Battery-powered electric heaters operate by connecting a wire element between the positive and negative power source terminals, similar to regular electric light bulbs. The wire’s resistance determines whether it glows red and produces heat -- if the wire is too thin, it will get too hot and melt, but if it’s too thick, it won’t get hot at all. Making a small heater using a 12 volt battery and a few strands of wire is an educational exercise, but remember the wire can get red hot, so be sure to protect your hands.
Measure the distance between the positive and negative 12 volt battery terminals and double the figure. An old car battery is a great battery to use as both the terminals are on top, making it easier to connect the heater element.
Cut a strip of wire the length you calculated earlier using a knife or wire cutters. Speaker wire is a good choice because there are ten or so strands of wire inside the protective outer coating that make an excellent heater element.
Remove all the plastic covering from the wire. Use a knife to carefully slice 2 or 3 inches along the length of the wire so the plastic splits. Peel back the plastic covering with your fingers. Hold the exposed strands of wire in one hand and continue to peel back the plastic the whole length of the wire until all the plastic is removed. You now have a length of about 10 bare wire strands.
Twist the strands of wire so they don’t separate. Attach one end of the wire to a battery terminal using electric insulating tape. It doesn’t matter which terminal you use. Place the wire on the terminal then wrap the tape so it is held in place.
Use a pair of pliers that open wide enough to go over the battery terminal. Open the pliers and place the opposite end of the bare wire onto one of the jaws of the pliers. Wrap a little strip of tape around the wire and the jaw of the pliers so the wire is held in place.
Put on your protective gloves in case you accidentally touch the bare wire. Carefully place the open jaws of the pliers over the battery terminal and clamp the jaws together. As soon as the wire touches the battery terminal, the electric circuit is complete. The wire starts to heat up and will glow red, producing heat using the 12 volt battery.
If the wire doesn’t glow red, the resistance is too high so you need to remove a couple of wire strands. This makes the wire thinner and it will get hot. If the wire breaks, it means the resistance is too low so you need to start over using more strands of wire.