A secondary battery supplying electrical power to a load comprises a battery unit, at least one PTC temperature sensing device and a load switch. The battery unit is encompassed by a battery shell. The PTC temperature sensing device is disposed on a primary surface of the battery shell to effectively sense a temperature of the battery unit. The load switch connects to the battery unit and a load and determines whether the battery unit and the load switch are in electrical connection or in an open circuit state upon resistance variation of the PTC temperature sensing device. When the resistance of the PTC temperature sensing device increases by a threshold times within a specific temperature variation or time period, the load switch receives a control signal and accordingly switches to an open circuit state.