Battery equalizer and battery equalization method

Abstract

The invention discloses a battery equalizer of a series battery pack. The battery equalizer comprises a plurality of unit circuits, an AC link and a DC / AC converter. The input ends of the plurality of unit circuits are connected in parallel and are connected with the AC link. And the output end of each unit circuit is connected with a corresponding battery in the battery pack. The DC / AC converter is connected with the AC link. Each unit circuit comprises an AC / DC converter, and the AC side of the AC / DC converter is coupled with the AC link capacitance of the input end of the unit circuit. Since the current drawn from the first stage is common to all cells, the charging or discharging of each cell is autonomously determined by the output current of the second stage.

Description

Technical Field

[0001] The present invention relates to a battery management system, and in particular to battery cell balancing in a battery system. Background Art

[0002] As electric vehicles become increasingly popular, it is foreseeable that a large number of retired electric vehicle batteries will accumulate worldwide[1]. If lithium-ion electric vehicle batteries are reused before recycling, their carbon footprint can be reduced by up to 17%[2]. These retired batteries still have usable capacity and can be reused in stationary energy storage systems[3]. Figure 1 As shown, solar and wind energy can be stored in large energy systems to power the grid and charge electric vehicles. However, extending the service life of retired batteries remains a major challenge. Long-term use can lead to inconsistent battery characteristics, reduce available capacity, and shorten battery life [4]. Therefore, cell voltage balancing is crucial to ensure safe and efficient battery operation. It can balance the charge and discharge rates of the battery, extend battery life, prevent damage or safety hazards caused by overcharging or discharging, and maximize the battery's energy storage capacity [5].

[0003] Battery balancing structures can be divided into two main types: DC-DC converters (DCSS) with selective switches, and multi-port circuits (MC) [6]. The structure of DCSS is simple (as shown in Figure 2a) because it utilizes existing DC-DC converters and multiplexers, resulting in a simple implementation method [7]. Reference [8] proposed a balancer based on a bidirectional converter, which achieved high integration and reliability. Reference [9] proposed a balancer based on a wide voltage range converter. This balancer achieves bidirectional energy flow over a wide voltage range while maintaining high energy conversion efficiency. However, DCSS usually requires a large number of switches to select the required battery cells for balancing. This requirement requires the use of complex control and drive circuits, especially when balancing a large number of batteries. In addition, DCSS has only one output. Therefore, when multiple batteries need to be balanced, the selection switch needs to be switched to select the target battery, and only one battery can be balanced at any time

[10] .

[0004] A multi-port circuit (MC) needs to be designed to replace the converter and the selection switch network, as shown in Figure 2b. Since the charge can be automatically transferred to the target battery, there is no need for a selection switch, which reduces the number of active components. In addition, MC allows multiple batteries to be balanced simultaneously, which greatly reduces the balancing time. Reference

[11] proposed an integrated equalizer based on parallel transformers. This method uses cascaded multi-winding transformers, where the secondary side of the main transformer is balanced within each module. This method achieves balancing functions within and between modules. Reference

[12] proposed a multi-layer voltage equalizer that can target multiple battery cells at the same time, so its balancing speed is very high. However, multi-winding transformers are often used in MC

[13] , as shown in Figure 2c. Typically, a transformer winding needs to be configured for each battery. If the number of batteries is large, the design and implementation of the transformer will become challenging.

[0005] References

[0006] The following references mentioned in this specification are indicated by number brackets, and their entire contents are incorporated into this application by reference.

[0007] [1]MAHannan et al., "SOC Estimation of Li-ion Batteries WithLearning Rate-Optimized Deep Fully Convolutional Network," IEEE Trans.PowerElectron., vol.36, no.7, pp.7349-7353, Jul.2021, doi:10.1109 / TPEL.2020.3041876.

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[0014] [8]X.Qi,Y.Wang,and M.Fang,“An Integrated Cascade Structure-BasedIsolated Bidirectional DC-DC Converter for Battery Charge Equalization,”IEEETrans.Power Electron.,vol.35,no.11,pp.

[0015] 12003-12021,Apr.2020,doi:10.1109 / TPEL.2020.2988661.

[0016] [9]W.Lujun et al.,“Efficient and Fast Active Equalization Method forRetired Battery Pack Using Wide Voltage Range Bidirectional Converter andDBSCAN Clustering Algorithm,”IEEE Trans.Power Electron.,vol.37,no.11,pp.13824-13833,Nov.2022,doi:10.1109 / TPEL.2022.3185242.

[0017]

[10] S.K.Dam and V.John,“A Modular Fast Cell-to-Cell Battery VoltageEqualizer,”IEEE Trans.Power Electron.,vol.35,no.9,pp.9443-9461,Sep.2020,doi:10.1109 / TPEL.2020.2972004.

[0018]

[11] K.Chen et al.,“Double-Layer Multi-Winding Transformer-BasedModular Integrated Equalizer for Extended Battery String,”IEEE Trans.PowerElectron.,vol.39,no.2,pp.2685-2695,Sep.2023,doi:10.1109 / TPEL.2023.3314931.

[0019]

[12] H.Nazi and E.Babaei,“A Modularized Bidirectional Charge Equalizerfor Series-Connected Cell Strings,”IEEE Trans.Ind.Electron.,vol.68,no.8,pp.6739-6749,Aug.2021,doi:10.1109 / tie.2020.3003661.

[0020]

[13] M.Liu,Y.Chen,Y.Elasser,and M.Chen,“Dual Frequency HierarchicalModular Multilayer Battery Balancer Architecture,”IEEE Trans.Power Electron.,vol.36,no.3,pp.3099-3110,Mar.2021,doi:10.1109 / tpel.2020.3015768.

[0021]

[14] R.C.Viero and F.S.dos Reis,“Dynamic modeling of a ZETA converterin DCM applied to low power renewable sources,”in 2011IEEE Energy ConversionCongress and Exposition,Sep.2011,pp.685-691.doi:10.1109 / ECCE.2011.6063836.

[0022]

[15] Z.Wei,F.Peng,and H.Wang,“An LCC Based String-to-Cell BatteryEqualizer with Simplified Constant Current Control,”IEEE Trans.PowerElectron.,vol.37,no.2,pp.1816-1827,2021,doi:10.1109 / TPEL.2021.3102627.

[0023]

[16] M.Uno and K.Tanaka,“Single-Switch Cell Voltage Equalizer UsingMultistacked Buck-Boost Converters Operating in Discontinuous Conduction Modefor Series-Connected Energy Storage Cells,”IEEE Trans.Veh.Technol.,vol.60,no.8,pp.3635-3645,Oct.2011,doi:10.1109 / TVT.2011.2165229.

[0024]

[17] N.Hasanpour,M.R.Mohammadi,A.Tavakoli,and S.Ali Khajehoddin,“Modular Voltage Equalizer Circuit with AC-bus Inter-Modules Connection,”IEEETrans.Transp.Electrification,pp.1-1,2023,doi:10.1109 / TTE.2023.3280063.

