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This paper describes a 6.6-kV battery energy storage system based on a multilevel cascade PWM (pulsewidth- modulation) converter with star configuration. It discusses design concepts with and without a linefrequency transformer for grid connection. The control system consists of state-of-charge (SOC)-balancing control and fault-tolerant control. The former is indispensable for effective utilization of battery energy while the latter is required for maintaining continuous operation during a converter-cell or battery-unit fault. A 200-V, 10-kW, 3.6- kWh laboratory system combining a three-phase cascade PWM converter with nine nickel-metal-hydride (NiMH) battery units is designed, constructed, and tested to verify the validity and effectiveness of the proposed control system.