A novel two-steps method for estimation of the capacity imbalance among in-pack cells

The battery management system (BMS) embedded with equalization function cannot ensure the capacity balance for the lithium ion battery system due to its limited equalization effect under the dynamic operating condition on electric vehicles. Vehicles loaded with the lithium iron phosphate (LFP) are always operated with capacity imbalance even without any alarm, because only voltage difference among in-pack cells is used as the indicator of the state of disequilibrium (SOD) but the plateau of LFP is too flat. Capacity of each single cell cannot be fully utilized in this case. A simple algorithm for SOD estimation of the LFP battery system is proposed in this paper, adopting a voltage compensation method. The data of the maximum and minimum cell voltage of the battery system is used to conduct a rough estimation, and it turns out that a value no larger than the true one is obtained. The data of each single cell voltage is used to conduct the precise estimation, and the error is confirmed to be lower than 1% for battery pack with true SOD within 5%, which is validated on fresh cell, cycled cell and virtual cell. Accuracy and simplification are ensured by combining the rough and precise two-steps estimation. It provides a novel alarm function against the capacity imbalance for electric vehicles, and can be easily realized in engineering application.