To rapidly detect a malfunction of a semiconductor switching element connected to a battery in series and transmit an abnormal signal to a connection equipment while it has a very simple circuit constitution.

A battery pack includes a chargeable battery 1, the semiconductor switching element 2 connected between this battery 1 and an output terminal 5, a protection circuit 3 to on-and-off control this semiconductor switching element 2, a temperature sensor 4 to detect a temperature of the battery 1, and a temperature terminal 6 connected to this temperature sensor 4. Furthermore, the battery pack includes a sub temperature sensor 7 to detect the temperature of the semiconductor switching element 2, and an impedance conversion switch 8 which is connected to the temperature sensor 4 in parallel or in series, and changes the impedance of the temperature terminal 6 in a state that a detection temperature of the sub temperature sensor 7 becomes a higher temperature than a set temperature.

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