In this article, an interleaved soft-switching bidirectional dc-dc converter (BDC) with high voltage conversion ratio (VCR) and low voltage stress is proposed for battery charging/discharging applications. A built-in transformer (BT) is integrated into conventional half-bridge BDC and three-level structure to achieve high VCR and reduced voltage stresses simultaneously. The feature of BT is that the balanced magnetic flux exists in the magnetic core, which allows for small size of the magnetic core. The current ripple in the battery side is reduced by adopting the interleaved structure. Moreover, the voltage-matching on both sides of the BT is realized by the adjustment of the duty cycle. As a result, even if the battery voltage changes, the BT always operates under the highest efficiency condition. Besides, by employing phase-shift control, power flow can be regulated monotonously. Soft-switching of all the switches within a wide range of battery voltage is obtained by employing the optimal design. The operational principles, characteristics, and design considerations are analyzed in detail. Experimental results from the BT-BDC prototype of 1-kW, 40-60 to 400 V verify the effectiveness.