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How to check holdingcapacity of rechargeable NiCd batteries

Discussion in 'Electronic Repair' started by SISC, Jul 20, 2005.

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  1. SISC

    SISC Guest

    Hi out there in cyberspace,
    got a bunch of power batteries for various batt.operated tools and other
    things.
    After some cycles of charging and discharging some deteriorations of holding
    capacity can be noted ;some before their
    time of reasonable lifecycle. Yes a good charger will prevent improper
    charging.

    However most of these powerpacks consist of numerous individual cell like
    subC etc.
    Do I assume correctly, that if a powerpack goes down ,not all individual
    cells are to blame and some me be still quiet
    allright und of further use?
    How can someone check out which one to toss(into recyclebin) and which one
    to reuse?
    Any idea if evaluation can be accomplished with DMM or any specific
    procedure?
    I think many friends out there might be intertested to learn about!
    Please reply to forum to enlighten (me) !
    Thank You MS
     
  2. Charge the battery normally, then put it under load until the voltage
    drops by about 10%. The poor cells will have lower voltage than the good
    ones.
     
  3. mike

    mike Guest

    Consumer grade devices are designed with the cheapest/worst quality
    cells available and charged with the cheapest/most abusive charger
    available. High drain applications like drills are particularly hard
    on batteries. By the time the drill starts to slow down, you've alredy
    reversed a cell. And we just gotta finish that hole...and only one
    more... and the pack is toast.

    I've wasted months of my life over the years trying to make one good
    battery pack out of two bad ones.
    If the drill drills holes, use it. If it don't get a new pack or replace
    ALL the cells with new ones. This will cost you more than a new pack
    if you use good quality cells.

    If you're intent on using dead cells, here's what I'd do.
    If there's any white powder around the contact, throw it out...it's vented.

    Measure the voltage of each cell. If it's zero, put it aside to worry
    about later. Anything but zero is good. Zero is ok if the battery
    hasn't been used for years, but it's usually a bad sign.

    Charge 'em all up at C/10. (That's 100 mA or so for a cheapo
    Sub-C cell). for 16 hours.
    Measure the voltages. If any are much lower than the rest, throw them out.
    Let the rest sit on the shelf for a week.
    Measure the voltages again. If any are much lower than the rest, throw
    them out.
    Build yourself a 1 Amp or so current source. Car tail light and a car
    battery in series works.
    Put your meter on the battery and measure the voltage.
    Then apply the current. Quickly note the new voltage. Subtract to get
    the difference. This is a measure of the internal resistance of the cell.
    This measurement is all very sloppy, but after you've measured a few,
    you'll get a feeling for the voltage difference for a good cell vs. a
    bad one.

    Throw out the ones that have significantly higher delta-V.
    This takes some experience and experimentation. Once you get
    a handle on what the numbers should be, it'll go quickly.

    If you have any cells left, you might have a chance of using them
    in something. It's almost impossible to tell the capacity of cells that
    look the same. When you put them in series, the weak one reverses first
    and things start going downhill fast. Fast charging is also a problem
    cause you can EXPLODE one before the charger senses termination.
    Ask me how I know...

    If you want to add to your pain, go back and work on the cells you
    culled out at various stages of the process.

    And DO NOT solder on cells. If you cut the tabs so there's a little
    left on each end, and they're solderable, you can solder to the tabs.

    There's gonna be a snowstorm of people telling you soldering is OK,
    there always is, and how many hundreds of cells they've soldered.
    I've soldered a few hundred myself, but results on the first few
    weren't all that good.
    My yield went WAY up when I got a tab spot welder.
    Just don't do it.
    mike


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  4. Jim Adney

    Jim Adney Guest

    You can do this, but you this process may still leave you with a few
    weak cells. Nevertheless, I do this frequently, and with excellent
    results.

    Just put the battery pack on the charger for a normal charging cycle,
    then remove it, wait a few hours to let them stabilize, and measure
    the voltage across each cell. Cells with 0-1V across them are
    effectively dead. Replace them.

    If you have a pack with a lot of cells in it, 90% of which are bad,
    you should probably just replace all of them.

    -
     
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