[0025]

[18] X.Qi,Y.Wang,M.Fang,H.Wang,Y.Wang,and Z.Chen,“A Family ofIntegrated Cascade Multiport Converters for Centralized Equalization Systems:Derivation,Analysis,and Verification,”IEEE Trans.Power Electron.,vol.38,no.6,pp.7398-7415,Jun.2023,doi:10.1109 / TPEL.2023.3246723.

[0026]

[19] Y.Shang,S.Zhao,Y.Fu,B.Han,P.Hu,and C.Mi,“A Lithium-Ion BatteryBalancing Circuit Based on Synchronous Rectification,”IEEE Trans.PowerElectron.,vol.35,no.2,pp.1637-1648,2019,doi:10.1109 / tpel.2019.2917390.

[0027]

[20] L. Liu, Z. Yan, B. Xu, P. Zhang, C. Cai, and H. Yang, “A Highly ScalableIntegrated Voltage Equalizer Based on Parallel-Transformers for High-VoltageEnergy Storage Systems,” IEEE Trans.Ind.Electron., vol.71, no.1, pp.595-603, Jan.2024, doi:10.1109 / TIE.2023.3241382. Summary of the Invention

[0028] Therefore, in a first aspect, the present invention provides a battery equalizer for a battery pack connected in series. The battery equalizer includes a plurality of cell circuits, an AC link, and a DC / AC converter connected to the AC link. The plurality of cell circuits are connected in parallel at their input terminals, and each cell circuit is adapted to be connected to a corresponding battery cell of the battery pack at its output terminal. The plurality of cell circuits are also connected to the AC link at their input terminals. Each cell circuit includes an AC / DC converter, the AC side of which is capacitively coupled to the AC link at the cell circuit's input terminal.

[0029] Preferably, the DC / AC converter comprises a voltage source, and an active switch connected between the voltage source and the AC link.

[0030] More preferably, the active switch is the only active switch in the cell equalizer.

[0031] Alternatively or additionally, the DC / AC converter further comprises a first inductor as a magnetizing inductance, wherein the first inductor is connected in series with the active switch between the two ends of the voltage source.

[0032] Preferably, the first inductor is the only magnetic component of the battery balancer.

[0033] In a variation of the preferred embodiment, the voltage source is a first capacitor adapted to be charged by a battery.

[0034] In another variant of the preferred embodiment, each unit circuit further comprises two coupling capacitors at the input end, and the AC / DC converter of the unit circuit is connected to the AC link via the two coupling capacitors.

[0035] In another variation of the preferred embodiment, the AC / DC converter in each unit circuit includes a second inductor and a diode connected in series between both ends of the corresponding battery cell of the unit circuit.

[0036] In another variation of the preferred embodiment, each unit circuit further comprises an output filter connected between the AC / DC converter and the corresponding battery cell of the unit circuit.

[0037] Preferably, the output filter is a third inductor.

[0038] In another variation of the preferred embodiment, each AC / DC converter is a ZETA-derived converter.

[0039] In another variation of the preferred embodiment, the cell equalizer further includes a transformer coupled between the DC / AC circuit and the AC link.

[0040] In another variation of the preferred embodiment, the duty cycle of the active switch is controlled based on the ratio between the pack voltage of the battery pack and the lowest cell voltage of each battery cell.

[0041] In another aspect of the present invention, a battery system is provided, comprising the above-mentioned battery equalizer and its various variations, and a battery pack comprising a plurality of battery cells connected in series, wherein the plurality of battery cells are connected to the battery equalizer.

[0042] In another aspect of the present invention, a method for cell balancing a battery pack connected in series is provided. The method includes the following steps: providing a first balancing current to a first battery cell in the battery pack, the first balancing current being based on an initial voltage difference between the first battery cell and the remaining battery cells in the battery pack; and providing a second balancing current to the first battery cell that gradually decreases from the first balancing current, the second balancing current being based on a decrease in the voltage difference between the first battery cell and the remaining battery cells in the battery pack. The second balancing current is further based on an AC voltage converted from a pack voltage of the battery pack.

[0043] Preferably, the AC voltage is obtained by using a DC / AC converter coupled to the battery pack. The DC / AC converter includes an active switch and a first inductor connected in series with the active switch.

[0044] Alternatively or additionally, the second balancing current is generated by an AC / DC converter coupled to the first battery unit, wherein the AC / DC converter is coupled to an output capacitor of the DC / AC converter.

[0045] In another aspect of the present invention, a battery balancer is provided, comprising a plurality of unit circuits connected in parallel, an AC bus connected to the plurality of unit circuits, and a DC / AC converter connected to the AC bus. Each unit circuit is adapted to be connected to a battery cell. Each unit circuit includes an AC / DC converter. The AC side of the DC / AC converter and the AC / DC converter are both connected to the AC bus.

[0046] In certain embodiments, the DC / AC converter includes an active switch that is the only active switch in the cell balancer.

[0047] In certain embodiments, the DC / AC converter further includes an inductor connected to the active switch.

[0048] In certain embodiments, the DC / AC converter further includes a coupled inductor connected to the active switch.

[0049] In some embodiments, the DC / AC converter, and the AC side of the AC / DC converter, are connected to the AC bus via capacitors.

[0050] In certain embodiments, each AC / DC converter is in an input parallel output series (IPSO) configuration.

[0051] In some embodiments, two coupling capacitors are connected to each AC / DC converter.

[0052] In certain embodiments, each AC / DC converter includes an inductor and a diode.

[0053] In certain embodiments, each AC / DC converter is adapted to be connected to its corresponding battery cell through an output filter.

[0054] According to yet another aspect of the present invention, a battery system is provided. The system includes the above-mentioned battery equalizer and a plurality of battery cells connected to the battery equalizer.

[0055] According to another aspect of the present invention, a method for performing battery balancing is provided. The method includes the following steps: providing an initial balancing current to a plurality of battery cells; and determining a voltage difference between the plurality of battery cells and gradually reducing the balancing current as the voltage difference decreases. The balancing current provided to each of the plurality of battery cells is determined by an AC / DC converter.

[0056] As can be seen, various embodiments of the present invention provide an autonomous cell balancer using a capacitively coupled ZETA-derived structure. This balancer eliminates the need for a selector switch, as the output current is automatically distributed based on cell voltage differences. Furthermore, the use of a single magnetic component for the entire battery pack significantly reduces circuit size and weight. The balancer incorporates only a single semiconductor switching device, reducing cost and simplifying control.

[0057] The above summary of the invention is neither intended to define the invention of the present application nor to limit the scope of the invention in any way. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, in which:

[0059] Figure 1 A battery-based energy storage architecture is shown.

[0060] FIG2 a shows a block diagram of a conventional DCSS-based equalizer.

[0061] FIG2 b shows a block diagram of a conventional MC-based equalizer.

[0062] Figure 2c shows the block diagram of an equalizer based on conventional multi-winding transformer coupling.

[0063] Figure 3 A cell balancing circuit according to one embodiment of the present invention is shown, connected to a battery pack.

