There is disclosed a universal power pack, power supply and battery harvesting device. Generally, each comprises a receptacle for receiving at least one battery or battery pack having a different shape and/or nominal output voltage. A controller circuit is provided for regulating the output voltages and/or currents of the battery or batteries with the regulated voltage and current available at an output. The devices allow other devices to be powered or recharged, or batteries to be recharged. In a particular embodiment, a master cell and slave cell are provided wherein the slave cell continues to power attached devices when connection with the master cell is not available.