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Active balancing of lithium‐ion battery cells using WPT as an energy carrier

2019; Institution of Engineering and Technology; Volume: 12; Issue: 10 Linguagem: Inglês

10.1049/iet-pel.2018.6177

1755-4543

Lizhou Liu, Wenbing Sun, Peibang Han, Ruikun Mai, Zhengyou He, Bo Luo,

Advanced DC-DC Converters

IET Power ElectronicsVolume 12, Issue 10 p. 2578-2585 Special Issue: Advanced Technologies Utilised in Wireless Power Transfer SystemsFree Access Active balancing of lithium-ion battery cells using WPT as an energy carrier Correction(s) for this article Manuscript No. PEL-2018-6177.R1 Volume 14Issue 13IET Power Electronics pages: 2303-2304 First Published online: August 11, 2021 Lizhou Liu, Lizhou Liu School of Electrical Engineering, Southwest Jiaotong University, Chengdu, 610036 People's Republic of ChinaSearch for more papers by this authorWenbing Sun, Wenbing Sun School of Electrical Engineering, Southwest Jiaotong University, Chengdu, 610036 People's Republic of ChinaSearch for more papers by this authorPeibang Han, Peibang Han School of Electrical Engineering, Southwest Jiaotong University, Chengdu, 610036 People's Republic of ChinaSearch for more papers by this authorRuikun Mai, Corresponding Author Ruikun Mai mairk@swjtu.edu.cn School of Electrical Engineering, Southwest Jiaotong University, Chengdu, 610036 People's Republic of ChinaSearch for more papers by this authorZhengyou He, Zhengyou He School of Electrical Engineering, Southwest Jiaotong University, Chengdu, 610036 People's Republic of ChinaSearch for more papers by this authorLuo Bo, Luo Bo orcid.org/0000-0003-0499-1078 School of Electrical Engineering, Southwest Jiaotong University, Chengdu, 610036 People's Republic of ChinaSearch for more papers by this author Lizhou Liu, Lizhou Liu School of Electrical Engineering, Southwest Jiaotong University, Chengdu, 610036 People's Republic of ChinaSearch for more papers by this authorWenbing Sun, Wenbing Sun School of Electrical Engineering, Southwest Jiaotong University, Chengdu, 610036 People's Republic of ChinaSearch for more papers by this authorPeibang Han, Peibang Han School of Electrical Engineering, Southwest Jiaotong University, Chengdu, 610036 People's Republic of ChinaSearch for more papers by this authorRuikun Mai, Corresponding Author Ruikun Mai mairk@swjtu.edu.cn School of Electrical Engineering, Southwest Jiaotong University, Chengdu, 610036 People's Republic of ChinaSearch for more papers by this authorZhengyou He, Zhengyou He School of Electrical Engineering, Southwest Jiaotong University, Chengdu, 610036 People's Republic of ChinaSearch for more papers by this authorLuo Bo, Luo Bo orcid.org/0000-0003-0499-1078 School of Electrical Engineering, Southwest Jiaotong University, Chengdu, 610036 People's Republic of ChinaSearch for more papers by this author First published: 24 June 2019 https://doi.org/10.1049/iet-pel.2018.6177Citations: 6AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract This study develops a novel equaliser by combining wireless power transmission (WPT) and switch array for series-connected batteries. The physical isolation achieved by WPT can offer unique benefits that traditional equalisers can hardly provide. Parameters of the WPT system are properly designed to achieve an equivalent constant-current output, which can maximise the speed of balance and ensure the health of batteries. This methodology can ensure each battery is connected independently to the inverter through the switch array circuit. Therefore, balancing for either a single battery or multiple batteries can be achieved. In the experiment, the prototype system is built. Experimental results show that the proposed equalisation system can achieve high overall system efficiency (above 87.2%) when balancing four series-connected battery modules. 1 Introduction Batteries are a major element for electric vehicles, uninterruptible power system, and energy storage systems [1, 2]. Owing to the high operating voltage demanded by the load, multiple batteries need to be connected in series. However, each battery may have some tolerance produced by chemical and electrical properties, leading to a voltage gap increasing during repeated charging (discharging) and decreasing the battery life and total capacity. Furthermore, voltage unbalance can cause the battery to be overcharged which may lead to fire and even explosion. Therefore, it is important to have a balanced system for the battery pack. Previous equaliser [3] can be divided into two categories: the passive balance method and the active balance method. The passive balance method [4] is used to shunt the cell which has a higher voltage through the bypass resistor. The advantage of this