[0064] Figure 4 Shows Figure 3 Key waveforms in the steady state during equilibrium for various circuit values ​​in the circuit.

[0065] Figure 5a Shown Figure 3 The cell balancer operates in this mode when the MOSFET is turned on.

[0066] Figure 5b Shown Figure 3 The cell balancer in operating mode when the MOSFET is off.

[0067] Figure 6 Shown Figure 3 The power loss of the battery balancer is related to the magnetizing inductance (L m ) relationship.

[0068] Figure 7a Shown Figure 3 Non-ideal model of DC / AC converter in .

[0069] Figure 7bshows the voltage spike generated when turning off MOSFETQ.

[0070] Figure 8a Shows the use of Figure 3 Simulation results of the four batteries in the battery balancer, with initial open circuit voltages of 3.756, 3.558, 3.360, and 3.201 V, respectively.

[0071] Figure 8b Shows the use Figure 3 Simulation results of the four batteries in the cell balancer, with initial open circuit voltages of 3.638, 3.435, 3.332, and 3.142 V, respectively.

[0072] Figure 8c Shows the use Figure 3 Simulation results of the four batteries in the battery balancer, with initial open circuit voltages of 3.756, 3.558, 3.360, and 3.201 V, respectively.

[0073] Figure 8d Shows the use Figure 3 The simulation results of the four batteries of the battery balancer show that under the discharge condition of 40Ω load, the initial open circuit voltages are 3.926, 3.728, 3.572 and 3.382V respectively.

[0074] Figure 9 shows the key waveforms measured from a prototype of the cell balancer, including the gate drive signal v gs , inductor current i L1 、i L2 and the output voltage v of the DC / AC converter x .

[0075] Figure 10a The experimental results of four batteries under single module balancing under idling conditions are shown.

[0076] Figure 10b The experimental results of four batteries under 1A current charging condition are shown.

[0077] Figure 10c The experimental results of four batteries under 2A current charging condition are shown.

[0078] Figure 10d Shows the experimental results of four batteries discharged under 16Ω load conditions.

[0079] Figure 11 A cell balancing circuit according to another embodiment of the present invention is shown, the circuit being connected to a battery string.

[0080] Figure 12FIG. 2 shows an overall circuit topology of a battery balancing circuit according to another embodiment of the present invention, which is connected to a battery string.

[0081] Figure 13a A cell balancing structure including multiple cell equalizers according to an embodiment of the present invention is shown.

[0082] Figure 13b Shows Figure 13a A specific implementation of the battery balancing structure.

[0083] Figure 14 The circuit structure of the mth module in the battery balancing structure is shown.

[0084] Figure 15a It shows that when the MOSFET is turned on, Figure 14 The working mode of the mth module in .

[0085] Figure 15b Shows Figure 14 The operating mode of the mth module when the MOSFET is turned off.

[0086] Figure 16a Shows Figure 14 The simplified circuit of each of the four unit circuits in the mth module.

[0087] Figure 16b A simplified circuit of the discharge unit is shown.

[0088] Figure 16c Shows the simplified circuit of the charging unit when the MOSFET is turned on.

[0089] Figure 16d Shows a simplified circuit of the charging unit when the MOSFET is off.

[0090] Figure 16e Shows Figure 14 Simplified circuit of the DC / AC circuit of the mth module.

[0091] Figure 17 Shows Figures 15a-15b The steady-state key waveform of the battery balancing module in the balancing process.

[0092] Figure 18a shows the key waveforms of MAVE, including the gate drive signal v when module M2 operates independently to balance four cells. gs2 , inductor current i L21 、i L22 And the coupling capacitor C 26 The voltage v C26 .

[0093] Figure 18b shows the key waveforms of MAVE, including the drain-source voltage v of Q1 when the two modules are used to balance eight cells. ds1 and Q2's v ds2 、The output voltage v of the DC / AC converter of module M2 x2 , and the inductor current i L11 .

[0094] Figure 19a The experimental results of eight cells under idling conditions balanced across two modules are shown.

[0095] Figure 19b The experimental results of eight batteries under 1A current charging condition are shown.

[0096] Figure 19c Shows the experimental results of eight batteries under discharge conditions of 32Ω load.

[0097] Before explaining any embodiments of the present invention in detail, it should be understood that the invention is not limited in its application to the details and arrangement of components of the embodiments described below or illustrated in the accompanying drawings. The invention is capable of other embodiments and of being carried out in various ways. Furthermore, it should be understood that the phraseology and terminology used herein are for descriptive purposes only and are not to be construed as limiting. DETAILED DESCRIPTION

[0098] A first embodiment of the present invention is an autonomous battery balancing circuit that performs energy routing and balances the battery voltages of a series-connected battery string (i.e., a type of battery pack). It utilizes a capacitively coupled, two-stage power conversion system. The first stage converts the DC voltage of the battery string into a high-frequency AC voltage, forming an AC link. The second stage consists of a diode rectifier, whose input is connected in parallel with the AC link and whose output is connected to the individual battery cells. The current drawn from the first stage is shared between the rectifiers in the second stage via the AC link. Because the current drawn from the first stage is shared by all battery cells, the charging or discharging of individual battery cells is autonomously determined by the output current of the second stage, which in turn is determined by the difference between the coupling capacitor voltage and the AC link voltage. This structure requires only a coupled inductor and an active switch (e.g., a MOSFET), reducing its size and weight and simplifying the balancer control circuitry. This structure does not require a selector switch, and the charge / discharge current is automatically distributed based on the differences in battery voltages within the battery string.

[0099] Figure 3The circuit diagram of the cell balancing circuit (or simply cell equalizer) in the first embodiment is shown. The circuit is connected to a battery string consisting of N series-connected cells, designated as Cell_1, Cell_2, ... Cell_N. Each of the N cells has its own cell voltage. Since N cells are connected in series, the string voltage is defined as the cumulative voltage of the N cells, i.e. Figure 3 The voltage between the positive terminal of Cell_1 and the negative terminal of Cell_N in the cell balancer is measured. The group voltage (DC voltage) is provided to the input of the cell balancer, specifically to the terminals of capacitor Cin in the first-stage DC / AC converter of the cell balancer. Capacitor Cin acts as a DC voltage source in the cell balancer, and capacitor Cin itself is charged by the group voltage. As described below, the input and output of the cell balancer are both DC voltages, but the cell balancer has an AC link that provides AC voltage. Therefore, in addition to the DC / AC converter on one side of the AC link, there is also an AC / DC converter on the other side of the AC link to convert the AC current of the AC link into a DC charging current for the battery string, which will be explained later.

[0100] In addition to the capacitor Cin, the DC / AC converter also includes an active switch, which is an N-channel MOSFETQ with a body diode and parasitic body capacitance, and an inductor Lm. The inductor Lm is connected in series with the MOSFETQ between the two ends of the capacitor Cin. The drain of the MOSFETQ is connected to one end of the capacitor Cin, while the source of the MOSFETQ is connected to the inductor Lm and the transformer. The gate of the MOSFETQ is connected to Figure 3 A controller (not shown) is external to the circuit and provides a drive signal to MOSFET Q. The transformer has a turns ratio of n:1 and can be considered a pair of coupled inductors. The primary side of the transformer is connected to MOSFET Q and inductor Lm, while the secondary side of the transformer is connected to the AC link. Inductor Lm acts as the magnetizing inductance.

