Design of Two-Stage Charge Equalizer with Modularized Cell Selection Switches for Series-Connected Lithium-Ion Battery

The equalizer is required to reduce the voltage imbalance of series-connected lithium-ion batteries to achieve efficient and safe operations. Repeated charging and discharging energy to the lithium-ion battery can cause a charge imbalance between the battery cells, as each battery cell has an unavoidable difference in chemical and electrical characteristics. An imbalance of the battery cells will reduce the total storage capacity and decrease the lifetime. This paper proposed a two-stage charge equalizer with modularized cell selection switches for voltage imbalance detection. Two-stage flyback converters were used along with three battery cells. The cell selection switches were used to avoid overcharging or undercharging in one of the cells during the charging process. In addition, the two-stage charge equalizer method operated properly since modulation of the cell selection switch of the lithium-ion battery charging process arranged in series has worked safely and made the battery life longer. The battery charge equalizer implementation worked well with a voltage difference of 10 mV for each battery cell. The duration of the equalization process depends on the voltage difference between the tested battery cells. Furthermore, the charging equalization can be optimally improved according to the experimental results.