Composite Active Equilibrium Method for Lithium Ion Battery Pack

The inconsistency of lithium-ion batteries can lead to the decrease of battery pack's capacity and lifetime, and can even seriously affect the electric vehicles' performance. A composite active equilibrium method, based on Cuk chopper circuit and flyback balancing circuit, is proposed in this paper. The key point of the proposed composite method is to group the battery into different battery subgroups and divide equalization process into two parts: intra-group equilibrium and inter-group equilibrium. The battery's state of charge (SOC) is taken as the equilibrium criterion to formulate the equilibrium strategy. We combine recursive least squares algorithm with forgetting factor and the extended Kalman filter algorithm to estimate the SOC of battery. The equilibrium model is built through Matlab/Simulink. The simulation results show that the composite active equilibrium method can decrease the balancing time by 47.35% and increase the equilibrium effect by 55.32% compared with the conventional cell to cell equilibrium method, and the method can effectively improve the battery pack's available capacity in charging or discharging process.