[0101] The AC link serves as an AC bus, so that the output AC voltage of the DC / AC converter can be shared by all unit circuits of the battery balancer. Figure 3Only three cell circuits, Cell_1, Cell_2, and Cell_N, are shown, but it should be noted that N can be any number equal to or greater than 2 (in the case of N=2, the cell balancer will only have Cell_1 and Cell_2), and all cell circuits in the cell balancer preferably have the same structure. The N cell circuits consist only of passive components and are interconnected in a parallel ZETA configuration. Therefore, MOSFET Q is the only active switch in the entire cell balancer. The input terminals of the multiple cell circuits are connected in parallel, and these input terminals are coupled to the AC link. On the other hand, the output terminal of each cell circuit is connected to the corresponding battery in the battery string. Therefore, the multiple cell circuits effectively form an input parallel output series (IPOS) circuit.

[0102] Each of the N unit circuits includes an AC / DC converter, which includes diodes D1, D2...D n and inductors L1, L2…L N The diode and inductor in each unit circuit are connected in series between the two ends of the corresponding battery Cell_1, Cell_2...Cell_N. In addition, the AC / DC converter in each unit circuit is capacitively coupled to the AC link on the AC side of the AC / DC converter. Capacitive coupling is achieved by two capacitors in each unit circuit, such as capacitors C1 and C2 in the first unit circuit including diode D1 and inductor L1. The two capacitors C1 and C2 are respectively coupled between the AC link and one of the two ends of diode D1. Note that Figure 3 The AC link in the exemplary embodiment has two wires, and each of the two capacitors C1 and C2 is coupled to a different one of the two wires of the AC link. The total number of capacitors used for capacitive coupling of the AC / DC converter is 2N. The capacitors coupled to the AC / DC converter function by decoupling the DC component in the AC link, thereby blocking DC current circulating between the AC / DC converters.

[0103] In addition, two inductors L are configured for each unit circuit. f1 -L f(N+1) , they act as output filters for the charging current supplied to the battery cells. L f1 -L f(N+1) The inductor is connected to each end of all N battery cells, so the inductor L f1 -L f(N+1) The total number is N+1. In addition to the inductor L f1 and L f(N+1) In addition, the inductor L f1 -L f(N+1) The other inductors in are shared by two unit circuits, such as inductor Lf2 Connect to the anode of the diode D1 in the first unit circuit and one end of the inductor L2 in the second unit circuit.

[0104] In describing Figure 3 After describing the structure and components of the cell balancer, we will now introduce the operating principle of the cell balancer. The DC / AC converter on the primary side of the transformer is used to generate a high-frequency output, which provides the AC voltage for the AC link after voltage conversion. Depending on the structure of the DC / AC converter, the cell balancing circuit operates in discontinuous conduction mode (DCM) or boundary conduction mode (BCM). During each switching cycle, the current flowing through the inductor L m The current in the battery equalizer reaches zero. The structure of the battery balancer is a ZETA-derived converter

[14] , in which the duty cycle of the MOSFET Q is determined by the ratio between the cell voltage and the group voltage. The cell voltage here refers to the voltage of a single battery cell in the battery string. The higher the ratio of these two voltages, the smaller the duty cycle of Q. The role of the transformer is to prevent the duty cycle from becoming too small when the number of batteries is large.

[0105] To simplify the analysis of circuit behavior, Figure 3 The cell balancing circuit in Figure 1 can be divided into two parts: MOSFET Q and transformer, and the passive component network (which contains the above-mentioned unit circuits). In addition, when analyzing the circuit balancing principle, all components are considered ideal components.

[0106] As mentioned above, the battery string provides the DC input voltage for the battery balancer. The key waveforms of the MOSFETQ drive signal and the various currents flowing through the components in the steady state during balancing are as follows: Figure 4 shown. Figure 5a and 5b shows the equivalent circuit of the balanced structure during one switching cycle, where Figure 5a shows the operating mode when MOSFETQ is turned on, while Figure 5b shows the operating mode when MOSFETQ is turned off. Figure 5a and 5b In the figure, only two cells in the battery string are shown. For the purpose of illustration, it is assumed that one of the cells (voltage is Cell out ) has a higher voltage than another cell (voltage Cell in ). Therefore, the voltages of the two cells need to be balanced.

[0107] For the entire battery string, the module voltage V module (i.e. the battery string voltage mentioned above) can be expressed as:

[0108]

[0109] Where N is the number of battery cells as described above, K is the number of net outflow cells, and J is the number of net inflow cells. The secondary side of the transformer provides an AC output, which can be in the form of a voltage source, a current source, or a combination of voltage and current sources. The peak voltage of the AC link is expressed as:

[0110]

[0111] Where n is the turns ratio of the transformer as described above.

[0112] In the passive component network, the input terminals of all unit circuits are connected in parallel, so the current i on the secondary side of the transformer is m is equal to the sum of the currents of all coupling capacitors. As mentioned above, there are 2N coupling capacitors in total, i m The relationship between the currents flowing through the coupling capacitors can be expressed as:

[0113]

[0114] For each unit circuit, at any time, one of the equations (4a) and (4b) will hold. For example, Figure 5a In the example, for the upper of the two cells in the figure, formula (4a) will apply, and for the lower of the two cells in the figure, formula (4b) will apply.

[0115] By using equations (2)-(4b), the output current of each unit circuit at any time can be given by one of equations (5a) and (5b). Figure 5a For the upper battery of the two batteries in the figure, formula (5a) will apply. For the lower battery of the two batteries in the figure, formula (5b) will apply.

[0116] The input current of the battery balancer (i M ) is the current drawn from the battery pack. Therefore, for each battery cell, the balancing current at any time is provided according to one of equations (6a) and (6b).

[0117] i B_out =i out -i M (6a)

[0118] i B_in =i in -i M (6b)

[0119] Based on equations (5a)-(6b), the power to balance each battery cell at any time can be expressed by one of equations (7a) and (7b).

[0120]

[0121] When MOSFETQ is in T on When the internal conduction Figure 5a As shown, the magnetizing inductance (i.e. inductance L m ) is charging. m The voltage across the terminals is:

[0122]

[0123] for Figure 5a and 5b The inductance L out For , its inductor voltage is expressed by formula (9a). Figure 5a and 5b The inductance L in For , its inductor voltage is expressed by formula (9b).

[0124]

[0125] When MOSFETQ is in T on When the internal conduction is on, all diodes D1, D2...D n are all closed, the coupling capacitor C out1 and C out2 The currents are expressed by equations (10a) and (10b) respectively.

