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This paper describes the design and operation of a 6-kW, full-bridge, zero-voltage switching, bidirectional isolated dc-dc converter for a 2-kWh lithium-ion (Li-ion) bank that is considered attractive as an energy storage system for applications to photovoltaic generation systems ranging from 10 to 30 kW. The dc voltage at the high-voltage side of the converter is controlled from 305 to 355 V as the battery voltage at the low-voltage side varies from 50 to 59 V. The galvanic isolation using a 20kHz transformer with a turns ratio of 6:1 provides flexibility in voltage matching and safety. This paper presents experimental verifications of bidirectional power flow into (battery charging), or out of (battery discharging) the Li-ion battery bank. The maximal efficiency of the dc-dc converter is measured to be 96.1% during battery charging, and 96.9% during battery discharging.