Battery energy storage is a well-known concept that has applications within the electricity supply industry. In a battery energy storage system (BESS), the d.c. energy in the batteries is exchanged with the d.c. utility using a power conversion system (PCS). The development of a generic low-cost PCS is identified as being critical to commercial viability of battery energy storage. The paper presents commercial and functional advantages of using a modular approach to power conversion in a BESS. Dynamic models are developed for both single and networked power converters. These are used to demonstrate the technical feasibility of the modular approach. The Wavedriver PCS is presented. This uses a modular network of generic power converters to provide higher functionality. Each power converter incorporates a higher level of systems integration than is usually. present in a traditional PCS. This approach changes the balance on the commercial viability of energy storage systems.