A handheld tool has an attached position in which the tool is removably attached to a battery pack. The battery pack has an auxiliary battery and output terminals for outputting electricity from the auxiliary battery. The tool includes a load and a primary battery. Input terminals of the tool are configured to connect to the battery output terminals when the tool is in the attached position. A circuit is connected to the load, the primary battery and the input terminals. The circuit is configured to power the load solely from the primary battery when the housing is not in the attached position, and to power the load from both the primary and auxiliary batteries in series when the housing is in the attached position.