A Multi-Cell-to-Multi-Cell Equalizer for Series-Connected Batteries Based on Flyback Conversion

This paper proposes a multi-cell-to-multi-cell (MC2MC) equalizer for series-connected lithium-ion battery strings based on flyback conversion and its control method. An onboard 12 V lead-acid battery is used as the energy transfer station to reduce a half of switches. Different from the traditional equalizer, this equalizer can directly transfer energy from the higher-voltage cells (i.e., the optimal discharging combination) to the lead-acid battery and deliver energy from the lead-acid battery to the lower-voltage cells(i.e., the optimal charging combination), which greatly improves the equalization speed. The pulse-width modulation (PWM) duty cycle can be adjusted according to the voltage of the optimal charging or discharging combination during equalization, so that the balancing current and speed will not decrease with the decrease of the voltage difference among cells. Moreover, the demagnetization for the transformer can be realized in the discontinuous mode of the bidirectional flyback converter, thereby improving the stability of the equalization system. The simulation results show that with the proposed equalizer, the cell voltage difference can be decreased from 540 mV to 2 mV within 1500s, leading to a fast balancing speed.