A battery module usable with a computer system includes a circuit to generate a power, and a controller to output the power of the circuit to an external device, for example, the computer system, and to prevent the power from being output from the circuit to the external device according to an external signal received from the external device. The computer system includes a circuit to receive the power from the battery module, a second circuit to generate another power other than the power of the battery module, and a second controller to generate a detection signal as the external signal according to existence of the another power of the second circuit. The circuit may include an adapter to receive an extern power from an external power source and a battery to receive the external power from the adapter or to generate an auxiliary power and the power.