The energy that is generated from renewable energy systems is usually stored in high-energy lithium-ion battery storage systems so that it is optimal to use high voltage batteries to decrease the current, and then eventually to have minimum power loss. Therefore, in this paper, 200V battery management system (BMS) hardware and software design are discussed. This paper proposes a 3-level flying capacitor DC-DC converter in the hardware design and SoC estimation algorithm in the software design by using simulation and test results. In this paper, the contribution is in the estimation method and in the DC-DC converter.