method is easy to implement, but the resistance will consume excessive energy which leads the system to be energy inefficient. Compared to the passive balance method, the active balance method is more energy efficient. The active balance method can be divided into two categories: using the energy storage electronic component or using the external power supply. The equaliser which uses energy storage components includes switched capacitor (SC) method [5, 6], zero-current switching (ZCS) method [7-9], and multi-winding transformer method [10-12]. The advantage of the classic SC method is that automatic equalisation can be achieved without using a monitor system for a battery string. However, energy can only be transferred from one battery to the adjacent one, so the more batteries between the batteries are being charged and the battery proving the power, the lower the balancing speed is [7]. Moreover, the batteries in the transmission path have to repeat charge and discharge. A double-tiered structure and a chain structure are proposed to reduce the energy transfer path and to improve balance speed. ZCS method [9] is proposed to reduce the switching loss. Furthermore, the inductance can increase balance speed when the voltage gap is small. However, the SC method and ZCS method cannot achieve cell-to-cell balance. Li et al. [11] proposed cell-to-pack equaliser based on forward and flyback transformer, which can achieve cell-to-cell balance. A charge balance type uses an external power supply to balance the lower-voltage battery. This method can reduce the time of subsequent battery replenishment. The active balance method that relies on an external power supply includes switch array method [13-15], flyback Transformer method [16], voltage-multiplier method [17-19], and resonantly coupled wireless power transmission (WPT) method [20, 21]. Chen et al. proposed charge equalisation based on switch array [14], so solar power can directly achieve cell-to-cell balance. However, in this topology, the model needs an additional battery for energy storage. Direct current (DC)–DC and additional energy storage modules lead to complex system control. Park et al. proposed charge equalisation composed of flyback transformer and buck–boost [17] circuit which reduces the number of transformers and leads to a small size. However, the high-voltage cell can balance only one cell at the same time, resulting in a slow balancing speed. Uno et al. proposed voltage-multiplier method [18], which can achieve equalisation without additional monitoring modules. However, repeated charging and discharging of capacitors in the topology results in low balance efficiency. Much of wireless power transmission (WPT) technologies have been proposed during the past few years, which can implement convenient and safe non-contacting charging. Especially, the WPT system parameters of the coils and the capacitors are optimised, which enables a constant-current (CC) or a constant-voltage (CV) output system. This feature simplifies the battery control method. Compared to the traditional DC–DC circuit, using the WPT technology can achieve electrical isolation. Moreover, the transmitter and the receiver can be separated, which improve the flexibility and modularity of the equaliser. When the mutual inductance is optimised, the core of the secondary side can be eliminated, further reducing the size and cost of the equaliser. The WPT-based charge equalisation was first proposed by Liu et al. [22]. This method realises a fast and safe conversion of electrical energy through non-contact balancing. However, using the LCL-S structure results in complexity, and CV charging mode needs an extra communication module to control the primary side which increases the complexity. Fu et al. [20] proposed multiple-receiver WPT-based battery cell equalisation system, the equalising current is supplied to the battery through the multi-receiving coil, and the equalising current is automatically adjusted by the reflected impedance of each battery to realise automatic balancing of the battery pack. However, for a long battery pack, the receiving coils interact with each other and produce mutual inductance, which leads the coil hard to design. The equaliser proposed in this paper is a combination of the WPT system and the switch array, benefiting from a direct balance of any battery cell in the pack by the switches array. The WPT system adopts the simplest series-series (SS) structure and the transmitting coil and the receiving coil can be separated. To have the load-independent output CC, the system parameters have optimised design. 