Research on Two-level Equalization Strategy of Lithium-ion Battery Based on Graph Theory

To solve the problem of inconsistency in the use of series-connected lithium-ion battery packs, this paper proposed a topological structure of dual-layer equalization based on a flying capacitor circuit and Cuk circuit, as well as a control strategy seeking the shortest equalization path. In this structure, batteries are divided into two forms: intra-group and inter-group; the intra-group equalization is the lower-level equalization while the flying capacitor circuit is used as an equalization circuit to achieve equalization between individual battery cells; the inter-group equalization is the upper-level equalization while Cuk circuit is used as equalization circuit to achieve equalization between battery packs; each battery pack shares a battery cell, thus to obtain more options on equalization path. The proposed strategy, with State of Charge as the balancing variable, represents topological structure of the circuit in form of graph by adopting graph theory control, seeks the optimal equalization path via ant colony optimization algorithm with global search, thus to improve the equalization speed and efficiency. At last, the structure and the strategy proposed in this paper were simulated in MATLAB/Simulink to compare with the maximum value equalization method in the condition of static, charging, and discharging. The result of the simulation experiments shows that the equalization method based on graph theory control reduces the equalization duration by approximately 17%, and improves the equalization efficiency by approximately 2%, which verifies the superiority and effectiveness of the structure and strategy proposed in this paper.