Drill pack nicad to Nimh conversion questions

[NigelBond]

Hello, I have an 18v drill that came with two battery packs, one pack is still good but the other has a cell in it that doesn't take charge. voltage of the offending cell after charging the pack is 0.18v

The battery packs are made up with 15 sub-c nicad cells with a crappy capacity of 1300mAh. I was thinking about just replacing the one bad cell. Batteryjunction has nicad sub-c at 2200mAh but im not sure that would be a good idea insterting a higher capacity cell with the lower cpacity ones. Is this a problem? Then i got to thinking that i could rebuild this pack with much higher capacity nimh cells. So now I need some advice on if my old charger will be able to charge it safely. The charger that came with the drill is just a constant on charger that has no cut off or even a timmer. You just pull the battery pack off after 5-6 hours. The wall wart plug has a label that says output is 24v 200mA. Would it work to use this charger and just adjust how long i let charger for?

 

[PhantomPhoton]

Having cells of different capacities is a big no-no. You'll just damage other cells that way.
Rebuilding the pack is an option. Sounds like you have a dumb charger though without any balancing and no cutoff. That will damage NiMh cells which need a properly terminated charge. If you're going to pend the money on 15 high(er) quality sub C's you should also invest in a smarter charger. Many hobby chargers can be set to a 15-cell setting and wil do a better and likely faster job of charging. The catch is you won't be able to just drop the pack into a charging cradle. You'll have to hook it up to the hobby charger. In the end is t worth the money? That's for you to decide.

 

[metlarules]

Also keep in mind that Nicad handles high drain loads(drill)better than nimh. If you go the nimh route make sure that they can handle the current load.

 

[Fallingwater]

The catch is you won't be able to just drop the pack into a charging cradle. You'll have to hook it up to the hobby charger.

Or you can do what I did for my cordless dremel clone: open up the cradle, disconnect everything, solder two wires to the terminals and connect the hobby charger to those. :)
You still need to turn on the charger and set it on the right setting, but at least you don't have to futz around with alligator clips and magnets and whatnot.

 

[TakeTheActive]

NiMH is NOT Necessarily Better Than NiCD...

...The battery packs are made up with 15 sub-c nicad cells with a crappy capacity of 1300mAh... ...Batteryjunction has nicad sub-c at 2200mAh... ...Then i got to thinking that i could rebuild this pack... ...The charger that came with the drill is just a constant on charger that has no cut off or even a timmer. You just pull the battery pack off after 5-6 hours. The wall wart plug has a label that says output is 24v 200mA...

CLICK on my Sig Line LINK and read the Theory articles on "...Best Battery" and "...Right Battery". NiMH is not necessarily better than NiCD - it depends on the application.

Plus:

1. Can the NiMHs handle the drill's current draw (recommended at =<0.5C per a quick glance)?
2. How are you going to avoid deep discharging the NiMHs (below 0.9V)?
3. NiCDs have a better Cycle Life than NiMHs.
4. You'll need a better charger to take care of NiMHs.

IMO, you should stick with NiCD and compare the costs of a new assembled pack from the drill manufacturer ($$$ vs Capacity?) or rebuilding a pack with 15 2200mAh cells. Keep in mind, though, at 200ma (0.09C), it's going to take 16+ hours EACH TIME to recharge your pack.

Maybe it's time to buy a new drill?

 

[jayflash]

In this particular case you may be able to get away with another NiCd cell even if it's of greater capacity, as it's unlikely to reverse charge a string of cells that big. Why not use another sub-C of similar capacity? I'd avoid substituting a NiMH cell, however.

If I'm wrong about this, perhaps another CPF member will help to educate us both.

 

[Fallingwater]

Or you could just use five or six A123 cells. No discharge and lifetime problems with those...

 

[jayb79]

Or you could just use five or six A123 cells. No discharge and lifetime problems with those...

charging them would be a problem with the nicad charger.

 

[LumenHound]

Isn't the OP asking about recharging NiMH cells with a charger that uses a nicad algorymn?

In a nutshell, you can charge NiMh cels with a old nicad charger but you would be best advised to not do so. (
Should you deside to charge anyways, the NiMH packs may need to sit on the old charger for twice as long as the regular nicad packs.
Twice the capacity means twice the recharge time.

