A laptop computer system executes automatic battery calibration to improve the accuracy of the battery's internal charge monitor. The calibration procedure drains the battery to approximately 0% of charge capacity during normal computer operation, permitting a user to operate the computer without risking power failure when the battery discharges. The computer returns to normal operation if either the AC power or the battery is removed. During the calibration procedure, the computer system prevents power management software from forcing shut down before the battery completely drains.