Battery chargers are comparable to transformers; they convert an alternating current of 110 volts into a low voltage direct current. Rechargeable 3.7-volt lithium-based batteries are great as while they are initially expensive to purchase, you save money over time, because you don't need to keep buying new batteries. If your lithium battery needs recharging and you don't have a suitable charger nearby, it can be a cause for concern. However, the chances are you probably have a transformer in the house. Many children's electrical toys use transformers such a car racing or railroad sets and most string or rope lights use transformers.
Check the voltage output of the transformer you are going to use to charge your 3.7-volt lithium battery. You find that transformers used to power children's toys often have built in rheostats, so you can control the voltage. It's important that the output voltage from the transformer isn't more than 5 volts, otherwise you can damage the battery and it can be dangerous.
Cut a strip of dual-core wire using a knife; 12 to 18 inches is fine. Use wire strippers to remove about 3 inches of the outer plastic off both ends of the dual-core wire. You see two coloured wires inside.
Strip ½ inch of the coloured plastic off the internal wires. This reveals the inner copper metal wire that connects to the lithium battery and the transformer.
Loosen the terminal screws on the transformer. Either use a screwdriver or if the screws have knurled edges, use your fingers.
Insert the two wires on one end of the strip of wire under the terminal screws on the transformer. Make a note of the colour of the wires that attach to the positive and negative terminals as you need to attach the same coloured wire to the corresponding terminals on the lithium battery. The terminals are likely to be labelled "+" and "-."
Attach the opposite end of the coloured wire that's attached to the negative terminal of the transformer onto the negative terminal of the lithium battery. Use a strip of electrical insulating tape to hold it in place. The negative terminal is on the bottom of the battery and is a flat metal plate.
Attach the opposite end of the coloured wire that's attached to the positive terminal on the transformer onto the positive terminal of the lithium battery using the same method. The positive terminal is on top of the battery and is a slightly raised small knob.
Put the plug on the end of the wire from the transformer into the power socket. Turn on the transformer. Leave the lithium battery to charge for about 30 minutes. Check the side of the battery every 10 minutes to feel how warm it's getting. Slightly warm is normal, but if it starts to get hot, remove one of the wires and let it cool completely.
Put your lithium battery into the unit it powers, after charging for 30 minutes. You'll find the battery has charged.
Only use this method of charging on a temporary basis. Rechargeable batteries are best charged using a purpose made charger.
Don't charge your battery for more than 30 minutes. Always let your battery go dead before recharging again.