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I built this simple charger / discharger for 3.7 volt Nickel Cadmium cordless telephone batteries. It could easily be scaled up to charge larger Nickel Cadmium battery packs. Those of you who work with these battery packs know that they must be completely emptied of charge before they can be recharged. This device makes that procedure easy.

Step 1: Materials Needed

You will need a number of simple materials and tools for this project. They include:
A wall transformer,or power supply; mine is 3.7 volts. The output of your charger will vary depending on the voltage rating of the battery you want to charge.
A small, incandescent lamp; this will use up the remaining power in the battery in discharge mode to prepare it for charging. I used a Christmas bulb, which works fine for 3.7 volts. You'd need a bigger one for a higher voltage.
SPST switch; for those who don't know, that stands for Single Pole Single Throw; it's the kind with three terminals on the bottom. I used a push button one, but any kind except momentary would work.
Wire; any kind that will carry the required voltage and current - which, in this case, isn't very much. You'll need about a foot, at most.
Clips or plug; You'll need something to connect the battery to the charger; my batteries terminated in bare wires, so I used clips (binder clips, as it happens, because I didn't have alligator clips). If your battery terminates in a plug, you could use a corresponding socket.
Something to hold everything, such as a small box. An altoids tin would work if you insulated it.
You'll also need something to cut the necessary holes in the box, and perhaps a soldering gun, although you could manage quite well without one.

Step 2: Wiring

Connect everything together as shown in the diagram; run a wire from one terminal of the wall transformer to one of the clips - it doesn't matter which one; you'll check the polarity later. The positive terminal is usually the inside of the round plug - you can tap it with a paper clip, or just a wire. If the plug is USB, you should probably just cut it off - although a USB charger would be too much power for the batteries I use. Also connect one lead of the discharge lamp to the same clip. Attach the other terminal of the transformer to one of the outermost tabs of the switch. Connect the other lead of the discharge lamp to the other outermost tab of the switch, and connect the other clip to the center tab. Box everything up, and the wiring is complete.

Step 3: Charging

With the charger set to charge and the transformer plugged in, use a voltmeter to determine the polarity of the charger and mark the clips accordingly. Unplug the charger and connect the battery, taking care to observe polarity (positive to positive, negative to negative). Flip the switch to discharge. The discharge lamp should illuminate. Wait until it goes out, indicating that the battery is fully discharged. Now plug in the transformer, and switch the switch to charge. My batteries take about an hour to charge - the cell phone transformer supplies 340 mA. You might need a different amount of time depending on the size and voltage of the battery and the current of the supply. You could scale this design to various sizes of batteries. Be safe, and don't charge batteries unattended. DON'T CHARGE OTHER TYPES OF BATTERIES WITH THIS DEVICE!
what i do is just switch the dead telephone battery with the one inside the telephone and set the telephone in its own charger. I have two sets of phones that take 3 AA and 3AAA, so im set.
I don't use the batteries to power cordless telephones anymore; I don't even have the phones they came from. One of them, I just found sitting loose in the trash. What you describe would work, but this solution is significantly less bulky. I only mentioned cordless telephones because that's where they came from.

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