2 Equaliser proposed In Fig. 1, the structure of the system shows the structure of the proposed equaliser. The system is divided into two parts: the first part is the WPT system which composed of the power supply, the inverter bridge, coils, and the rectifier bridge. The second part is composed of the switch array. The advantages of the proposed CC equaliser are as follows: (i) Energy can be directly transferred to any unbalanced batteries by switch array. (ii) The proposed circuit is a CC system; therefore, the balance current is independent of the voltage distribution of cells and the number of cells. (iii) The receiving coil and the transmitting coil are separately designed, and the transmitter is movable. If the cells do not need to be balanced all the time, different receivers are able to share the same transmitter. Fig. 1Open in figure viewerPowerPoint Structure of the system 3 Theoretical analysis 3.1 CC system analysis The circuit of the WPT system as shown in Fig. 2a is an SS compensation topology [21]. Primary inductor–capacitor circuit is powered by the AC power supply Uin. LP and CP are referred to the self-inductance of the transmitter coil and the transmitter coil compensation capacitance, respectively. LS and CS are the self-inductance and the compensation capacitance of the receiver coil, respectively. The mutual inductance between the first primary coil and the secondary coil is represented by M. Fig. 2Open in figure viewerPowerPoint Proposed battery equalization schemes (a) Equivalent circuit of the proposed WPT, (b) Simplified systemconfiguration ZP, ZS, and ZM are the impedances of the transmitter, the receiver, and the mutual inductance, respectively. Moreover, ω is the angular frequency. The impedance of each branch is shown in the equation below: (1)To simplify the analysis, set RP = 0 and RS = 0. As shown in Fig. 2b, according to Kirchhoff's voltage law, the WPT system can be described by (2)Uin and Ii are input voltage and current of the inverter, respectively; IO is the output current of the receiver; and R is the equivalent load impedance. After solving (2), the output current can be expressed by the following equation: (3)Then, the ratio of the input voltage to the output current is as follows: (4)To have the load-independent output current, it must eliminate the effect of load R change on the output current. Obviously, the load-independent output current can be achieved when ZP = ZS = −ZM. The values of CP and CS can be chosen by the following equation: (5) (6)When both sides of the inductance and the compensation capacitor are resonant, simultaneously (5) and (6) are brought into (3) to solve (7)Io is the output current. From (7), it is clear that the Io is related to frequency, coupling mutual inductance, and input voltage source. Therefore, when the input voltage source, system frequency, and coupling mutual inductance are determined, and the CC system can be achieved. With a voltage source as input, the CC output can be obtained which is independent of the impedance of the load. The output efficiency needs to consider the parasitic resistance. According to (1), (3), (5), and (6), the output voltage and output power of the system under a resistive load are expressed as (8)The input power of the system can be expressed as (9)From (8) and (9), the WPT system efficiency ηe considering the equivalent internal resistance of the coil and capacity can be obtained as (10) 3.2 Optimal load analysis As shown in Fig. 3, to find the optimal AC load resistance corresponding to the maximum efficiency of the system, derive (10) and let its derivative be 0 (11) Fig. 3Open in figure viewerPowerPoint Equivalent circuit of SS compensation topology The optimal AC load resistance can be solved by the equation below: (12)when the circuit resonates, the rectifier bridge input voltage is (13)where rectifier input voltage Urec and the output voltage is Uout, The DC output current filtered by the DC output filter Req can be obtained as follows: (14)where IS is the effective value of the secondary current. According to (13) and (14), the AC equivalent resistance Req of the rectifier bridge is obtained when the input voltage and current are as follows: (15)According to (15), the AC equivalent resistance of the rectifier bridge is only 8/π2 times of the actual load resistance. 