Flashlight Power sources
The most common power source for flashlights is the battery. Many types of batteries are suitable for use in flashlights, such as button cells, alkaline batteries, lithium batteries and rechargeable NiMH and lithium ion batteries. The choice of batteries will depend on the light source used, and will usually play a determining role in the form factor of the flashlight.
Some flashlights are solar powered, using the energy generated from a solar cell to charge an on-board battery for later use.
Mechanical power
Some flashlights have an electrical generator built into them. One type of dynamo-powered flashlight has a winding crank connected to a small alternator that feeds several diode bridges with their outputs connected in parallel feeding a field effect transistor that charges a capacitor that connects to one or more LEDs. Others generate electricity using electromagnetic induction. They use a strong permanent magnet that can freely slide up and down a tube, passing through a coil of wire as it does. Shaking the flashlight will charge a capacitor or a rechargeable battery that supplies a current to a light source, typically a light-emitting diode or, more rarely, an incandescent light bulb. Such flashlights can be useful during an emergency, when batteries may not be available.