Abstract: This paper proposed an efficient and fast active equalization circuit and its control strategy for echelon use of battery pack. The unbalanced battery energy in the retired battery pack was transferred to the energy transfer unit composed of super capacitors through the four-switch bidirectional converter, and then the energy in the super capacitor was distributed by the converter to the battery cells with lower energy in the battery pack to achieve the balance of battery packs connected in series. The main switches of the four-switch bidirectional converter worked in the synchronous rectification mode. While realizing the bidirectional flow of energy, the converter itself had a high energy conversion efficiency. The K-Means clustering algorithm was used to analyze the retired battery. The non-adjacent single cells in the group were firstly balanced, and then the adjacent "battery clusters" within the battery group were balanced, so as to realize the efficient and fast balancing of the battery packs in echelon use. In order to verify the effectiveness of the proposed equalization method, software simulation was carried out in MATLAB/Simulink software, and an experimental platform was built to perform equalization experiments on 6 series battery cells. Experimental results show that, compared with the traditional battery balancing method, this method has shorter balancing time and higher balancing efficiency.