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.
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.
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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