Very often there are situations when the battery on the phone is dead, and there is no power outlet at hand. Or, for example, you are going camping for several days, and you constantly need a work phone. Buying spare batteries for your phone is expensive and unreasonable. But finger-type batteries are cheap and you can find them in any stall. To make a device that charges the phone with ordinary batteries, you only need a piece of wire with a charging plug, a 2 ohm resistor and four AA batteries.