You can charge a depleted rechargeable battery by inducing a direct flow of electricity from a charging device, such as toy transformer, to the battery. A chemical reaction in the electrical flow creates a movement of electrically charged particles inside the battery that causes the battery to become charged. However, electrical pressure, known as voltage, produced from the device that charges the battery, must exceed by a couple of volts the electrical pressure produced by the battery being charged. For example, if your battery produces 6 volts, your charger must induce voltage pressure of about 8 volts.
Check your children's playroom or bedroom to look for toys that use transformers to power them, such as an electric train set or car racing set. A battery charger is essentially a transformer by another name; transformers reduce the local power company's alternating current that runs your household electricity from 120 volts to variable, low-voltage direct current that runs battery-powered devices.
Check the output voltage of your rechargeable battery, then the output voltage from the transformer. The transformer voltage must be greater than the battery voltage, but don't go too far. For example, laptop batteries usually produce about 10 volts. So if your transformer produces 12 volts, that's fine, but a transformer that produces 15 volts is too high, will damage the battery and could be dangerous. A variable-voltage transformer is a good option, as you can alter the voltage to the correct level.
With a knife, cut a strip of AWG 16-gauge wire; it contains two colored inner wires. Cut the strip long enough to connect between the transformer and the rechargeable battery; a one-foot strip is fine. Using wire strippers, strip one inch off the outer plastic coating to reveal the two inner colored wires. Strip about ¼ inch of plastic off the ends of the two colored inner wires so the copper metal is exposed.
Connect the end of one colored wire to the positive terminal of the transformer labeled "+," and then connect the other colored wire to the negative terminal of the transformer labeled "-." Most transformers have screw terminals. Loosen the screw with a screwdriver and insert the wires under the screws; then retighten. If you can't find a way to attach the wires to the terminals, use two strips of electrical insulating tape. Place the wires on the terminals; then put the tape over the wire and terminal to hold them in place.
Attach the opposite ends of the two colored wires to the rechargeable battery terminals, using strips of electrical insulating tape. Attach the same colored wire that's connected to the positive transformer terminal to the positive battery terminal labeled "+." Connect the other colored wire from the negative transformer terminal to the negative battery terminal labeled "-." For example, if you have a red colored wire and one end is connected to the positive terminal of the transformer, ensure you connect the opposite end of the red wire to the positive terminal of the battery.
Plug your transformer into a wall socket. Set its voltage to about two volts more than the output from the rechargeable battery. Turn on the charger. Let the battery charge. Check the battery every 15 minutes; it starts to get warm as it charges. If it gets too warm, turn off the transformer and let it cool. Charge the battery for about 30 to 45 minutes; then check whether it's charged by attaching it to the device it powers.