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This paper discusses a Dual Active Bridge (DAB) DC-DC converter utilizing to three-level flying capacitor topology for a battery conversion system. With applying to a multi-level topology, the voltage stress is reduced in all switches. In addition, the low order harmonic components are reduced due to five-level waveform. In a two-level DAB converter, the efficiency becomes low when the input voltage is not closed to the nominal voltage. In order to improve the efficiency at the input voltage fluctuation, this paper also proposes the operation mode of the three-level DAB converter at the input voltage fluctuation. The output voltage waveform of the inverter is determined by the relationship among the efficiency of the two modes which are so called the full-bridge mode and the half-bridge mode. In this paper, the changing point between the half-bridge and the full-bridge mode is derived using the loss calculation. In the simulation, the ZVS range and the inductor current are compared with the two-level and the three-level inverter. It is confirmed that the ZVS range is extended by 60%. In addition, the fundamental operation of the proposed DAB converter is confirmed by the experimental results.I.