Abstract: In order to weaken imbalance of each single cell in battery units and improve battery life and efficiency, an active balancing control system of mine-used Li-ion battery units was designed. The system detects voltage of each single cell in battery units real-timely, when the maximum differential voltage of each single cell exceeds threshold value, the balancing circuit starts to trickle charge the battery with the lowest cell voltage. The experiment results show that the system can make the maximum differential voltage of each single cell within 30 mV after battery units is charged, which improves efficiency of battery units.