Automatic charge equalization circuit based on regulated voltage source for lithium-ion batteries

Abstract

Nowadays, electric vehicle (EV) or hybrid electric vehicle (HEV) usually adopts a lithium-ion battery string as a power source with high voltage. In the lithium-ion battery string, the charge imbalance of each battery makes many problems such as reduction of life time and explosion. Hence, the charge equalization circuit is required to guarantee life time and safety. In charge equalization circuit, the cost of a cell voltage sensing module is relatively high, but the equalization performance gets lower in case the sensing module is removed. For higher equalization performance without the cell voltage sensing module, a new automatic charge equalizer based on regulated voltage source is proposed. One regulated voltage source with average voltage of batteries is implemented from a bidirectional DC/DC converter. And switching block exists for connection between battery cells and regulated voltage source. The charge equalization can be achieved by a periodic connection between battery cells and regulated voltage source without the cell voltage sensing module. The equalization current of the proposed equalizer can be increased by reference voltage modulation and the equalization time of the proposed equalizer can be reduced by skips of unnecessary operations. In this thesis, the theoretical analysis and design considerations of the proposed equalizer are presented. And, to confirm the validity of the proposed scheme, experimental results with 7Ah batteries are demonstrated.