[0126] i Cout1 =-i Lout (10a)

[0127] i Cin1 =i Lin (10b)

[0128] When MOSFETQ is in T off When the inner is closed, Figure 5b As shown, the unit charging circuit (for Cell in ) and unit discharge circuit (for Cell out ) has different operation modes, such as Figure 5b As shown, and L m Discharging. m The voltage across the terminals is:

[0129]

[0130] At this time, the diode D (such as Figure 5b The voltage of the AC link is clamped, and other diodes (such as D out )Close (such as Figure 5b(shown in medium grey). V Lout and V Lin It can be expressed as

[0131]

[0132] V Lin =-V Cellin (13)

[0133] In addition, the relationship between the currents of the coupling capacitors can be expressed by the following formula:

[0134] i Cout1 =-i Lout (14)

[0135]

[0136] Since the cell balancing circuit operates in BCM, the period during which the inductor current remains constant can be ignored.

[0137]

[0138] Formula (16) shows that C in1 The principle of capacitor ampere-second balance, where D is the duty cycle of the switch, I Lout_min and I Lin_min They are inductance L out and L in According to Kirchhoff's Current Law ("KCL"):

[0139] I Lout_min +I Lin_min =-nI Lm_min .(17)

[0140] During the balancing process, assuming that the current of K battery cells is a net outflow and the current of J battery cells is a net inflow, formula (17) can be expressed as:

[0141] KI Lout_min +JI Lin_min =-nI Lm_min .(18)

[0142] By using (15), (16) and (18), L out The minimum current can be derived as:

[0143]

[0144] Where L = L out =L in Similarly, according to the capacitor C out1 Ampere-second balance, L outThe minimum current can be derived as:

[0145]

[0146] Inductor L in The average current can be calculated as:

[0147]

[0148] Magnetizing inductance (i.e. inductance L m ) is the discharge current of the battery pack during the balancing process, and it can be obtained that:

[0149]

[0150] Combining formulas (6), (21) and (22), the balancing current of each battery can be expressed as:

[0151]

[0152] Therefore, due to the balancing current (i balancing ) is different, balancing of all battery cells in the battery pack can be achieved.

[0153] In the next section, we will describe Figure 3 As mentioned above, for a battery balancer, during the battery cell balancing process, the balancing current gradually decreases until the voltage difference between the battery cells is eliminated. Therefore, the initial value of the balancing current can only be calculated based on the initial voltage distribution of the battery cells before balancing. From formulas (22) and (23), it can be seen that the balancing current is affected not only by the duty cycle and voltage distribution, but also by the inductance value. The inductance value here refers to the secondary side diodes (D1, D2...D N ) next to the inductor (L1, L2...L N ) inductor value, for example, 10uH. Higher currents increase balancing speed, but the presence of the diode increases conduction losses. Furthermore, the voltage ripple of the coupling capacitor affects balancing speed and efficiency. Here, the relationship between component values ​​and losses is investigated. Furthermore, the number of cells needs to be considered when designing the transformer. Zero voltage switching (ZVS) of the switches is also considered.

[0154] Figure 3 The cell balancer in a circuit exhibits two primary loss sources: diode losses and magnetic component losses, which determine the efficiency of the cell balancer. Magnetic component losses can be categorized as coil losses and core losses. In circuit design, a trade-off must be made between conduction losses, switching speed, and component size.

[0155] The diode losses can be calculated as:

[0156]

[0157] Among them, V D is the forward voltage drop of the diode. The coil loss of the magnetic component can be calculated as:

[0158] P coil =R coupL_coil I discharge 2 +R L_coil I L 2 (25)

[0159] Therefore, the conduction loss can be expressed as:

[0160] P loss =P diode +P coil (26)

[0161] Figure 6 Shows the conduction loss and magnetizing inductance (L m ). It can be seen that when the magnetizing inductance (L m ) is greater than 200μH, the conduction loss is significantly reduced. In addition, when the inductance (L k ) exceeds 10μH, the loss reduction is not obvious.

[0162] Due to the non-ideal nature of the transformer in the experimental setup, the effect of leakage inductance is investigated. Figure 7a The non-ideal model of the DC / AC converter is shown. Figure 7b As shown in Figure 1, when switch Q is turned off, leakage inductance will cause a voltage spike to appear. The spike voltage can be calculated as:

[0163]

[0164] The high voltage spike caused by the current suddenly dropping to zero may damage the MOSFET. Therefore, it is necessary to design C oss To suppress voltage spikes. The measured leakage inductance is 2uH. According to ΔQ=C oss V spike ,C oss was chosen to be 2000pF.

[0165] To verify Figure 3 Analysis and design of battery balancer in A circuit to balance four mismatched capacitors is designed in PSIM. In order to shorten the simulation time, four 0.1F capacitors in series are used to simulate the battery cell. Figure 8aAs shown in Figure 1, the initial voltages of the series capacitors are 3.756, 3.558, 3.360, and 3.201 V. After 0.2s of equilibrium, the voltage of the series capacitors is 3.410 V. Figure 8b As shown in Figure 1, the initial voltages of the series capacitors are 3.638, 3.435, 3.332, and 3.142 V, respectively. After 0.2s of equilibrium, the voltages of the series capacitors converge to 3.351 V. Figure 8c As shown in Figure 1, the initial voltages of the series capacitors are 3.756, 3.558, 3.360, and 3.201 V, respectively. The battery pack is subjected to a 0.5 A charging current. After 0.15 s of balancing, the voltages of the series capacitors converge to 4.005 V. The initial voltages of the series capacitors are 3.926, 3.728, 3.572, and 3.382 V, respectively. Figure 8d As shown in Figure 2, the battery pack is discharged through a 40Ω load. After 0.2s of balancing, the voltage across the series capacitors converges to 2.910V. The results of the four simulations demonstrate that the battery balancing circuit is suitable for balancing batteries during both charging and discharging.

[0166] Subsequently, according to Figure 3 A cell balancer prototype was constructed and tested. Four retired 2600mAh Samsung ICR18650 lithium-ion batteries were used in the prototype. The circuit parameters are summarized in Table 1. The transformer turns ratio in this prototype is 4.5:1. The number of unit circuits or module layers can be adjusted based on the number of battery cells. Furthermore, to improve the balancer's conversion efficiency, a low forward voltage power Schottky diode (30BQ015) was used. Voltage data was recorded using a data logger (Keysight 34970A).

[0167] Table 1 Key design parameters

[0168]

[0169] Figure 9 Figure 10 shows the key waveforms of the balancer prototype described above. Due to the varying battery voltages, the inductor in the AC / DC converter exhibits different DC biases. Figure 11 shows the voltages detected across four battery cells during the experiment. These four cells were combined into a battery string and balanced by a balancer module (i.e., the prototype). Figure 10a The figure shows the balancing results of four batteries under static conditions. The initial voltages of the four batteries are 3.452V, 3.442V, 3.419V and 3.153V respectively. The balancing circuit reduces the voltage difference from 299mV to 18mV in 4300s. Figure 10b In Figures 1 and 2, equilibration experiments were performed at 1A and 2A charging currents. The initial voltage differences were 479mV and 229mV, respectively. After equilibration, the voltage difference was less than 20mV. Figure 10d The figure shows the equalization results during discharge under a 16Ω load.

