A more universal charger. By Lee Davison.

I seem to collect tools of all kinds and battery drills are no exception to this. They don't come to me whole, they come to me as broken and neglected things and I give them new life, at least for a while.

One such drill was grabbed for use one fine afternoon and found to be almost completely flat. No problem, plug in the '4 HOUR Fast Charger', drop the battery in it and go mow a lawn or two while I wait for it to charge. So I spend a while mowing one lawn and then return inside to move the mower lead to the front of the house. I immediately noticed something amiss, whether it was the slight metallic, acrid smell or the inability to see across the kitchen for the thick smoke that clued me in I'm not sure but I had one of those "Oh FU..!" moments before reaching for the wall sockets and pulling the power on everything.

After letting the room clear and everything cool down a bit the source of the smoke was quickly revealed to be the obviously poorly 25uF, 400V capacitor shown here that, up until its smokey demise, had been employed as a lossless current limiter in this fast charger. Bother!

Another battery drill I have comes with a 'universal' charger that unfortunately only works for very small values of universe, the metabo universe. The charger that had just died in its own fire was not part of this universe so its battery doesn't fit as you can clearly see if you compare the two batteries and the battery charger socket.

Apart from the gross physical differences though there is no reason this charger shouldn't be able to do sterling service as the charger for the Faithfull battery. So, after a quick check on polarity, three croc clip leads were pressed into service to connect the two together.

The connection proved successful and the Faithfull battery, recently charged by a forty eight hour connection to a slow but safe-for-anything trickle charger, was quickly topped up to full charge.

That all worked well as it went but suffers from a fairly large drawback. It's a huge pain to get the three croc clip leads down into the metabo charger socket to connect them to the three terminals and, once you have got them all secured, to get back in to remove them afterwards.

I did toy briefly with the idea of cutting the battery socket from the Faithfull charger and grafting it onto the metabo case but that itself suffered from two drawbacks. Firstly it would destroy the Faithfull charger so, should I ever find a replacement for the capacitor, I would never have both chargers available again and secondly it looked like a whole heap of work to do.

The solution to this is to provide another set of terminals, in this case three 4mm banana socket clones, to the metabo charger that can be easily accessed to make connecting other batteries much less of a struggle.

The site chosen to mount the banana sockets is the small shelf on the left in the battery socket. This has two advantages, nothing stands proud of the original outline, so there aren't any new sticky out bits to crush or snag, and you can't accidentally connect two batteries at once. When there is a battery in the socket the banana sockets are covered and when the banana sockets are in use you can't just drop a battery into the charger socket.

As can be seen in the picture the pilot holes for the sockets were lined up by eye and drilled through both the battery adapter and the battery charger body itself. This was to ensure that the holes lined up on reassembly.

The adapter was then removed and the holes in the charger body were drilled out to 3.5mm, just enough to clear the banana sockets' mounting threads. Each socket was pushed through its hole, an eyelet that had been soldered on the end of a wire and secured in place with a nut.

As the charger body was fairly thick there was not enough thread to use the original, plastic locking washer or to be able to use a star washer but as there won't be any routine twisting forces on the sockets in use, I can't get my fingers in there to twist them, this shouldn't cause any problems.

The pilot holes in the battery adapter were drilled out enough to just clear the banana sockets and then it was push fitted back into the charger body.

The wires from the banana sockets were cut to length and soldered to the same points on the charger PCB as the built in charging terminals.

With this done the case was replaced over the PCB, taking care not to trap the new wiring, and the charger was reassembled.

Now flying leads, either croc clip or banana plug ended for the charger end, can easily be used to connect any suitable NiCd or NiMH battery pack to the charger.

If you try this make sure that any leads that you use can easily carry the maximum charge current. Just using any old leads, say from one of those cheap croc clip leads packs, can quickly result in more burning smells and some melted wires.

How do I know? Take a guess.

Some more images of the project.

Last page update: 27th June, 2011.

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