Control of a series-input, parallel-output cell balancing system for electric vehicle battery packs

Cell balancing circuits are required in battery packs to equalize the state-of-charge (SOC) of series-connected cells. This paper presents a control strategy for a modular cell balancing architecture based on low-voltage bypass dc-dc converters that perform real-time active cell balancing using a shared low-voltage (LV) bus. Each bypass converter includes an autonomous controller, which employs a faster loop with a standard PI compensator to regulate the LV bus voltage and a slower loop with droop control to regulate SOC of each cell. The control strategy results in balanced LV load current sharing among bypass converters. Design and stability analysis details are provided for the cell balancing system and bypass converters. The control approach is verified by experimental results for a three series-connected Li-ion NMC battery cell system with three digitally controlled dual-active bridge bypass converters.