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Why are cheap Nicads rejected by intelli-charger

Discussion in 'Electrical Engineering' started by -dave-, Nov 1, 2009.

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  1. -dave-

    -dave- Guest

    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?
     
  2. Rich Webb

    Rich Webb Guest

    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.
     
  3. 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.
     
  4. James Sweet

    James Sweet Guest


    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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