US Patent 8698457 Battery pack with protection circuit

A battery pack includes a current control device that has a thermostat and a positive temperature coefficient device whose resistance increases as the temperature rises connected in parallel, the current control device being inserted into a discharge current path of a battery cell and a protection circuit that detects a voltage of the battery cell, detects an overcharge and over-discharge of the battery cell, and generates an overcharge detection signal and an over-discharge detection signal. In a normal state, a discharge current of the battery cell flows through the thermostat of the current control device. When the discharge current is large enough to damage the battery cell, the thermostat turns off and the discharge current flows through the positive temperature coefficient device. When the positive temperature coefficient device produces heat, a resistance of the positive temperature coefficient device increases and the discharge current is limited or interrupted.