Abstract: The battery pack is provided with a rechargeable battery that can be charged, and a protection device to protect the rechargeable battery when an abnormality occurs. The protection device is provided with an externally exposed first output terminal connected to the first electrode of the rechargeable battery, an externally exposed second output terminal connected to the second electrode of the rechargeable battery, and an externally exposed third output terminal for signal detection as output terminals, and a contact switching section that can switch contact when a rechargeable battery abnormality is detected. The protection device is formed as a single piece with the output terminals and contact switching section. The contact switching section has one end connected to the second electrode of the rechargeable battery, the other end is connected to allow switching between a contact connected to the second output terminal and a contact connected to the third output terminal.

Abstract: The battery pack is provided with a rechargeable battery that can be charged, and a protection device to protect the rechargeable battery when an abnormality occurs. The protection device is provided with an externally exposed first output terminal connected to the first electrode of the rechargeable battery, an externally exposed second output terminal connected to the second electrode of the rechargeable battery, and an externally exposed third output terminal for signal detection as output terminals, and a contact switching section that can switch contact when a rechargeable battery abnormality is detected. The protection device is formed as a single piece with the output terminals and contact switching section. The contact switching section has one end connected to the second electrode of the rechargeable battery, the other end is connected to allow switching between a contact connected to the second output terminal and a contact connected to the third output terminal.

Abstract: The battery pack for electric power tools of this invention has a circuit breaker connected in series with the batteries to protect those batteries from abnormal temperature or abnormal current by cutting-off the current. The circuit breaker is provided with a manual reset mechanism wherein even if the temperature sensing element is cooled, automatic reset will not occur. The circuit breaker must be manually reset. After the circuit breaker detects battery abnormality and cuts-off current, the current cut-off state is maintained until the manual reset mechanism is operated.

Abstract: The dangers of excessive battery current and battery heating are effectively prevented by maintaining a heat sensitive cut-off device in the cut-off state as long as an over-current situation exists. An inside case positioned in the enclosed space between the batteries and the outside shell houses both the cut-off device and a parallel connected heating resistor. Double insulation allows the heating resistor to efficiently transfer Joule heat to the adjacent cut-off device. Effective use of space and heat transfer properties also reduces post-cut-off battery current and results in no battery pack volume increase.