A built-in battery controller for a personal telecommunication device. The battery controller includes a push switch that is switched on and off by a user, a power control unit that is connected to the push switch and allows a leakage current to flow through the power switch, a battery pack including a protection circuit module (PCM) for turning off the battery when the leakage current flows through the power switch under the control of the power control unit, and a charging circuit for charging the battery pack. The battery controller can be simply operated and can more reliably turn off the battery without affecting the outer appearance of the device.