Restoring dead Ni-MH batteries

It wasn't neglect, I swear!

So, I'm on holidays. An excellent opportunity to get back into robotics, I thought, whilst I eyed my dust entrenched Lego NXT prototype SumoBot built several months prior. It was a uni project at the time, and ever since, I've been busy with other studies. So I quickly brushed off the dust and hit the on button, only to find that the batteries were dead. "That's OK" I thought, "I'll just charge them up again." I was wrong. You see, when you don't charge batteries for a long period of time, they tend to over-discharge. I proceeded to place the batteries in their designated charger, switched it on, and noticed that the status bars for each battery were not changing. In fact, the charger didn't realise there were batteries inserted! Why was this so? Could the issue be resolved? I investigated further...

Google was my first port of call. At this stage I was unaware of the over-discharging phenomenon, so my ability to find relevant results was quite dismal. However, luck was on my side (eventually) and I came across an interesting PDF file. I had to try this. So I found my digital multimeter (albeit in pieces and without a fuse),  proceeded to short circuit the fuse connection, and finally did some testing. Either the multimeter was playing silly, or these batteries were giving off zilch voltage. That couldn't be right, I thought. So I skipped to the fun part, electricity!

Note: I take no responsibility for any damage to you or your equipment as a consequence of following this guide. Electricity can be dangerous - don't take risks.

Before we start, keep in mind that these batteries were not dead due to wear and tear. They were fairly new to start with, but had lost enough charge to make them unrecognisable by the charger. Whether this works with worn out batteries or not is another story; one which I am yet to discover. I have my doubts. Also note the title of this guide. It clearly says Ni-MH. Will it work on other types of batteries? I don't know.

The first step was to find a suitable power supply. Keep in mind that the output should be no more than 12V, and the current supply 0.5A or thereabouts. Your best bet is a small wall-wort transformer, such as those used to charge mobile phones. All we need is to get enough charge in the battery so that the charger can recognise it and finish the job. After a quick look around my room, I came across an old Nokia 3200 phone charger. Its output is rated at 7V and 355mA. Not quite as much as 12v and 0.5A, but it was the closest I could get with what I had. The voltage and current ratings should be printed on the power supply device. If it isn't, and you're unsure, assume it's unsafe for this purpose.

As you can see, the plug used is a little awkward to wire up. We want a negative and positive terminal to work with. In order to achieve this, I fabricated a state-of-the-art positive terminal using a mere paperclip - MacGyver style. This particular paperclip is coated in plastic, so the tips were altered using scissor blades to reveal the conductive material inside. If you don't have a paperclip, a sewing needle is also a good alternative. For the negative terminal, a small strip of aluminium foil (as used in cooking) was folded up to create what one could only describe as strategic mess. Luckily the shape doesn't matter, as long as it works. More on that later.

If you haven't guessed already, the paperclip is inserted into the centre of the power plug. Push it in as far as possible to ensure electrical contact. It should look similar to the following:

On the other hand, the aluminium foil is a little trickier. The metal area on the outside of the plug is the negative terminal. We don't want the positive and negative terminals to touch, so if you're not using an insulated paperclip (i.e. covered in plastic) you'll need to take care. Shape the foil so that it's relativity snug around the plug, and ensure there's enough protruding so we can connect it to the battery. You may find it easier to attach the aluminium foil terminal before the paperclip.

Now it's time for the fun! Ensure the two terminals are not touching, and plug in the power adapter. The voltage and current should be safe, but avoid touching both terminals at once regardless. Attach the battery so that the positive terminal of the battery touches the paperclip, and the negative touches the aluminium foil. Only leave it connected for approximately 3 seconds! Too long may damage the battery, or possibly yourself.

And there you go! The battery should now charge in its purpose designed charger. :)