An improved equalization technique for fast charging of Electric vehicles

Abstract

This paper describes a lithium-ion battery (LIB) hybrid equalization (HE) structure. There are two primary strategies for voltage equalization (EQ): passive and active. Active equalization (AE) moves energy between the LIBs, whereas passive equalization (PE) wastes energy. PE strategy has the benefit of being easy to put into action, but at the same time, it requires more time to equalize and results in a loss of energy. Although it is difficult to implement, AE has a shorter EQ time and less energy loss. Combining these two methods simultaneously controls a pack of LIBs and equalizes voltage. This article offers recommendations on a cell level (CL) and between stacks. Some LIBs were outfitted with varying state of charge (SOC) levels to simulate a cell imbalance. Different topologies were tested, including combined AE and PE at the pack level and PE and AE topologies at the CL. Response times and SOC levels are used to compare the results. Compared to strictly AE or PE strategies, the HE showed encouraging results regarding EQ time and autonomy. This combination offers a solution that enables one to achieve minimal HE time while maintaining appropriate SOC.