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This paper presents a simple circuit for balancing series-connected battery cells. The circuit is composed of one low-voltage metal-oxide-semiconductor field-effect transistor (MOSFET) for each cell and a symmetrical multiwinding transformer for a group of cells. Only one control signal is needed for all MOSFETs, and energy can be directly transferred from higher voltage cells to lower voltage cells. A small capacitor is added to form a resonant circuit with the transformer magnetizing inductance so that soft switching can be achieved in some operation range to obtain high efficiency. The circuit balance function and soft-switching condition are analyzed and verified by simulation and experiments. A circuit for balancing a 4- and 12-cell battery group is tested. The experimental results showed the effectiveness of the circuit. The energy transfer efficiency between cells can reach up to 93%. The circuit can be easily scaled to battery strings containing up to 12 battery cells, directly applicable to consumer electronics application where the number of cells is typically less than 12. A battery pack containing a large number of battery cells, such as the ones used in electric vehicles, can be balanced in groups, for example, in modules containing 12 cells each and with one additional circuit to balance among the modules.