Modularized charge equalization converter with high power density and low voltage stress for HEV lithium-ion battery string

This paper proposes the modularized charge equalization converter for hybrid electric vehicle (HEV) lithium-ion battery cells, in which the intra-module and the inter-module equalizer are implemented. Considering the high voltage HEV battery pack, approximately above 300 V, the proposed equalization circuit modularizes the entire M*N cells; in other words, M modules in the string and N cells in each module. With this modularization, low voltage stresses of all the electronic devices, roughly below 64 V, can be obtained. In the intra-module equalization, the current-fed type DC/DC converter with cell selection switches is employed for concentrated charging of the specific under charged cells. On the other hand, the inter-module equalizer makes use of the voltage-fed type DC/DC converter for bi-directional equalization. In the proposed circuit, high power density can be achieved by employing the optimal power rating design rule. In addition, small size and low cost can be accomplished by sharing the MOSFET switch by the intra-module and inter-module equalizer and employing no additional reset circuitry at the inter-module equalizer. Experimental results of an implemented prototype show that the proposed equalization scheme has the promising cell balancing performance for the 7Ah HEV lithium-ion battery string while maintaining low voltage stress, low cost, small size, and high power density.