Why are cheap Nicads rejected by intelli-charger

After only a few cycles, some very cheap AA NiCads I had bought were being rejected by my "intelligent" charger.
Is there something about the chemistry of these cells which causes them to present the wrong electrical characteristics for the charger?
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With no specifics to go on -- charger model, no-load & loaded voltage for the cells that it rejects -- this is speculation. However, some chargers check the terminal voltage before starting a charge and refuse to start a charge on cells below a threshold.
This can occur where several cells were run in series (a common setup) and they were not all of similar capacity and at similar states of charge. It may be that the "cheap AA NiCads" didn't all quite meet the spec'd capacity; some may have been much less.
Regardless, if you have a benchtop power supply with a constant current setting, set the current limit to 200 mA and give the rejected cell a few moments of charge. You'll want to see the terminal voltage rise up to around 1.2 V. If it doesn't pretty quickly then the cell may indeed be beyond recovery. If it does, toss it into the charger and it will probably charge okay.
--
Rich Webb Norfolk, VA

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Smart fast chargers reject high-impedance cells (it's not safe to blast 'em @ 1C--too much heating). Such chargers can false on overly discharged cells, or, sometimes, dirty contacts.

Yep.
-- Cheers, James Arthur
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-dave- wrote:

It's possible the nicads need to be zapped. Look up nicad zapper online. I've donet his successfully for a few batteries. It always brings them back to life but in some cases they die out pretty quickly. The method I used was to take a 12V battery and go ground to ground and + to +. You only touch the + for about 1 second so. It will get warm. It can get pretty hot if you leave it on their longer. Which I've done and just about burned up my jumper wires(they are pretty cheap though).
The idea is to force enough current into the cell to burn up small fiberous connections that build up between the anode and cathod. You can check and see if this is the case by checking the resistance of the bettery. In my case they were all reading 0. (when it should be non-zero)
I tried making a circuit at first but simply did not have the capacitance to do anything(the recommended was about 15mF and it just wasn't enough).
In many cases I made several "taps" of about 1s each with an off time of about 1 to 2 seconds. It will charge them up(voltage wise anyways) almost instantly and should allow the charger to work(it's checking for a voltage). In some cases I simply could not get some cells to stay charges. I put them all in the charger(it was a 24V back of 20 cells but I zapped each one individually) and the pack was charged. I then checked the next day and some cells were much less than 1.2 while others, that were original "bad" kept their charge. The ones that wouldn't keep their charge potentially may just need to be zapped longer but I didn't want to try it.
The method I used may be dangerous though so only try it if your willing to take responsibility for the outcome.
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Yes. Use a transparent shield or face shield and do it where the alkaline products won't damage stuff when the cell bursts.
DB
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Like the other guys said, plus this:
Never connect cells with different histories in series. The weakest nicad with be forced into a voltage reversal and may blow up, if it's not internally fused.
DB
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-dave- wrote:

I've had this happen when a cell became too deeply discharged. I keep a cheap "dumb" charger around for this reason, pop the cells in that for a few minutes and then the good charger will recognize them.
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-dave- wrote:

The charger is intelligent enough to see and reject certain brands.. No joke; i have one (different that whatever you have) that only accepts one brand of cell for charging.
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