A new adaptive output voltage controller for fast battery charger method is proposed in this paper. The charging power demanded is increased with larger battery capacity to shorten the charging time. To overcome the longer charging time with the constant voltage charger, the charging power may vary from high power to low power for the fast battery charger to shorten the battery charging time. In this paper a new adaptive output voltage control circuit is proposed. And the conversion efficiency of flyback converter under various power conditions is analyzed. Then the output voltage of flyback converter is controlled with the output power level to maintain high efficiency power conversion to achieve fast battery charging and higher efficiency. Finally, a 30W experimental prototype with output voltage range 5V∼15V is designed to verify the feasibility of the proposed controller. The maximum efficiency reaches 84.5% and the ligh load efficiency at 0.5W is 71%.