[0170] In summary, the above description and Figure 3-10d The embodiment shown in the present invention provides an autonomous battery balancer using a capacitively coupled ZETA-derived structure. The balancing circuit does not require a selection switch because the output current is automatically distributed according to the difference in battery voltage. In addition, the use of a single magnetic component for the entire battery string can significantly reduce the size and weight of the circuit. The balancer contains only one semiconductor switching device, which reduces cost and simplifies control. The operation of the MOSFET as an active switch adopts pulse width modulation, which simplifies the overall circuit design and reduces control complexity. In traditional battery balancers, the state of health (SOH) of the battery cells is adversely affected by the alternating current during the balancing process. However, according to the balancer of the exemplary embodiment described, a DC charging or discharging current is provided, thereby greatly reducing the damage to the battery cells and maintaining the SOH. In addition, the battery balancer can be modular. Balancing can be achieved within a module or between modules (M2M). Therefore, the structure is scalable and can be applied to systems of various voltage levels.

[0171] It should be noted that although Figure 3 The cell balancer in FIG. 1 includes a transformer (coupled inductor) before the AC link, but the transformer is only optional and the present invention should not be limited by the configuration of the transformer in the circuit. Figure 11 Another embodiment of the present invention is shown, and its circuit structure is similar to Figure 3 The circuit structure in is roughly similar, but without the transformer. As mentioned earlier, when the number of battery cells in the battery string is large, the transformer can prevent the duty cycle from becoming too small, but it is not absolutely necessary in all cases, especially when the number of battery cells is small.

[0172] Come and see Figure 12 , the figure shows Figure 11 A more generalized representation of the cell balancer circuit in . Figure 12 In this design, not only are there coupling capacitors connected to the AC / DC converter within the unit circuit, but two coupling capacitors are also connected between the DC / AC converter and the AC link to decouple the DC component from the DC / AC converter's output. The AC link is shared among multiple modules (i.e., multiple unit circuits), with each module connected to the AC link via two capacitors. Figure 12 The battery string shown has N AC / DC converters coupled via 2N capacitors. These AC / DC converters in the unit circuit are connected in an IPSO configuration. The multi-winding transformer is replaced by capacitors, effectively reducing the circuit size and making it easier to implement in large battery systems.

[0173] Come and see Figure 13a , the figure shows the block diagram of the multi-module battery balancing structure, where each module is based on Figure 12 The configuration of the battery balancing circuit shown in Figure 13 does not require an external energy source and is used to balance the voltages of multiple battery cells, namely Cell_11, Cell_12, ..., Cell_1N, Cell_21, Cell_22, ..., Cell_2N, ..., Cell_m1, Cell_m2, ..., Cell_mN, ..., Cell_K1, CellK2, ..., Cell_KN, which are connected in series to form a battery string. The batteries are divided into K groups, each with N battery cells. Each group is controlled by a battery balancing module. Therefore, there are a total of K modules, labeled M1, M2, ..., and MK.

[0174] will be Figure 13a The structure and operation of one of the four modules in the figure are explained as an example. This module is the third module Mm in the figure. Module Mm consists of a single front-end DC / AC converter "CON_Mm" and multiple AC / DC converters (i.e., CON_m1, CON_m2, ..., CON_mN). The input terminal of CON_Mm is connected across the battery string consisting of battery cells Cell_m1, Cell_m2, ..., Cell_mN. The input current i Mm The output of CON_Mm provides an AC output, which can be in the form of a voltage source, a current source, or a combination of a voltage source and a current source. It is connected to the capacitor C mA and C mB Connected to the bus, i.e. the AC bus, for separating the DC component from the output of CON_Mm. The bus is shared by multiple modules, each connected to the bus via two capacitors.

[0175] Figure 13a Each of the four modules in the 1000 series consists of N AC / DC converters, labeled CON_11, CON_12, ..., CON_1N, CON_21, CON_22, ..., CON_2N, ..., CON_m1, CON_m2, ..., CON_mN, ..., CON_K1, CON_K2, ..., CON_KN. Each converter is connected through a plurality of series capacitors (C m11 、C m12 、C m21 、C m22 ..., C mk1 、C mk2 ..., C mN1 、C mN2) are connected to the busbar. These capacitors are used to eliminate the DC component on the busbar, thereby blocking the DC current between the AC / DC converters. Each AC / DC converter is used to control the charge or discharge state of the connected battery cell. For example, for cell k in module m, assuming the output current of the AC / DC converter is i o,m,k Then, if i o,m,k Greater than i Mm , unit k will enter the charging state. On the contrary, if i o,m,k Less than i Mm , cell k will enter the discharge state. Therefore, the structure acts like an "energy router and redistributor", allowing each battery cell to discharge or charge other battery cells through the bus. By programming the input current of each module (i.e., i Mm)， The amount of energy from the associated battery pack to the bus that is processed by the DC / AC converters, as well as the output current of individual AC / DC converters, can be controlled to control the charging or discharging process described above. For illustration purposes, assume that battery cell Cell_m2 needs to be charged. All AC / DC converters except CON_m2 will output zero average current. The input current (i.e., i M1 、i M2 、……、i MK ) are controlled to deliver energy to the bus.

[0176] The energy routing and redistribution structure described above is universal, allowing individual batteries to be programmed to specific voltage levels. It is also applicable to other types of energy storage devices, such as capacitors, where individual capacitors may require voltage control. It should also be noted that the number of battery cells in all modules does not have to be the same.

[0177] according to Figure 13a A specific implementation of the battery balancing structure has been developed, such as Figure 13b shown. Figure 13b The cell balancing structure in the embodiment includes a coupled inductor (forming a transformer) in each of the four modules, and the coupled inductor is arranged between the DC / AC converter and the two capacitors of the DC / AC converter. Specifically, Figure 13b Each DC / AC converter consists of an input capacitor C m,in , semiconductor switch Q m , coupled inductor T m And two decoupling capacitors C mA and C mB The input of each DC / AC converter is connected to a set of batteries through two filter inductors, while the output is connected to the AC bus. m With magnetizing inductance L m,m . Figure 13bEach AC / DC converter consists of a diode D mk 、Inductor L mk And the filter inductor L mfk and L mf(k+1) composition.

[0178] By using pulse width modulation technology to drive MOSFET (Q m ), it is possible to achieve energy redistribution of the battery cells connected to each module. This circuit can operate in either DCM or BCM mode. The operating principle of a single cell circuit is similar to that of a ZETA converter.

