A computer including a system part, a battery mounted from the outside to supply power to the system part, a detector to detect a residual amount of the battery, and a controller to calculate a decreased residual amount of the battery based on the residual amount of the battery, and to control power supplied to the system part if the decreased residual amount of the battery is lower than a predetermined value, and if the residual amount of the battery is lower than a predetermined critical residual amount.