Optimal Active State-of-Charge Balancing with Temperature Control for Lithium-Ion Battery Cells Considering the Aging Effect

Abstract

This article presents an active State-of-Charge (SoC) balancing control method that considers the aging effect on the Lithium-Ion battery cells in a series connection implemented by a bidirectional CuK converter circuit. The purpose of balancing the SoC process for cells in the series ensures that, in all operating conditions, the current and temperature of the cells do not exceed the technical limits corresponding to the degree of aging of the cells. The nonlinear optimal control problem is established based on constraints on the balancing current, temperature, and the effect of cell aging in the dynamic model of the SoC balancing system. The sequential quadratic programming (SQP) method is used to solve the optimization problem at sampling time to determine the optimal duty cycle of the pulse width modulation (PWM) pulse to be applied to the balancing circuits. The SoC balancing results for cells are compared between the case where all cells in the series are new and the case where cells in the series have different levels of aging, showing differences in the control of balancing current and cell temperature in the SoC balancing process. The application of the optimal active SoC balancing method for cells in the series proposed in this article allows the cells in the series to operate safely, extend the life of the battery cells, reduce costs for energy storage applications aimed at popularizing the use of renewable energy sources and promoting the development of the electric vehicle industry, accelerating the implementation of global carbon reduction and environmental protection.