Battery pack and method of detecting battery pack abnormalities

The battery pack is provided with a rechargeable battery 4 that can be charged, and a protection device 3 to protect the rechargeable battery 4 when an abnormality occurs. The protection device 3 is provided with an externally exposed first output terminal 6A connected to the first electrode of the rechargeable battery 4, an externally exposed second output terminal 6B connected to the second electrode of the rechargeable battery 4, and an externally exposed third output terminal 6A for signal detection as output terminals 6, and a contact switching section 12 that can switch contact when a rechargeable battery 4 abnormality is detected. The protection device 3 is formed as a single piece with the output terminals 6 and contact switching section 12. The contact switching section 12 has one end connected to the second output terminal 6B, the other end is connected to allow switching between a contact connected to the second electrode of the rechargeable battery 4and a contact connected to the third output terminal 6C. When no abnormality is detected the other end is connected to the contact connected to the second electrode of the rechargeable battery 4, and when an abnormality is detected the other end switches to the contact connected to the third output terminal 6C. The contact switching section 12 may be a bimetallic strip.