3.3 Analysis of the efficiency of the system As shown in Fig. 4, Iout and Uout = are the output current and the voltage of the battery in balancing. Iin and Uin are the input voltage and input current of the CV source, respectively. Fig. 4Open in figure viewerPowerPoint WPT system with SS compensation topology The balance efficiency ηn can be shown by the equation as follows: (16) 4 Operation principles The switch array circuit structure of the system is shown in Fig. 5. The two switch arrays connect the respective battery cells connected in series with the + BUS and −BUS of the WPT system. By controlling switch on/off, it is flexible to achieve balanced switching of a single battery or a series battery. Only one switch can be turned on in a positive (negative) direction switch array to prevent shorting of the batteries. The output of the system is a CC. Therefore, secondary-side open-circuit results in high open-circuit voltage which may break down the receiving coil. Therefore, the load side must form pathways. The function of the S9 is to shorten the + BUS and −BUS, so the power of the system is shortly after the load is short circuited. When the target switch completes an operation, S9 switches off. Diode D1 in −BUS is to prevent current backflow during switch S9 switches on. Fig. 5Open in figure viewerPowerPoint Mode of switching network circuit To simplify the principle of the switch array, it is assumed that the cell voltage VB2>VB1>VB4>VB3. In the initial state, S9 is in the ON state, as shown in Fig. 6a: State I (t0–t1): At time t0, switches S5 and S8 are turned on and switches S1, S2, S4, S5, S6, and S7 are turned off. When the switching operation is completed, the switch S9 is turned off, and the WPT module is connected in series with the Battery2–Battery3 through the switches S5 and S8. As shown in Fig. 6b, the balanced path is constructed to achieve the CC balance of Battery3–Battery4. When the voltage of Battery4 reaches the threshold, the switch S9 is turned on. Switches S5 and S8 are off. State II (t1–t2): At time t1, switches S5 and S6 are on, and switches S1, S2, S3, S4, S7, and S8 are off. When the switching operation is completed, the switch S9 is turned off, and the WPT module is connected in series with the battery Battery3 through the switches S5 and S8. As shown in Fig. 6c, the balanced path is constructed to achieve CC balance Battery3. When the state of charge (SOC) of the Battery3 in the battery string reaches the threshold, the switch S9 is turned on. Switches S5 and S8 are off. State III (t2–t3): At time t2, the switches S1 and S2 are on and the switches S3, S4, S5, S6, S7, and S8 are off. When the switching operation is completed, the switch S9 is turned off, and the WPT module is connected in series with the Battery1 through the switches S7 and S8. As shown in Fig. 6d, the balanced path is constructed to achieve a CC balance of the Battery1. When the Battery1 SOC in the battery string reaches the threshold, the switch S9 is turned on. Switches S5 and S8 turn off, complete CC balance. Fig. 6Open in figure viewerPowerPoint Operating states of the proposed equalisation under the assumption of VB2>VB1>VB4>VB3 (a) Start, (b) State I, (c) State II, (d) State III 5 Implementation A proposed algorithm [23] can measure state of charge (SOC) of each cell, and then the total charge QAll that needs the power supply to provide is estimated as follows: (17)where n is the number of cells in the pack and Qmax is the highest-voltage cell charge. ηn is the energy efficiency transferred from the power supply side to the cell. Qk is the actual amount of charge of each cell. The flowchart of the cell balance algorithm is shown in Fig. 7. The charge equaliser algorithm consists of five procedures. (i) First, measure the SOC of the n batteries. (ii) Calculate the absolute value difference σ between the highest battery voltage and the lowest-voltage battery in the battery pack. If σ is less than the threshold, the balance operation does not work; otherwise, start the balancing operation. (iii) Qmax is used as the target charge for cell balance and position in the battery string. (iv) Divide the adjacent unbalanced battery to a module battery (i, j), i and j are the beginning cell number and the end cell number in the module, respectively. (v) Use the WPT system to charge the target module and stop balancing when a battery voltage reaches SOCmax in the battery module. (vi) Re-divide the battery in the module and perform a new round of equalisation. Until all battery SOCs of the module reaches the target state. Fig. 7Open in figure viewerPowerPoint Control flowchart of the proposed system Repeat 1–6 after completing the module balancing operation. 