 

[NigelBond]

Ok thanks for the information everybody. I've decided not to bother rebuilding the battery as it just insn't cost effective or safe. The only reason I decided to ask was because I was going to make an order for some cr123 cells and while browsing, I saw the nimh sub-c cells and thought it would be neat to make a pack that has 4 times the original capacity. As it is, i still have one good battery pack for the drill and I have another newer cordless drill and two corded drills that work just fine so it's not as though I really need to do this.
I found sites with information one reviving dead nicad cells by zaping them with a capacitor or a series of batteries. I might give it a try if I can find the parts nesessary laying around the house. If not, I'll just leave it alone.

Why not use another sub-C of similar capacity? I'd avoid substituting a NiMH cell, however.

If I'm wrong about this, perhaps another CPF member will help to educate us both.

Well I was browsing battery junction for 123cells and thats the kind of nicad sub-c cells that they have. They got 4/5 sub-c cells that have the right capacity but it would be the wrong size. Probably wouldnt matter though.






Anyway, so now another quick question. Which cr123 batteries would you get? titanium or rayovac? I want the one with better shelf life as I don't normally use primaries. I usually use rechargables but I like to have some primaries on hand just in case. The last batch of primaries i had was the old made in china batterystation cells and they didn't seem to have great shelf life.

 

[TakeTheActive]

...I've decided not to bother rebuilding the battery as it just insn't cost effective or safe. The only reason I decided to ask was because I was going to make an order... ...I found sites with information one reviving dead nicad cells by zaping them with a capacitor or a series of batteries. I might give it a try if I can find the parts nesessary laying around the house....

Been there, done that, don't bother...

It'll work for a short time, but, IMO, for the amount of effort involved (with a pack vs a single cell) you'd be better off replacing the one bad cell with *ANY* equal or higher capacity NiCD (as long as you're placing an order ;) ). Yeah, it's NOT the recommended procedure, but, consider it an experiment - that's how we all learn.

Please let us know what you decide to do and how it works out.

Good Luck!

 

[jayflash]

Just bypass the bad cell. Missing one out of 15 is no biggie. However, new ~1500 mAh sub-C NiCds are commonplace. That 2200 mAh NiCd is OK to use too; it just won't be utilized to full capacity, but that's really no problem in your case.

 

[jayb79]

http://www.voltmanbatteries.com/servlet/StoreFront

 

[Fallingwater]

Just bypass the bad cell. Missing one out of 15 is no biggie.

Makes sense. A 1.2V reduction shouldn't drop the tools' effectiveness too much.

 

[NigelBond]

I had to try the battery zapping thing to test it. I cut out the bad cell and was about to start ripping capacitors off of an old mother board to build a device when i realized that I could do it with another drill battery pack. I just used some wires and an elastic band to hold the wires to the dead battery. Then I did several quick passes on the terminals of a fully charged 18v battery pack. Initally the dead battery which was reading a voltage of 0.08 volts shot right up to 1.12v Then I charged up that cell on a nicad AA charger using the wire leads and some aligator clips. The voltage went up to 1.3v which was a promissing result but now that i check it the next day, the battery is dead again.
While the dead battery was charging. I went ahead and tried to bypass the dead cell in the battery pack and surprisingly the drill is very strong with only one cell missing. It feels like almost full power. I dont completly understand this behavior. When the dead cell was in place, the whole battery pack measured over 18v on a full charge but the drill had no power. With the dead battery removed, the whole pack registers a lower voltage but the drill has almost full power.

I thought of another solution to rebulding the battery. There are cheap house brand drills that sell for 20 bucks at some stores. These cheap drills have cheap battery packs that sell for 10 bucks. The cells wont be the highest of quality but for 10 bucks you could not even buy the cells seperatly for that price. I might buy one of these batteries and transplant the cells.

 

[Fallingwater]

Why transplant anything if it works fine with the bad cell bypassed?

To answer your question: the dead cell was acting like a big fat resistor in your pack, dropping the whole thing's voltage under load. Now that you've removed it, the other healthy cells are doing their job fine, and the drill doesn't particularly feel the voltage variation.
To answer your other question: take one cell out of a series of three and the device using them is likely to become rather unhappy, but take one out of fifteen and the device won't much care. It's all relative. :p