Synthesis of Balancing Topologies for Parallel-connected Battery Cells by Principle of Duality

In electric vehicle and battery energy storage system, battery cells are connected in parallel to prolong the working time. However, the current of each battery cell is unequal due to the imbalance of battery impedance and varied state-of-charge (SOC) rate. As a result, problems with reliability and safety may occur and battery capacity is not fully utilized. To eliminate the imbalance issue, this paper proposes two balancing methods which are transformed from series balancing techniques by the principle of duality. They use the switching capacitor circuit in ring and star structure to transfer energy from a high SOC cell to a low one. To verify the proposed method, simulations for 4 parallel-connected 18650 battery cells (3.7V/2.6Ah) have been implemented in Matlab/Simulink.