A new technique based on buck-boost topology is proposed to equalize a series-connected battery stack in this paper. The proposed scheme transfers the energy from fully charged battery cell to the weakest charged battery using buck-boost operation. This operation maintains batteries at the same charge and voltage level. Unlike previous battery equalizing schemes, the new battery equalizer uses only one magnetic component, resulting low cost and small size. Experiment results are provided to verify the operation of the proposed battery equalizer.