Equalisation strategy for serially connected LiFePO 4 battery cells

Owing to different temperature gradients, charge/discharge rates, and coulomb efficiencies, various releasable capacities of each cell have significant impacts on a pack capacity. Therefore, the equalisation system is an essential part in a battery pack, especially for a high‐power application such as electric vehicle. In this study, the authors first review several traditional equalisation strategies. Subsequently, a new equalisation strategy (ES) based on the real ‘state’ of a battery is proposed. They further establish the destination state‐of‐charge versus open‐circuit voltage (SOC–OCV) cluster which is the target of the ES. Moreover, not only each SOC in serially connected battery cells, but also the SOC error according to the destination SOC–OCV cluster can be estimated. To avoid over‐equalisation, the SOC error caused by the dispersion of OCVs is taken into account during the calculation of equalisation SOCs. A battery pack with five lithium iron phosphate cells in series is employed to verify the method, and results show that the capacity of battery pack is increased by 11% with equalisation.