Lithium-ion or Li-ion batteries produce higher voltage than other rechargeable batteries such as nickel-cadmium or nickel-metal hydride: One Li-ion battery produces between 3.6 and 3.7 volts, compared to 1.2 volts for the other two. If you need even more voltage to power your device, linking Li-ion batteries together into a pack is a good choice. Two Li-ion batteries wired in series will produce 7.4 volts.
Cut three lengths of AWG 18 gauge wire using a knife. The length of two of the wires depends on the distance between where you intend to position your Li-ion battery pack and the device it powers. Measure the distance using a ruler and then cut the appropriate lengths. The third wire only needs to be about 2 inches long to connect the two batteries.
Strip about ¼ inch of outer plastic coating from the ends of the three wires using wire strippers so the inner copper metal is visible.
Heat a soldering iron. Put the end of the soldering iron onto the exposed metal wire on the end of one of the wire strips. Melt the solder so it coats the copper wire. Remove the soldering iron and solder and let the wire cool. Repeat the process on the five remaining ends.
Place the end of one of the long soldered wires onto the positive terminal of the first Li-ion battery. The positive terminal is labelled "+." The other end is attached to the device you want the Li-ion battery pack to power.
Touch the battery terminal and soldered wire using the soldering iron. Remove the soldering iron from the battery terminal and wire as soon as the solder melts. Hold the wire on the terminal until the solder solidifies. Check the wire is firmly attached.
Place one end of the short wire strip onto the negative terminal of the first battery. The negative terminal is labelled "-." Put the end of the soldering iron onto the wire and battery terminal as before and let the solder melt. Remove the soldering iron and allow the solder to harden so the wire is secured on the battery terminal.
Attach the oposite end of the short wire strip to the positive terminal of the second battery using the soldering iron as before.
Attach one end from the remaining long wire strip onto the negative terminal of the second battery using the soldering iron. The opposite end is attached to the device you want to power. Turn off the soldering iron.
Wrap electrical insulating tape around the two Li-ion batteries to form a neat battery pack, ensuring the soldered wires and battery terminal are sealed. Be sure you leave the two lengths of wire exposed from the pack. You need to connect these to your device.
Connect the end of one of the wires from the Li-ion battery pack to your device. Connect the end of the remaining wire to the other terminal on your device so your Li-ion battery pack completes the circuit and can power the device.
You can add as many batteries to your pack as you need to power your device. For example, if your device operates on 12 volts, then wire three Li-ion cells in series. However, don't build a battery pack that produces more voltage than your device needs, as you may damage it. Always follow the manufacturer's instructions for charging the Li-ion battery.
The chemical composition of a 3.7 volt Li-ion battery means it must only be recharged using a specifically designed charger. Li-ion batteries contain a power surge breaker that prevents overcharging, which can result in the battery overheating and catching fire.