This paper focuses on the loss analysis and optimized design of DC-DC converter for battery module. By using the data measured from the experiment study of inductor loss and component datasheets to derive the inductor, MOSFET, and capacitor loss equations of bidirectional buck converter, boost converter, and buck-boost converter through numerical analysis. In addition, the charging and discharging characteristics of the lithium-ion phosphate (LiFePO4) battery are used to get the one cycle charging and discharging efficiency. Finally, the characteristics and cost of battery module in different battery series/parallel conditions with bidirectional buck converter, boost converter, and buck-boost converter are analyzed by the proposed method. There are 60 LiFePO4 batteries cell (3.3 V/1.0 Ah) used as the battery bank, the bus voltage is 48 V, and maximum output power is 200 W. The cycle efficiency of buck converter and boost converter are above 96%, and buck-boost converter is 87.6%.