Nickel metal hydride (NiMH) batteries are rechargeable and have 50 per cent more capacity than nickel cadmium (NiCad) batteries and two or three times the capacity of alkaline batteries. Each battery produces 1.2 volts when fully charged. To build a 7.2-volt NiMH battery pack, you need to wire six batteries in series. Wiring in series combines the voltage from each battery but maintains the same capacity.
Place six NiMH batteries flat on a suitable work surface. NiMH batteries are the same or similar size to regular AA batteries and have a positive terminal on top and a negative terminal on the bottom.
Turn three NiMH batteries 180 degrees so you have three positive and three negative terminals at each end. Label each battery, from "1" to "6." Wrap electrical insulating tape around the batteries to form a neat, tight battery pack.
Cut seven strips of wire using a small knife. Use AWG gauge 16 or 18. Two strips need to be long enough to reach the device you intend to power; the other five need to be long enough only to reach between their assigned battery terminals. Use wire strippers or a small knife to remove ¼ inch of plastic coating from the ends of each wire and expose the metal.
Attach one long strip of wire to the positive terminal of battery "1." Use a thin strip of electrical insulating tape to hold the wire in place. Attach the other long strip of wire to the negative terminal of battery "6," using the same procedure.
Attach the end of a strip of short wire to the negative terminal of battery "1" and the opposite end to the positive terminal of battery "2."
Repeat the process of attaching a wire to the negative and then positive terminals for the other four strips of wire, using the numbered batteries sequentially. Your last wire will connect from the negative terminal of battery "5" to the positive terminal of battery "6." Check that each battery terminal has one wire attached.
Wind electrical insulating tape around the battery pack to cover all the terminals. This will ensure that the wires stay attached. Make sure that none of the wires touches another wire. Ensure that the two long wires protrude and that you correctly label the wires "+" and "-."
Connect the wire labelled "-" to the negative terminal of the device you want to power. Connect the wire labelled "+" to the positive terminal of the device you want to power. Your 7.2-volt NiMH battery pack is complete.
Ensure when you charge your 7.2-volt NiMH battery pack that the charge rate is set to the correct voltage.