A Modularized Two-Stage Active Cell Balancing Topology with Reduced Balancing Time for Series Connected Li-Ion Battery String

This paper introduces a modularized two-stage active cell balancing topology utilizing an improved buck-boost converter for a series-connected lithium-ion battery string. The proposed topology adopts a modular structure where each module comprises three cells, two inductors, and four MOSFET switches. The voltage monitoring circuit controls the switches to ensure each cell has same voltage by transferring charge from a source cell to target cell. This approach enables module-to-module balancing through a module equalizer while simultaneously targeting two cells within a module through a cell equalizer. Using modularization technique in the proposed topology, the balancing time is reduced significantly compared to cell equalization circuit. Moreover, using a combination of cell and module balancing, the balancing time is reduced effectively compared with performing cell balancing only under dynamic charging/discharging conditions. This methodology substantially reduces cell equalization time and enhances system performance with minimal components. Proposed topology is verified theoretically and experimentally with a five-module battery string under static and dynamic conditions.