Balancing Method of Retired Battery Pack Based on Variable Domain Fuzzy Control

Abstract

For the problem of the performance gap between individual cells in retired lithium batteries after group use, which affects the usable capacity of the battery pack, a grouping bidirectional equalization method based on variable domain fuzzy control is proposed. Equalization circuits based on a single inductor and LC oscillation circuit are respectively used for inter-cell and inter-cell group, to achieve inter-cell equalization and inter-cell group equalization. Variable domain fuzzy control strategy is used to determine the reasonable range of operating current according to state of health (SOH) of the battery, combined with the relationship between the capacity decay coefficient, and the average operational range of state of charge (SOC); the equalization current is dynamically adjusted according to its mathematical relationship with the operating current. To verify the effectiveness of this equalization method, an experimental platform was built and verification simulations were performed. The result of experiments shows, the equalization speed is increased by 25%, compared to the fixed equalization current control strategy; the capacity decay is reduced by 6% and the service life is extended after experiments of 1200 charge/discharge cycles, compared to traditional fuzzy equalization strategy.