Pack-to-Multicell Equalization of Lithium Battery String Based on Series-Parallel Topology of Switched-Capacitor Circuit

Abstract

This paper proposes a novel pack-to-multicell topology to equalize the voltage distribution of a series lithium battery pack. Switched-capacitor converters are implemented in a series-parallel configuration to simultaneously allow energy transfer from the entire pack to every cell. Those switched capacitors act as intermediary energy storage on equalization paths between each cell. The series-parallel term refers to two equalization phases of the switched-capacitor. At the series-connected phase, each capacitor is connected to its corresponding cell, so the cell’s voltage and capacitor’s voltage would be equalized. Subsequently, in the parallel-connected phase, all capacitors are connected so that they are set to the desired voltage value regulated by a DC/DC converter. The experimental setup of the proposed system has been tested on four series-connected 3000 mAh lithium cells, which have a large spread of initial voltages (2.69-3.92 volt). The experimental results show convergence of the cell’s voltages without over-equalization and a charge transfer efficiency of about 87%.