A battery powered computer system includes an add-on device, such as a modem, with a power supply that uses power from a telephone line to provide power to the modem. In one embodiment the power supply is an adaptive power supply that uses power from both the telephone line and the battery of the computer system. The adaptive power supply determines the maximum power available from the telephone line and delivers that power to the modem. If the power from the telephone line is not sufficient for the modem, the adaptive power supply supplements the power with the power from the computer system. In one embodiment, the adaptive power supply uses power time division multiplexing to share the power from the telephone line and the computer system.