6 Experimental verification analysis Tables 1 and 2 show the voltage equaliser parameters based on the WPT system and polymer–lithium (Li) battery parameters, respectively. The voltage equaliser consists of a DC power supply, an inverter, a transmitter coil, a receiver coil, a rectifier bridge, and a switch array. Fig. 8 shows the CC equaliser platform. The voltage source input DC is converted into the high-frequency alternating current with a frequency of 85 kHz through a full-bridge inverter. Through the principle of electromagnetic coupling, the primary coil conducts energy to the secondary coil and achieves electrical isolation. The secondary rectifier bridge rectifies the high-frequency power into DC through the switch array for CC equalisation of the specific battery. Table 1. Dimensions and simulated values of the chosen inductive coupler Parameter Value Parameter Value Cp 62.23 nF Lp 56.34 μH Cs 100.04 nF Ls 35.02 μH M 14.07 μH K 0.3227 gap 18 mm f 85 kHz Table 2. Polymer–Li battery parameter Parameter Value capacity 1800 mAh maximum voltage 12.5 V minimum voltage 10.5 V maximum current 2.4 A Fig. 8Open in figure viewerPowerPoint Experimental prototype and associated instruments The air gap between the transmitter and receiver is 18 mm, and insulation is taken into account. The coil parameter design is designed according to the power of four batteries charged at 3 A and the power is 150 W. Both the transmitter coil and the receiver coil have a single-layer double-ring structure. Each D structure coil (coil diameter = 75 mm) has ten turns (0.04 mm/1500 strands in litz wire) and the PC40 ferrite core is placed under the transmitter coil. The battery cell voltage is sampled by the HIOKI voltage collector. In the balancing process, the voltage source is fixed to the input voltage, and the output current value at the load side is constant, so no feedback is required to control the magnitude of the equalisation current. Verify the performance of the WPT equaliser CC. Fig. 9 shows the inverter input voltage Uin and the input current Ii, the charging voltage UO, and the charging current Io in the CC mode when S3 and S8 are turned on. Fig. 9Open in figure viewerPowerPoint Experimental waveforms of Uin, Ii, Uo, and Io in CC mode Here, η1, η2, and η3 represent the balance efficiency of a single battery balance, two batteries balance, and three batteries balance, respectively. As shown in Fig. 10, when the Io increases from 0.1 to 1.25 A, η1 increases from 60.5 to 85.6%. When the Io increases from 1.25 to 2.5 A, η1 decreases from 85.6 to 81%. The measured peak efficiency is 85.6% at the Io of 1.25 A. When the Io increases from 0.1 to 2.5 A, η2 increases from 60.5 to 85.6%. When the Io increases from 0.1 to 2.5 A, η3 increases from 50.4 to 91.35%. However, when the output current is 1.6, the efficiency is close to 90%. These results show that the proposed equaliser can work with high efficiency over a wide range of loading conditions. To prove the improvement of the balancing efficiency of the proposed structure, the experience chosen to charge current is 1.7 A which η1 = 85.4%, η2 = 90.2%, and η3 = 89.6% of batteries and loads. Fig. 10Open in figure viewerPowerPoint Equilibrium efficiency of different numbers The charging curve is shown in Fig. 11a, in the beginning, VB1>VB2>VB3>VB4, as shown in Table 3, the switches S3 and S8 are turned on, and the remaining switches are in the off state, which is equivalent to the CC charging of the battery. When VB2 = VB1, switch S9 is turned on to make BUS + and BUS− shorted. Since the secondary side is in CC mode, the load cannot be disconnected, and each time the switch is switched, the bus must be shorted. Switches S3 and S8 are open. Turn on switches S5 and S8. The CC charging of the battery 3 and the battery 4 is achieved. In 4000 s, when the voltage drops to 0.02 V, the current balance is completed. The balance efficiency is 87.7%. Table 3. Initial voltages of battery cells Case VB1, V VB2, V VB3, V VB4, V 1 11.74 11.45 11.09 10.62 2 11.45 11.74 10.62 11.09 3 11.09 11.45 11.74 10.62 4 11.45 11.74 10.62 11.74 Fig. 11Open in figure viewerPowerPoint Experimental result of the simplified equaliser for four Li iron phosphate cells As shown in Fig. 11b, the battery voltage distribution VB2>VB1>VB4>VB3. The system first turns on the S6 and the S8 to charge the battery 4 and the battery 3. By turning on S9 after charging is completed, switching the switch to S1 and S2, the Battery1 is equalised to the target voltage. The balance time, in this case, is 4800 s and the efficiency is 87.2%. Case 3 and case 4 are shown in Figs. 11c–d; same as the previous equalisation logic, the efficiencies are 86.1 and 85.8%, respectively. Table 4 illustrates a comprehensive comparison of the proposed equaliser with other active equalisers. The comparison focuses on cost, size, and efficiency, whether simultaneous balancing, balancing cycles, energy flow, balancing speed, implementation feasibility, complexity, modularisation, switch voltage stress, and switch current stress. Each parameter is fuzzified into six fuzzy scales, e.g. '0' represents the worst performance and '5' represents the outstanding performance [7]. Table 4. Quantitative systematic comparison of the proposed battery equaliser with the conventional ones Balance performances P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 