[0179] Figure 14 Shows Figure 13a The circuit structure of the mth module in the battery balancing structure. The entire battery pack provides input voltage for a single module. Module voltage Vmodule It can be expressed as

[0180]

[0181] The output of the DC / AC converter CON_Mm provides an AC output, which can be in the form of a voltage source, a current source, or a combination of a voltage source and a current source. The output voltage v of CON_Mm mx Can be expressed as:

[0182]

[0183] Where n is the voltage ratio of CON_Mm. All AC / DC converters, i.e. CON_m1 to CON_mN, have their inputs connected in parallel. Therefore,

[0184]

[0185] For each AC / DC converter:

[0186] v mx i i,m,k =i o,m,k V Cell_mk (31)

[0187] By using equations (29)-(31), the output current of the AC / DC converter can be expressed as:

[0188]

[0189] The balancing current of each battery cell is expressed as:

[0190] i B,m,k =i o,m,k -i Mm (33)

[0191] According to formulas (32) and (33), the balancing power of each battery cell can be expressed as:

[0192]

[0193] During the balancing process, the output power of the battery pack is equal to the input power. Therefore:

[0194] V Cell_m1 i B,m,2 +…+V Cell_mk i B,m,k +V Cell_mN i B,m,N =0(35)

[0195]

[0196] According to formula (34), the balancing current can be expressed as:

[0197]

[0198] Formula (37) can also be expressed as:

[0199] i B,m,k =Ai i,m,k (38)

[0200] where i B,m,k =[i B,m,1 i B,m,2 … i B,m,N ] T ,i i,m,k =[i i,m,1 i i,m,2 … i i,m,N ] T and

[0201]

[0202] AC / DC Converter i,m,k The input current can be calculated from the input impedance of the circuit, which is related to the circuit.

[0203] Figure 15a and 15b The equivalent circuit of the balanced structure during one switching cycle is shown. Figures 16a-16e A simplified circuit diagram of each AC / DC circuit is shown. For simplicity, the filter inductor is ignored in the analysis. Figure 17 The key waveforms are shown. Due to the change in the initial battery voltage, the AC / DC converter operates in different modes. Assume that Cell_m1 has a net current outflow and Cell_m2 has a net current inflow. Figure 15a The “ON” period (Ton ), the switch is turned on, causing the inductor L m2 As the current increases, the inductor L m1 The current decreases to zero and then increases. m When turned on, all diodes are closed, such as Figure 16b As shown. Figure 15b The “off” period (T off ), the switch is closed, causing the inductor L m2 The current decreases, the inductor L m1 The current decreases to zero and then increases. Only the diode corresponding to the battery cell that needs to be charged (such as D m2 ) turns on, while the diode associated with the higher voltage battery cell remains off.

[0204] The entire battery string provides input voltage to a single module. The module's input current is equal to the current flowing from the battery string. When the battery voltage is higher than the average voltage of the battery string, the diode in the AC / DC converter connected to the battery does not conduct, and the inductor L mk (i o_mk ) is zero. This means that the average current flowing from the AC / DC converter to the battery is zero, and the battery discharge current is i Mm At the same time, the battery with the lowest voltage is being charged. Figure 16e The simplified circuit of each DC / AC circuit is shown. According to formula (28), the input voltage of the circuit can be expressed as:

[0205]

[0206] Under steady-state conditions, the maximum voltage stress on the capacitor is:

[0207]

[0208] In addition, the voltage difference between the two capacitors in each unit circuit is equal to the battery voltage, as

[0209] V C(2k-1) -V C2k =V Cell_k (41)

[0210] Self-balancing means that the balancing current decreases as the voltage difference decreases. When the voltage difference between the battery cells is eliminated, the balancing current will also decrease to zero.

[0211] When the switch is turned on (T on ) period, if Figure 15a As shown, the magnetizing inductance (L m,m ) is charged. L m,m Voltage is

[0212] VLm,m =V module (42)

[0213] The inductor voltage can be expressed as

[0214]

[0215] All diodes are off and the current in the coupling capacitor is:

[0216]

[0217] When the switch is in T off When the internal circuit is closed, the charging circuit and the discharging circuit have different working modes, such as Figure 15b As shown, where L m，m Discharging. m，m The voltage across the two ends is

[0218] V Lm,m =-n(V Cm3 -V Cm4 +V CmA -V CmB )=n(-V Cm1 +V Cm2 +V Lm1 +V Cell_m1 +V CmA -V CmB ) (45)

[0219] At this time, the diode D in the unit circuit with the lowest voltage m2 The AC bus voltage is clamped and other diodes (such as D m1 )closure.

[0220] V Lm1 and V Lm2 can be expressed as:

[0221]

[0222] V Lm2 =-V Cell_m2 (47)

[0223] In addition, the relationship between capacitor and current can be expressed by the following formula:

[0224] i Cm1 =-i Lm1 (48)

[0225] i Cm1 +i Cm3 +…+i Cm(2k-1) =ni Lm,m (49)

[0226] Since the circuit operates in BCM, the time during which the inductor current remains constant can be ignored.

[0227]

[0228] Formula (50) shows that C m3 The principle of capacitance ampere-second balance, where D is the duty cycle of the switch, I Lm1_min and I Lm2_min They are inductance L m1 and L m2 The minimum current. According to KCL,

[0229] I Lm1_min +I Lm2_min =-nI Lm,m_min (51)

[0230] During the balancing process, it is assumed that the current of a battery cell is a net outflow and the current of b battery cells is a net inflow. Formula (51) can be expressed as:

[0231] aI Lm1_min +bI Lm2_min =-nI Lm,m_min . (52)

[0232] By using equations (49), (50) and (52), the minimum current of L2 can be obtained as:

[0233]

[0234] Where L = L m1 =L m2 Similarly, according to the capacitor C m1 Ampere-second balance, L m1 The minimum current can be expressed as:

[0235]

[0236] The average current of inductor L2 is:

[0237]

[0238] Magnetizing inductance L m,m The average current is the discharge current of the battery pack during the balancing process, and it can be obtained:

[0239]

[0240] The balancing current can be calculated using equations (33), (55) and (56).

[0241] In order to demonstrate the effectiveness of MAVE, Figure 13a and 13b A laboratory prototype of the IPOS-based balancing system shown was built and tested. The prototype consists of two battery balancing modules. The balancing system supports eight series-connected battery cells, which are divided into two battery packs. Eight retired 2600 mAh Samsung ICR18650 lithium-ion batteries were used in the prototype experiments. The balancing module can be expanded to accommodate 15 cells. The circuit parameters are summarized in Table 1. An STM32F407 controller was used to control the MOSFETs. The coupled inductor turns ratio was 4.5:1. The number of cell circuits or module layers can be adjusted based on the number of connected batteries.

[0242] In addition, to improve the conversion efficiency of the modular equalizer, a low forward voltage power Schottky diode (30BQ015) was used. The voltage data was recorded by a data logger (Keysight 34970A).

[0243] Figure 9 、 18a -18b shows the key waveforms of module A, module B and the two-layer structure respectively. Figure 9 、 18a -18b shows that the switching frequency is 200kHz. Figure 9 and 18a As shown in Figure 2, the inductor exhibits different DC biases due to changes in battery voltage. Figure 18b The figure shows the voltage across the MOSFETs of two modules when a 2000pF capacitor is connected in parallel. Soft switching is achieved through the resonance of the parallel capacitor and the leakage inductance of the coupled inductor.

