Series connections of energy storage cells, such as lithium-ion cells and electric double-layer capacitors (EDLCs), require cell voltage equalizers to ensure years of operation. Conventional equalizers require multiple switches, magnetic components, and/or secondary windings of a multi-winding transformer in proportion to the number of series connections, and therefore, they are usually complex, expensive, bulky, and less extendable as the number of series connections increases.
A double-switch series-resonant equalizer using a voltage multiplier is proposed in this paper. The double-switch operation without a multi-winding transformer achieves simplified circuitry and good modularity at reduced size and cost, compared to conventional equalizers. Operation analyses were separately performed for the following two functional parts of the proposed equalizer: a series-resonant inverter and a voltage multiplier. The mathematical analyses derived a dc equivalent circuit of the proposed equalizer, with which simulation analyses of even an hour's duration can be completed in an instant. Simulation analyses were separately performed for both the original and the equivalent circuits. The simulation results of the derived circuit were in good agreement with those of the original circuit, thus verifying the derived dc equivalent circuit. A 5-W prototype of the proposed equalizer was built for eight cells connected in series. An experimental equalization was performed for series-connected EDLCs from an initially voltage-imbalanced condition. Voltage imbalance was gradually eliminated in course of time, and the standard deviation of the cell voltages decreased to approximately 5 mV at the end of the experiment, thus demonstrating the equalization performance of the proposed equalizer.