In this paper, a two-stage equalization topology based on Zero-Voltage Switching Quasi-Resonant Converter (ZVS QRC) is proposed to solve the problems of long equalization time and low energy utilization of series lithium-ion battery packs. The equalization topology is divided into two forms: intra-group and inter-group, ZVS QRCs are adopted in both intra-group and inter-group, which can equalize any single cell within a group, and the equalization circuit within each pack can equalize simultaneously, which greatly reduces the equalization time, and reduces the switching loss during the equalization process by introducing quasi-resonant circuit to improve the energy utilization. Taking the State of Charge (SOC) as the equalization target, a group equalization strategy based on the Credibility Factor (C-F) inference model is proposed. The strategy defines the SOC unbalance degree of a single battery, gets the group unbalance degree according to the C-F inference model, and gets the group equalization state from the unbalance degree, to control the group for equalization. Matlab/Simulink software is used to build the model and simulate it. The experimental results demonstrate that the proposed topology saves approximately 25% of the equalization time compared to the traditional Buck-Boost group equalization topology. Under the same static and charge/discharge conditions, the equalization time is reduced by about 21% and the energy utilization is improved by about 6% after using the C-F inference model for equalization compared to the fixed-threshold method.

In this paper, a grouping equalization circuit based on the Single Ended Primary Inductor Converter (SEPIC) circuit is proposed, which can transfer energy between any single cell or grouped cells. Compared with the traditional equalization circuits that transfer energy between adjacent cells, the SEPIC circuit can directly connect any two batteries that need to be equalized; the number of circuit equalization paths is calculated based on a directed graph, then used as a basis for grouping the batteries to improve the equalization efficiency. In the charging or discharging condition, the amount of charge remaining in the battery to be charged or discharged is used as the control variable for equalization, and intra-group equalization is completed before inter-group equalization starts. To ensure the equalization efficiency of the battery, the equalization current is controlled by fuzzy logic control (FLC). Taking 10 single cells as an example based on the calculation of the number of equalization paths, two 5-cell groups can be confirmed as the optimal solution. Experiments were performed on Matlab/Simulink simulation platform, and the results show that compared with the traditional adjacent inductance equalization circuit, the equalization circuit proposed above reduces the time needed for equalization by 35.8%; Compared with the traditional average difference method, in charging and discharging conditions, the FLC algorithm saves times by 20.5% and 31.3% respectively, and energy loss is reduced by 9.1% and 5.5% respectively, which verifies the feasibility of the proposed equalization scheme.