Individual cell equalizer using active-clamp flyback converter for li-ion battery strings in an electric vehicle

The necessity of a charge equalizer is increasing for series-connected lithium-ion cells in electric vehicles. It is used to enhance the battery's safety and life cycle. For these reasons, the cell voltage levels should be individually monitored and effectively balanced in the battery string. However, individual cell balancing has the problem of size as well as the control complexity for a large number of battery cells. To alleviate these drawbacks, this paper proposes an individual cell equalizer with simple structure. In the proposed equalizer, each cell has its own active-clamp flyback DC-DC converter. A main switch and clamp switch in the converter are shared by every separated transformer through parallel-primary connection. This converter is utilized as a selectable current source by using selection switches in the primary winding of transformer. With this configuration, the simple structure and easy control can be effectively achieved for each cell even if the proposed equalizer has separated transformers as for the counts of battery cells. Furthermore, high equalization efficiency and outstanding equalization performance can be obtained for a high capacity battery. In this paper, a prototype for eight lithium-ion battery cells is optimally designed and implemented. Experimental results verify that the proposed equalization method has a good cell balanced performance.