Abstract: A rechargeable lithium battery protection device uses an external circuit to control charging or discharging a rechargeable lithium battery for preventing an overcharge or an overdischarge of the battery. The rechargeable lithium battery protection device includes a diagnostic circuit, a light coupling circuit, a balance circuit, and an identifier circuit. The battery diagnostic circuit checks the voltage of the chargeable battery and outputs a detect signal, and then the balance circuit determines whether or not the detect signal is an overcharge signal and discharges the battery. Meanwhile, the light coupling circuit adjusts an impedance of the light coupling circuit according to the detect signal for disconnecting the battery from the external circuit and the identifier circuit provides the position of a battery that sends out the signal.