Author Topic: mismatched nicads  (Read 2136 times)

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karlb

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mismatched nicads
« on: August 03, 2013, 08:28:46 AM »
I have a heap of nicads lying around of different capacities, mostly AA's and D's. Can I combine cells of different capacities to make a battery or will that be a disaster?
Can I compensate by parallelling two AA's before connecting in series with a D?
Will bypass diodes help? If so which way around do I connect them?
Thanks
KarlB.

Bruce S

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Re: mismatched nicads
« Reply #1 on: August 03, 2013, 09:53:24 AM »
NiCds can be mixed but not generally a good idea if building a battery pack. NiCds are constant current devices that can swell and even exploded (see some of my previous posts and pics!).
IF you really MUST do this then have a charger that is built to charge to lowest capacity battery (say 700ma) then only charge the pack for that max capacity.
Then of course to make sure the others in the pack are getting at least 80% completely charged you'll need to say in a range that no more than around 1260ma.

It would be best to first make sure ALL the NiCds are in good shape, hold their rated charge and build accordingly.
Post more info about them and your electronics skills and we should be able to help further.
do a search for Ghurd and Norm, too. Norm is a guru on rebuilding these gems :).
Best of luck!
Bruce S
PS>> Pics help too  ;)
A kind word often goes unsaid BUT never goes unheard

Norm

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Re: mismatched nicads
« Reply #2 on: August 12, 2013, 10:04:54 AM »
Good morning (here)
   first of all .....organize ! exactly how many NiCads of each size do you have?
  NOT a good idea to mix sizes .
  First of all just get all the AA's together.....then next size and etc.
I like to make a holder that holds 10 of one size , which makes 12 volts nominal.
( a hunk of old hula hoop tubing works pretty well for AAs......3/4 thin wall PVC for C size )
This makes it handy to figure things out .....take a multimeter to check the voltages
getting the ones that are closest to the same voltage to charge in a batch.
I like to charge AAs or Cs with a small wall wart for charging 12 volt packs 600-800Ma.
even for the inexperienced should be able to safely check on them every half -hour.

On zapping  0.00 v to 0.3 (more or less ) touch the leads of the 12 volt charger
........quickly....very quickly for just an instant to the battery terminals at the right
polarity.......be very cautious!  I recommend wearing a face shield  treat these like
a firecracker that someone is holding a match very close to the fuse .

I've found that zapping very seldom gives good results......like 1 out of 10 ?
usually if you have to zap it more than once to get any results ...just recycle it

Well anyway this should get you started. If you have any size less than 10 in number
just disregard them.
wishing you the best of luck  :)
Looking forward to your response  :)
Norm .



 

karlb

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Re: mismatched nicads
« Reply #3 on: August 16, 2013, 10:24:11 PM »
Thanks, a couple of very informative responses.

I have 10 AA's which I arranged into a 12v pack that charges from my panels and powers some LED lighting, and means I don't have to use mains lighting in a few rooms of the house. This works great.
I have another 8 AA's and 7 D's spare. And was maybe thinking of trying to arrange them into another 12v pack somehow, but now that I think about it, it's just skimping. The AA's will run out first, and I could try and use blocking diodes to stop the D's from reverse charging the AA's, but I might as well just get another two AA's and another 3 D's and make two more packs. It's worth the money to not have the hassle and the chance of it blowing up.
Hopefully this thread will help others too having the same idea.

KarlB.