An apparatus and method for controlling battery discharging in a portable terminal having a detachable external battery are provided. The method includes determining if the external battery has been connected with the portable terminal when the portable terminal is booted, receiving power from the external battery when the external battery is connected with the portable terminal and receiving power from an embedded battery of the portable terminal when the external battery has been completely discharged. Furthermore, in charging the batteries, the embedded battery of the portable terminal is first charged when battery charging current is input in a state that the external battery has been connected with the portable terminal, and the external battery is charged when the embedded battery has been completely charged. Power from the external battery is first used when a portable terminal assembled with the external battery is used, and an embedded battery is first charged in charging batteries. Therefore, the embedded battery capacity of the portable terminal can be maintained as much as possible. Furthermore, although the external battery is separated from the portable terminal during battery charging/discharging, the embedded battery capacity can be maintained as much as possible. As a result, even if only the embedded battery is used, a using time of the embedded battery can be increased. Therefore, it is possible to improve the user's convenience.