Applications for use with a rechargeable power unit. In one embodiment, the rechargeable power unit is configured to be releasably connected to one or more interchangeable attachments having an electric functionality. In one embodiment, an electric device is configured having a compartment and connection portion configured to receive and use the rechargeable power unit as a power source. In some embodiments, the rechargeable power unit can be obtained or exchanged at a battery exchange machine.