With the development of the electromagnetic launch technology, the energy storage system has become an important part of electromagnetic launch system. Taking the power and energy demand as the traction, and the volume weight fitness of the energy storage system as the optimal target, the demands of battery were studied. By comparing the existing kinds of batteries and taking safety, power density, energy density and cycle life into consideration, the lithium battery was selected as the candidate for electromagnetic launch system. Research on circuit topology to satisfy current and voltage demands from different equipment, the battery groups can realize serial-parallel conversion in this circuit topology. It is necessary to keep consistency between the battery groups for realizing the parallel connection of them. So the estimation of SOC (state of charge) was needed and the balance of battery groups was also needed. Aimed at solving the difficulty of SOC accuracy under the condition of short pulse, high rate and continuous discharging, the battery pack was taken as the research object and a dynamic identification method was employed to estimate the SOC of batteries. Unbalance happens as battery groups are connected in parallel, because there is discordance between battery groups, which will lead to limits for some groups. Therefore, a balance method is put forward to solve this problem.