The wide application of energy storage battery makes it more and more demanding for charging equipment. The charger suitable for battery pack with different scale will usher in new challenges. Taking into account the charging requirements of different types and sizes of battery packs, the advantages and disadvantages of the existing charging circuits are summarized, and a design idea of a modular two-stage charger suitable for different scales and series-parallel applications is proposed. Space average method modeling and analysis, the charger based on two-stage series Buck topology is designed, the simulation is built by PSCAD and the prototype is made, and the theoretical simulation and experimental verification are carried out. The experimental results show that the charger output voltage ripple rate is small, the output voltage accuracy is high, the stability of the input power requirements are low, the performance requirements of the device is low.