P11 FAVG SW L C T dissipative equaliser [4] N 0 0 0 5 1 5 1 5 3 5 5 3 5 1 3.54 SC [5] 2n 0 n−1 0 5 2 5 5 2 2 5 4 3 5 2 3.63 ZCS SC [9] 2n n−1 n−1 0 5 3 5 5 2 3 4 4 3 5 2 3.72 wave trap [24] 2 0 N n 2 2 5 1 3 1 1 2 1 1 1 1.82 forward conversion [11] N 0 0 1 3 5 5 5 5 4 3 1 5 5 4 4.09 flyback conversion [12] N 0 2n−2 1 3 3 5 1 3 2 2 3 3 2 4 2.81 WPT-voltage multiplier-based [20] 1 0 n + 4 1 3 3 5 5 4 4 4 5 4 5 5 4.27 proposed equaliser 2n + 1 0 2 1 3 4 5 5 5 5 3 4 5 5 5 4.45 Parameters of comparison. Components: SW (switches), L (inductors), C (capacitors), and T (transformers). P1: cost (1: expensive, 5: cheap). P2: efficiency (1: low, 5: high). P3: application (1: only allows low power, 5: allows high power). P4: energy flow path (1: less, 5: more). P5: speed (1: low, 5: high). P6: implementation possibility (1: difficult, 5: easy). P7: size (1: big, 5: small). P8: electrically isolated (1: worst, 5: best). P9: scalability (1: big, 5: small). P10: switch voltage stress (1: high, 5: low). P11: switch current stress (1: high, 5: low). P1 and P7: The balance circuit size and cost mainly depend on the number of components including metal–oxide–semiconductor field-effect transistors (MOSFETs) (M), resistors (R), inductors (L), capacitors (C), diodes (D), and transformers (T). Particularly, a quantitative cost comparison can be achieved by calculating the total price of each equaliser. P2: 'Balancing efficiency' is evaluated according to the average energy conversion efficiency for one switching cycle and the average switching cycles to transfer energy from the source cell to the target one. 0: balance efficiency is from 0 to 50%, 1: balance efficiency is from 50 to 60%, 2: balance efficiency is from 60 to 70%, 3: balance efficiency is from 70 to 80%, 4: balance efficiency is from 80 to 90%, and 5: balance efficiency is more than 90%. P3: 'Application' is evaluated according to the voltage and current level of the cell, if the circuit allows the higher the power, the circuit obtains high fuzzy scales [R9]. P4: 'Energy flow' is evaluated according to the balancing paths, e.g. 20%: adjacent cell-to-cell, 40%: direct cell-to-cell, 60%: pack-to-cell or cell-to-pack, 80%: cell-to-pack-to-cell, and 100%: any-cell-to-any-cell. P5: 'Balancing speed' is determined by the equalisation current, the number of cells involved in balancing at the same time, energy flow, and average switching cycles to complete the charge transportation from the source cell to the target one. P6: 'Implementation feasibility' is evaluated according to the practical application possibility of the equaliser for a long series-connected battery string in electric vehicles. P8: 'Electrically isolated' represents the electrical isolation representative is to electrically isolate the voltage source from the target battery electrical circuit. Equilibrium circuits with electrical isolation have higher security and ensure independent and safe operation of each DC. P9: 'Scalability' is evaluated according to the number of elements for the equalisation among modules and the expandability. P10–P11: 'Switch voltage stress' and 'switch current stress' represent the ratio of the voltage to withstand to the switch withstand voltage at both ends of the switch, and the ratio of the current flowing through the switch to the current resistance of the switch. The greater the stress that the switch needs to withstand, the shorter the service life of the switch. Therefore, the higher the voltage or current value, the lower the fuzzy scales. By averaging the performance of all aspects, the average score Savage is obtained in the last column of Table 4, and the average score can be relatively intuitively compared. It can be seen that the proposed equaliser has the highest average score, has the advantages of high speed, low-voltage stress, and good scalability. However, the parameter weight should be modified according to the actual application, which helps to select the appropriate balance topology according to different applications. 7 Conclusion In this paper, the battery charge equaliser based on the WPT system and switch array is proposed and studied through analysis and derivation. First, the working principle of the WPT system is analysed. The system adopts SS topology and the CC output is achieved by compensating coil inductance. The influence of system parameters on output current is studied. Then, discuss the control method of the switch array and realise multi-cell series equalisation and cell balancing through the switch. Moreover, research is done on the equalisation capability and efficient distribution of the CC equaliser to find the best equalisation current of the battery. 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