US Patent 8148946 Battery pack having protection circuit for secondary battery

A battery pack has first through third external terminals connected to positive and negative power supply terminals and a voltage detection terminal, respectively. A secondary battery is connected between the first external terminal and the third external terminal. A protection circuit controls ON/OFF of first and second switching elements provided on a wiring between the secondary battery and a load or a charge device by detecting an overcharge, an overdischarge and an overcurrent of the secondary battery. A first thermistor is connected between the second external terminal and the third external terminal. A series circuit containing a second thermistor and a resistor is provided in parallel to the secondary battery. A third switching element is connected between the second external terminal and the third external terminal. The protection circuit turns on the third switching element and short-circuits between the second external terminal and the third external terminal when a detection is made by the second thermistor that a temperature of the secondary battery exceeds a predetermined temperature.