New On-Line Approach for Lithium Iron Phosphate Battery Pack Balancing Based on State of Charge

Lithium iron phosphate batteries (LiFePO4) are becoming one of the main power resources for electric vehicles (EVs), and the non-uniformity of cells in a battery pack has become the bottleneck to improve battery usable capacity. Many active balancing approaches are proposed to transfer charge among the cells to achieve the uniformity based on terminal voltage. This paper proposes a new balancing approach based on the battery state of charge (SOC) to equalize the cells in the LiFePO4 battery pack in charging process. The hybrid extended Kalman filter is adopted to dynamically estimate the cell SOCs which serve as a balancing criterion. A current sensor is added to detect balancing current for the SOC estimation. Simulations are carried out to compare the proposed approach with terminal voltage approaches and show that the proposed approach has charged more capacity into the pack in charging process.