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I have some cordless power tools which all require a certain 12V nickel-cadmium battery pack. My battery packs are dead, but the good news is they're held together with screws, so I took them apart to have a look. The bad news is the cells inside are all shot; they have the little crystals on the outside telling me they're done. enter image description here So, the dead cells are going to the official hazardous waste drop-off site.

  1. What should I replace them with? I've wired up battery packs for RC cars and stuff before, and I'm confident I can put it back together properly. The problem is I'm having a hard time finding the nicad cells now. did they stop making them? I see a few for sale and they're like $3-5 per cell now (I need like 50 of them).
  2. Is it possible to salvage the good cells from several different dead ni-cad packs to create one working battery?
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Will you charge the rebuilt pack with the tool vendor's battery charger or with something hand-built? Battery chemistries other than NiCd and possible NiMH are probably off-limits unless you build your own charger due to differing voltage profile & charge termination characteristics. – Matt B. Jun 6 '12 at 4:16
I would like to use the oem charger for simplicity, however, it does not shut off after the battery is fully charged. Perhaps I'll wire in a timer. – okayGraphics Jun 6 '12 at 4:47

3 Answers

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RE: 1.

Nickel-Cadmium (NiCd) cells are falling out of favor, because Cadmium is environmentally unfriendly and usually not compliant with RoHS. Nickel-metal hydride (NiMH) are replacing NiCd. NiMH and NiCd have similar characteristics (more thorough comparison of NiMH and NiCd here).

You can buy both NiCd and NiMH cells from industrial electronics distributors. Here, for example.

RE: 2.

It depends on the actual packs and how you're planning to charge. In a general case, I wouldn't mix and match used cells of unknown pedigree. It wouldn't produce the best results, probably. I would buy a uniform batch of new cells.

P.S. Of course, when you say "power tools battery pack", most folks knows what you're talking about. However, a photo of your tool's battery pack would help this question a lot, especially if you've already disassembled it. Please post it if you can.

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up vote 3 down vote

A "trick" that can work extremely well is to meld a 12V sealead lead acid (SLA) battery with the drill connection part of your old battery pack. This does not have to be overly pretty to work - as long as you value functionality above looks.

I tried attaching one of the industry standard 12V 7Ah "brick" batteries as used in alarm systems, to a drill whose battery pack had dies. While the 7Am 12V battery was heavier than the original I was agreeably surprised at the comfortable "heft" that was achieved and how usable the drill was. YMMV*.

Another "solution" which may suit depending on circumstances is to make a short cord to a battery pack with a quick remove connector on the drill. This is of course not as convenient as true cordless but is far more flexible than amains powered drill.

NiCd cells can be bought as Nick notes. The big things for a long life are to NOT overcharge them and to not run any cell "flat". The latter really requires you to stop discharge before any cell has a chance to be exhausted. More on that if of interest.

I have heard of people refitting drill packs with LiIon cells BUT that is a task for the brave and competent.

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I like your alternatives. How do you charge the SLA pack? Will the 12V SLA work with my automotive trickle charger? – okayGraphics Jun 6 '12 at 5:12
Some years ago, I bought a couple of 12V cordless drills precisely because they were 12V rather than, say 18V, (At Canadian Tire on sale for $9.95 each!), knowing their batteries would die at some point. When the first one did, I replaced the battery pack with a cable with battery clips on the end, and voila! a (admittedly, corded) drill to use on the boat or anywhere a automotive type battery is available but AC or a charger is not. I left all but one of the old cells in, to keep the balance and for ease of attachment. In case you decide rebuilding is not worth the trouble... – mickeyf Jun 13 '12 at 13:33
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@okayGraphics 12V SLA will work with trickle charger. Some chargers will be less suited than others but most should be OK. Measuring how it behaves at end of charge would be useful. Worst case it reduces lifetimes somewhat but probably not fantastically badly. – Russell McMahon Jun 13 '12 at 14:45
up vote 2 down vote

If you scavenge cells from different corners, thay will have different characteristics, some will charge earlier than other, then overcharge, some will discharge earlier than other, then overdischarge and eventually your new battery pack will be dead again.

You could do with a bunch of new cells, but they are not produced equal, so the same applies. You need to find a matched set of NiMH cells, these are available from various sources on the net.

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Thanks for the advice. I was just about to buy some too! Out of curiosity, how would I go about matching a set of cells? How many random cells of the same brand/model would I need to buy to get a matching set of 10 (with a 95% confidence interval)? – okayGraphics Jun 6 '12 at 5:03
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no, you can get a set that is already matched. matching them is hitec and is done at factory. – miceuz Jun 6 '12 at 7:26

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