An AC adapter including an automatic battery voltage selection circuit that senses which battery pack is installed in an electronic device and regulates the AC adapter output voltage at an appropriate level. A battery voltage selection signal providing circuit provides the battery voltage selection signal indicative of the particular battery pack present. An output voltage sensor senses the output voltage of the AC adapter and provides a proportional sense signal to a feedback circuit which controls the AC adapter output. A switching circuit coupled to the output voltage sensor and to the battery voltage selection signal providing circuit modifies the sense signal depending upon the state of the battery voltage selection signal, so that the AC adapter output voltage is regulated at an appropriate level for the particular battery pack installed. If a first battery pack is installed, the AC adapter regulates the output voltage at a first level and if a second battery pack is installed, the AC adapter output voltage is regulated at a second, higher voltage level. The switching circuit is preferably implemented with a transistor switching circuit.