A Modular Fast Cell-to-Cell Battery Voltage Equalizer

Voltage equalizers are used to ensure that there is no overcharge or overdischarge of cells in series-connected battery stacks. A novel fast multicell-to-multicell voltage equalizer topology is proposed in this article. The equalizer has a modular structure consisting of one half-bridge converter leg, one inductor, and one capacitor for each battery. Only battery voltage sensing is sufficient for controlling the equalizer. The proposed equalizer can transfer charge from multiple batteries to multiple batteries in a string simultaneously. The equalization current does not depend on the battery voltage differences and, hence does not reduce as the battery voltages approach each other during voltage equalization. A modularization method is proposed for this topology with two additional passive elements per module. This method provides a direct charge transfer path from any cell in one module to any cell in another module. Thus, the equalization time does not increase with the number of modules. The proposed topology is theoretically analyzed and experimentally verified with a prototype equalizer in a series-connected lead-acid battery string, a Li-ion battery string, and a hybrid ultracapacitor string. The proposed modularization method is verified experimentally with two modules tested together in a battery string.