As unbalance state of charge (SOC) of storage units usually leads to the decrease of lifetime, SOC balancing control is essential. In this chapter, a decentralized SOC balancing method is introduced to balance the SOC of series-type energy storage system (CESS). Since the method does not rely on any communication, it possesses higher reliability. As is well known, SOC is a slowly changing variable compared to other variables such as voltage and current. Thus, the studied system has obvious two-time scale characteristic. In addition, the stability analysis of the system based on the singular perturbation theory is carried out. Simulation and experiments are performed to verify the validity of the SOC balancing scheme.