Hybrid series–parallel structure provides an effective mean for large-scale energy storage system (ESS) integrating low voltage level energy storage units (ESUs). In ESS, the SOC balancing control plays an essential role. In this chapter, a local-distributed and global-decentralized SOC balancing control strategy is introduced for hybrid series–parallel ESS. Under the proposed scheme, the SOC of ESUs in each local string converge to the same value driven in a distributed manner and the SOC balance among different strings and power sharing of the ESS under both the charging and discharging modes are realized in a decentralized manner. Because there are no communication links between dispersed ESU strings, the scalability of the ESS is improved. Small-signal stability analysis is carried out, which provides a guide for parameter design. Simulation results verify the effectiveness and correctness of the proposed method.