Double‐Switch Series‐Resonant Cell‐Voltage Equalizer Using a Voltage Multiplier for Series‐Connected Energy Storage Cells

SUMMARY 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 multiwinding transformer in proportion to the number of series connections, which usually makes them complex, expensive, bulky, and less extendable with increasing series connections. A double-switch series-resonant equalizer using a voltage multiplier is proposed in this paper. The double-switch operation without a multiwinding transformer achieves simplified circuitry and good modularity at reduced size and cost, compared to conventional equalizers. Operational 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 equivalent circuits. The simulation results of the derived circuit correlated well 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 and an experimental equalization was performed for series-connected EDLCs from an initially voltage-imbalanced condition. The voltage imbalance was gradually eliminated over time, and the standard deviation in the cell voltages decreased to approximately 5 mV at the end of the experiment, thus demonstrating the equalization performance of the proposed equalizer.