If your flashlight runs out of batteries, you still may have a shot at using it. If you have smaller batteries that are charged, wrap the sides in tape, paper or cardboard to get the right battery diameter. Then, use aluminum foil or stacked coins to increase the battery length with conductive metal. With this trick, you can substitute AAA batteries for AA, and C cells for D cells. This isn’t just for flashlights, either. Any battery operated device can work on modified batteries, providing the replacements are the same voltage.