[0244] Figures 10a-10d The detected voltages of the four battery cells in the experiment are described separately. These four battery cells are combined into a battery pack and balanced by a balancer module. Figure 10a The figure shows the balancing results of four cells under static conditions, whose initial voltages are 3.452, 3.442, 3.419, and 3.153 V. The balancing circuit reduces the voltage difference from 299 mV to 18 mV in 4300 seconds. Figure 10b and 10c In the experiment, balancing experiments were performed at 1A and 2A charging currents. The initial voltage differences were 479mV and 229mV, respectively. After balancing, the voltage difference was less than 20mV. Figure 10d The figure shows the equalization results during discharge under a 16Ω load.

[0245] In order to further verify the performance of the modular equalizer, Figures 19a-19cThe results of a multi-level balancing experiment are shown in Figure 2. Two balancing circuits are coupled via an AC bus to balance eight battery cells connected in series. In addition, the two modules share a controller, and a 200kHz PWM signal drives two MOSFETs simultaneously. Figure 19a As shown in Figure 2, the modular equalizer reduces the voltage difference from 458mV to 20mV within 3060s. Figure 19b and 19c In the balancing experiment, the battery pack was balanced at a 1A charging current and a 32Ω load. The maximum voltage difference of the battery string after balancing was less than 20mV and 30mV respectively.

[0246] To illustrate the advantages of the above-mentioned MAVE, Table II provides a comprehensive comparison of MAVE (labeled as “proposed method” in Table II) with other similar equalizers. In

[15] , an equalizer based on an LCC resonant converter is proposed, which achieves constant current balancing through open-loop control. However, 2n relays are required as multiplexers. The large number of active switches leads to increased control complexity and reduced reliability. In

[16] , a stacked buck-boost converter is proposed, which only requires one switch for the battery pack. However, the circuit cannot be modularized because the voltage stress on the switch increases significantly when the number of batteries increases. The methods in

[17] ,

[18] and

[19] can achieve synchronous balancing of multiple batteries. However, the large number of MOSFETs leads to low reliability and high cost. A multi-winding transformer is used in

[20] , but this method is difficult to implement when the number of batteries is large.

[0247] Table 2 Comparison of different equalizers

[0248]

[0249] n is the number of battery cells, m is the number of modules

[0250] In summary, it can be seen that MAVE can be designed in a modular manner. Furthermore, each module requires only one MOSFET, eliminating the need for selector switches. As the number of battery cells increases, only the number of modules needs to be increased, without a corresponding increase in switch voltage stress. The MAVE system consists of two stages: a DC / AC converter, which can be a voltage source, a current source, or a combination of both; and a set of capacitively coupled AC / DC converters capable of autonomous energy distribution. Compared to traditional balancing circuits, the entire system does not require selector switches to target specific balancing cells. Furthermore, using a single magnetic component within a single module across the entire battery pack significantly reduces circuit size and weight. More importantly, each module contains only one semiconductor switching device, reducing cost and simplifying control.

[0251] Thus, the embodiments of the present invention have been fully described above. Although specific embodiments have been mentioned in the description, it will be apparent to those skilled in the art that the present invention may be implemented in variations of these specific details. Therefore, the present invention should not be construed as being limited to the embodiments set forth herein.

[0252] Although the embodiments have been described in detail in the drawings and the foregoing description, they should be considered illustrative rather than restrictive, as they only show exemplary embodiments and do not limit the scope of the invention in any way. It is understood that any feature described herein can be used in combination with any embodiment. The illustrative embodiments are not mutually exclusive, nor do they exclude other embodiments not described in this specification. Therefore, the present invention also provides embodiments that include combinations of one or more of the above-mentioned illustrative embodiments. The invention described in this specification may be modified and varied without departing from the spirit and scope of the invention, and therefore, only the limitations described in the accompanying claims should be applied.

Claims

1. A battery equalizer for a battery pack connected in series, comprising: a plurality of unit circuits connected in parallel with each other at their input terminals; each of the unit circuits being adapted to be connected at its output terminal to a corresponding battery cell of the battery pack; an AC link to which the plurality of unit circuits are connected at their input terminals; and a DC / AC converter connected to the AC link; Each of the unit circuits includes an AC / DC converter, and an AC side of the AC / DC converter is coupled to the AC link capacitor at the input end of the unit circuit.

2. The battery equalizer according to claim 1, wherein: The DC / AC converter includes a voltage source and an active switch connected between the voltage source and the AC link.

3. The battery equalizer according to claim 2, wherein: The active switch is the only active switch in the cell balancer.

4. The battery equalizer according to claim 2, wherein: The DC / AC converter further includes a first inductor as a magnetizing inductance, the first inductor being connected in series with the active switch between the two ends of the voltage source. 5 . The battery balancer of claim 4 , wherein the first inductor is the only magnetic component of the battery balancer.

6. The battery equalizer of claim 2, wherein the voltage source is a first capacitor adapted to be charged by the battery pack. 7 . The battery equalizer according to claim 1 , wherein each of the unit circuits further comprises two coupling capacitors at the input end; and the AC / DC converter of the unit circuit is connected to the AC link via the two coupling capacitors. 8 . The battery balancer according to claim 1 , wherein the AC / DC converter in each of the unit circuits comprises a second inductor and a diode connected in series between both ends of a corresponding battery cell of the unit circuit. 9 . The battery balancer according to claim 1 , wherein each of the unit circuits further comprises an output filter connected between the AC / DC converter and the corresponding battery cell of the unit circuit.

10. The battery balancer of claim 8, wherein the output filter is a third inductor.

11. The battery balancer of claim 1 , wherein each AC / DC converter is a ZETA-derived converter. 12 . The battery balancer of claim 1 , further comprising a transformer coupled between the DC / AC circuit and the AC link.

13. The battery equalizer according to claim 2, wherein: The duty cycle of the active switch is controlled according to a ratio between a pack voltage of the battery pack and a lowest cell voltage of each of the battery cells. 14 . A battery system comprising the battery balancer according to claim 1 , and a battery pack comprising a plurality of battery cells connected in series, the plurality of battery cells being connected to the battery balancer.

15. A method for battery balancing of a battery pack connected in series, comprising the following steps: providing a first balancing current to a first battery cell in the battery pack, the first balancing current being based on an initial voltage difference between the first battery cell and remaining battery cells in the battery pack; as well as providing a second balancing current gradually decreasing from the first balancing current to the first battery cell; the second balancing current is based on a decrease in a voltage difference between the first battery cell and the remaining battery cells in the battery pack; The second balancing current is further based on an AC voltage converted from a pack voltage of the battery pack.

16. The method according to claim 15, wherein The AC voltage is obtained by using a DC / AC converter coupled to the battery pack; the DC / AC converter includes an active switch and a first inductor connected in series with the active switch.

17. The method according to claim 15, wherein: The second balancing current is generated by an AC / DC converter coupled to the first battery unit; the AC / DC converter is coupled to an output capacitor of the DC / AC converter.

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