A Single-Capacitor Equalizer Using Optimal Pairing Algorithm for Series-Connected Battery Cells

In a series string, the behaviors of batteries are different due to mismatch of impedance, initial state-of-charge, and self-discharging rate. Among various equalization methods, the switched-capacitor equalizer seems to be the most promising method due to automatic equalization, low-cost, and small size. However, the equalization speed becomes slower and the power loss gets higher when the number of series cell increases. This paper proposes an equalizer that uses only one capacitor and a fishbone-shaped switch-matrix to transfer energy between any cells by an optimal pairing algorithm that makes the switching decision. By adopting one extra current sensor, the equalization speed increases while the power loss is reduced. For verification, simulations for six series-connected 18650 Li-ion battery cells have been implemented, which shows that the state-of-charge of all cells are equalized within 4% difference with high speed and the total power loss in the equalizer is reduced to 52% of conventional methods.