A battery pack that determines battery temperature via a protection circuit which detects electrical resistance change in a temperature sensor thermally joined with the rechargeable batteries. The protection circuit is provided with a timing circuit that sets timing for battery temperature detection, or it is provided with a current detection section that detects current flowing in the batteries and detects battery temperature when battery current is greater than a set current. A protection circuit, provided with a timing circuit, measures battery temperature with a fixed period. A protection circuit with a current detection section, detects battery temperature when battery current is larger than the set current.