherein, The first excitation current is positively correlated with the capacity of any battery cell in the plurality of battery cells.

9. The battery pack of claim 8, wherein, The first excitation current is 2A to 30A.

10. The battery pack of any one of claims 1 to 9, wherein, The frequency range of the first excitation current is 0.01 Hz to 8 kHz.

11. The battery pack of any one of claims 1-10, wherein, The first excitation current is composed of at least two sine signals of different frequencies.

12. The battery pack of any one of claims 1-11, wherein, The battery management system is further configured to report the impedance of each battery cell in the plurality of battery cells to a cloud server, so that the cloud server detects the health degree of each battery cell in the plurality of battery cells.

13. An energy storage system characterized by, The energy storage system comprises a shell and a plurality of battery packs according to claim 1, and the plurality of battery packs are arranged in the shell.

14. The energy storage system of claim 13, wherein, The battery pack further comprises a capacitor. The second end of the DC / DC conversion circuit is connected in parallel with the capacitor, and the DC / DC conversion circuit is configured to alternately charge and discharge the capacitor to provide the first excitation current to the plurality of battery cells. When the DC / DC conversion circuit charges the capacitor, the first excitation current flows from the negative electrode to the positive electrode of the battery pack; when the DC / DC conversion circuit discharges the capacitor, the first excitation current flows from the positive electrode to the negative electrode of the battery pack.

15. The energy storage system of claim 13, wherein, The second ends of the DC / DC conversion circuits in the plurality of battery packs are connected in parallel, the DC / DC conversion circuit in a first battery pack in the plurality of battery packs is configured to output power to a second battery pack and receive power input by the second battery pack to provide the first excitation current to the plurality of battery cells in the first battery pack.

16. An energy storage system according to any one of claims 13 to 15, wherein, The battery pack further comprises a control chip. The control chip is configured to send a synchronization signal to the battery management system, the synchronization signal is a digital signal, the frequency of the synchronization signal is the same as the frequency of the first excitation current, and the phase of the synchronization signal is the same as the phase of the first excitation current. The battery management system is configured to calibrate the phase of the second excitation current based on the phase of the synchronization signal, and determine the impedance of each battery cell in the plurality of battery cells at the frequency of the synchronization signal based on the calibrated second excitation current and the voltage of each battery cell in the plurality of battery cells.

17. An energy storage system according to any one of claims 13 to 16, wherein, The current sampling circuit comprises a current sensor and an operational amplifier; The current sensor is connected between the first end of the DC / DC conversion circuit and the positive electrode or the negative electrode of the battery pack, and is configured to convert the second excitation current flowing through the current sensor into a voltage signal; An input end of the operational amplifier is connected with the current sensor, and an output end of the operational amplifier is connected with the battery management system, so as to amplify the voltage signal and transmit the amplified voltage signal to the battery management system.

18. An energy storage system according to any one of claims 13 to 17, wherein, The first excitation current is positively related to the capacity of any one of the plurality of battery cells.

19. An energy storage system according to any one of claims 13 to 18, wherein, The first excitation current is composed of at least two sine signals with different frequencies.

20. The energy storage system of any one of claims 13 to 19, wherein, The energy storage system further comprises a cloud server. The cloud server is configured to receive the impedances of the plurality of battery cells reported by each battery pack, and detect the health degree of each battery cell based on the impedances of the plurality of battery cells.

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