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I have this Ryobi 12V NiCad cordless drill that was sitting in a drawer for the past 6 years or so due to dead batteries (2 of them). At least that's what I tought at the time but looking at it more closely this week-end, I could hear a rattle in the charger and could see a slight bump melted in the bottom of the charger.

I opened it up and was amazed at what I found. There is a single small PCB with a barrel connector for the 12V dc adapter, a small resistor for the "Charging" led and a big ceramic resistor for the charging circuit and that's it!!

The PCB was burnt out and cracked in two and the ceramic resistor had fallen off and was rattling in the case. The led was working though, giving the appearance of a working unit. Now I'm thinking that maybe my batteries are still good, and I only need a new charger. Would building the one mentioned in the answer here: http://electronics.stackexchange.com/a/21279/11014 work?

Specifically, could I use it to charge all cells in my 12V batteries at the same time? What resistor values would I use? Maybe there is a faster, safer, cheaper (or all three!) alternative?

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One more requirement that's important is how long you want to use the batteries. Batteries which are charged slowly in general tend to last longer than the ones that are quickly charged –  AndrejaKo Aug 21 '12 at 11:24
    
You might be interested in this Peter Hayles post where he did a similar thin. –  Dean Aug 21 '12 at 11:43
    
Sounds like you had a really poorly made charger. The batteries might be ok, but charging them with that thing will likely have dramatically shortened their life. –  Grant Aug 21 '12 at 12:27
    
@Grant - that's not really a valid assumption at all - the requirements for a basic low rate charger are pretty crude compared to those of a power supply to run electronics. Battery life is determined mostly be charge rate and usage. A 1/10th capacitor charger used <16 hours at a time will give pretty good life, even if it's a crude design. OTOH, many tool chargers are higher rate, and if they are left connected and do not have a cutoff circuit, that will shorten battery life. –  Chris Stratton Aug 21 '12 at 14:37
    
@Dean: Excellent link! I'm not a pic master myself but it should be feasible to code that for an avr... –  Zot Ditz Myo Aug 22 '12 at 10:05

4 Answers 4

Your batteries are gone. Yes, there are some tricks for reviving NiCd batteries, but 6 years is a very long time for a NiCd battery even if it were treated properly and not stored discharged. Even if you "revive" the batteries by blowing the internal dendrites that have most likely formed by now, they will at best hold a small fraction of their rated charge. In practise, they will be useless whether revived or not.

Forgeddaboudit.

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I've seen this twice, the first time on a electronic lantern and the second on a mini Hoover vacuum. In both of those times I only investigated because the batteries were shot. If the batteries are still good the charging circuit is a good idea but I'm doubtful.

The wall warts provided with these units are normally matched to the current capacity, this being said it means the voltage is slightly higher than needed since the rated current droops the voltage somewhat. When fully charged and little current is flowing this higher voltage causes voltage depression which destroys the battries.

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Do you think I could just hook up the wall wart directly to the battery? I would probably need a current limiting resistor at least no? too bad mine is burnt out too much to see the markings or take a reliable measure... –  Zot Ditz Myo Aug 22 '12 at 10:06
    
Well isn't that what caused the problems in the first place? If you have a voltage source that's no higher than the battery and use a resistor for current limiting your battery will survive for a while. The problem is that wall warts don't have a clean output and most will harm your batteries, there's a good reason why most charging is done with a constant current source... –  s3c Aug 30 '12 at 15:56
    
For an "overnight" NiCd charger you want a current of about 1/10th the capacity rating, though that shouldn't be left permanently plugged in - for always connected trickle maintenance you want a smaller current. Crude transformer/rectifier solutions are fine, and "clean" output is not needed. Charging circuits really only get complicated when you want to charge at faster than the C/10 rate, or auto switchover from a faster charge to a trickle maintenance, rejuvenate damaged cells, or charge more sensitive chemistries (lithium, etc). –  Chris Stratton Sep 21 '12 at 15:47
    
Plenty of wall warts do this fine. But if you have a 4 or 5 hour charge wall wart with no auto cutoff/cutback circuit, and routinely leave that connected longer, you will be damaging the battery. –  Chris Stratton Sep 21 '12 at 15:50

It is very possible the batteries went bad (shorted) and the charger melted because it had no protections. It's also likely that NiCd cells left in a drawer for a year (not to mention six!) will also be unusable.

The most sensible thing to do is to reconstruct the original circuit--easy to do, especially if the parts themselves are still good. Just wire them together, then try to charge the batteries. If the batteries come back, you can stop and build a charger suitable to your liking. If not, you've wasted very little time.

Generally, it isn't the charging from these cheap chargers that hurts the batteries; it's the fact that they don't stop when they're finished that cooks them.

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This battery may not be dead. Most probably it is deep discharged and internally short circuited, but this does not mean, it is dead. Many people don't know that, but you can easily "revive" the cells. Disassemble battery to access cells and measure voltage of every one, when voltage reads 0.00V, cell are in short circuit. NiCD cells, mostly old ones in bad condition are tend to fall into internal short circuit when being deep discharged for some time, conductive bridges grow there between electrodes causing short circuit, we need to burn them by high current, because such dead cell can not be charged - all charging current flows through these bridges like through wire and it is not charging the cell. Find source of high current, I have used 5 mF capacitors before, but it's not enough for some cells, now I am using 3 LiIon cells in series from old notebook battery, they are almost dead and useless, but still can deliver more than 10 amps for a short while, perfect for the purpose. Any other battery can be used, voltage is not important, but high current capability is. Connect + of current source to + of dead cell and briefly connect - to -, it should produce spark, don't let it be connected more than second! And never ever connect + to - and vice versa, or you will damage the cell. After that procedure, measure cell voltage again, it should be normal now, or at least non zero. If it's still absolute zero, try again or use stronger current source. After you revive all "dead" cells, let battery rest few minutes and then you can normally charge it and it should work almost like before. I am doing it regularly and I have saved a lot of batteries so far. But yes, so old battery could be somewhat weak and low capacity for the drill now and probably won't hold the charge long, but better than nothing.

And about charging, yes, you can build simple charger, best and safest is to use 1/10 C current for 16 hours like said before, or use higher current, but you will do good, if you will use timer for safety (and battery life span). This is the simplest way. But there are better ways, if I can recommend you something, buy some charger like people in RC uses, it's cheap enough, can charge almost every battery in the world, has automatic end of charging when battery is fully charged, lot of safety features, adjustable charging current, shows voltage, current, capacity, time, can measure battery capacity, some also internal resistance, can cycle or regenerate the battery... and has very simple and convenient use. I am using HobbyKing ECO eight and I have thrown away all other chargers from drills, lamps... this one charges it all.

Good luck and I hope, this will help. LQd

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