I. Introduction
Lithium-ion battery cells are usually connected in series to increase the pack terminal voltage. Because of manufacturing and environment variance, degradation with aging, imbalance between charge and discharge, and differences in thermal conditions, internal impedances and the self-charging rate, the energy (or charge) stored in battery cells is not equal. The imbalance between different battery cells in a pack reduces the lifetime, efficiency, and capacity of the system and also increases the chance of an explosion or fire. Cell balancing circuits (equalizers) are thus developed to correct the imbalance between battery cells and thus improve overall performance [1]–[3]. Most of cell equalization methods rely on cell voltage as the indication for balancing control [4]–[10].