Research on Double Closed Loop Equalization Control Strategy of Lithium-ion Battery in Electric Vehicle Based on Multivariable Coordination

In trying to address the discrepancy between single batteries in series connection of power li-ion battery packs for electric vehicles, a multivariable coordinated equalization control scheme is proposed according to the OCV -SOC curve, which is characterized by “steep at both ends and gentle in the middle”. When the State of Charge (SOC) is no less than 20% and no more than 90%, SOC is nominated as a variable in the course of equilibrium; When the SOC is less than 20% or more than 90%, voltage is selected as a variable in the equalization process. Therefore, on the basis of the cascaded cuk equalization circuit, a double-closed-loop equalization control strategy based on multivariable coordination is designed. The outer loop adopts variable theory division fuzzy control, and the inner loop adopts fuzzy PID control. The balance current is dynamically adjusted by modifying the duty ratio of MOSFET in the balance circuit. The analog results revealed that the equalization strategy can accurately control the equalization current under three different conditions of resting, charging and discharging, to achieve effective equalization of the battery group and the desired balancing effect.