Direct Cell-to-Cell Equalizer for Series Battery String Using Switch-Matrix Single-Capacitor Equalizer and Optimal Pairing Algorithm

Abstract

In series battery strings, cell-inconsistency is caused by the state-of-charge (SOC) mismatch, nonidentical battery impedance, or different self-discharging rates, and this leads to overcharge and overdischarge. Practically, switched-capacitor equalizers are the most promising means to eliminate the cell inconsistency by virtue of automatic equalization, but the performance is heavily dependent on the initial cell voltage distribution and the number of series connections due to inefficient switch utilization. This article proposes a direct cell-to-cell equalizer for a series-connected battery using a switch-matrix single-capacitor converter to further improve the switched-capacitor equalizer in term of performance consistency and Coulomb efficiency. By adopting one extra current sensor and an optimal pairing algorithm, energy is transferred directly between the highest-SOC cell and the lowest-SOC cell to eliminate the impact of battery-impedance difference and false voltage measurement in battery monitoring integrated circuit caused by polarization effect. The experimental results verify the feasibility of the proposed scheme. Real-time tests are also implemented to fairly compare the proposed with the conventional methods. It is found that the performance of the proposed method is independent from initial voltage distribution in a series string, where the performance indices are consistent regardless of initial conditions. Besides, the energy loss of the proposed equalizer is further reduced and its overall efficiency is high in all test scenarios.