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reverse biased Ni-Cad cells

Discussion in 'Electronic Design' started by aurgathor, Dec 21, 2004.

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  1. aurgathor

    aurgathor Guest

    A quick check on a 6 pack containing D size Ni-Cad
    revealed one with -0.4V, another with -0.1V. I know
    that's not a very good thing, so here are my questions:

    a) is there some simple circuit that could be used
    to prevent this? I'm thinking of a parallel Schottky
    diodes, but preferebly with a Vf under 0.1V.
    How low can Schottkies go, BTW?
    b) how much damage can reverse voltage cause,
    and how quickly?
    c) what should be terminal voltage when discharing
    Ni-Cads? I plan to build a discharger from a battery
    holder where they would discharge through some
    series diodes and a resistor.

    TIA
     
  2. Tim Wescott

    Tim Wescott Guest

    a) Probably not.

    b) Think "trashed battery".

    c) 0.9V/cell.

    A better discharger would that takes a constant current until the
    battery voltage reaches the discharge point, then stops.
     
  3. w_tom

    w_tom Guest

    A voltage reversed NiCd is typically due to a weaker cell
    being in a stack of other stronger cells when the entire
    battery pack is too discharged. Sometimes a negative cell
    could be corrected by hitting the cell with higher current in
    the reverse direction. On older cells, this would
    (theoretically) burn out the reverse section. Then that cell
    would be recharged. This technique has not been very
    successful for me with newer technology NiCds. But then you
    would only be restoring that reversed cell so that operation
    can continue until a new battery pack is obtained.

    Again, the most probably reason for such a destructive
    failure - battery pack was permitted to discharge well beyond
    what should have been its lowest limit causing its weakest
    cell to become reverse charged.
     
  4. aurgathor

    aurgathor Guest

    OK, that's 2 diodes + current limiting R
    Other then faster discharge, is there any definite advantage of
    a constant current discharge?

    TIA
     
  5. aurgathor

    aurgathor Guest

    The pack was composed of cells from 2 different
    manufacturer.
    I took it apart and recharged the reversed ones individually.
    No problem as yet, but I'll be replacing all of them soon; until
    then I just keep an eye on them.
     
  6. Dave Platt

    Dave Platt Guest

    A quick check on a 6 pack containing D size Ni-Cad
    I don't think you can protect individual cells against this with a
    simple circuit. Reverse-biasing tends to occur as a result of
    overdischarging a battery pack... one cell drains to zero before the
    others, and the others continue to push current through the drained
    cell, in effect "charging it backwards". You can't prevent this with
    a simple diode protector before some amount of damage occurs, I fear.

    The best thing to do is stop discharging the pack when the voltage
    drops to a certain threshold. I've heard that 1.0 volts per cell is a
    really good place to stop... there's very little useful charge
    remaining in the pack at this point, and this is usually high enough
    to stop the discharge before any one cell reaches 0 volts.
    I understand that it causes the growth of dendrites (thin fingers of
    metal) inside the cell, through the separators. In effect, the cell
    develops internal short-circuits, which can cause the cell to
    self-discharge quite rapidly each time you recharge it.
    If you *must* discharge NiCd packs for some reason, don't go below 1.0
    volts per cell.

    My understanding is that full discharge of NiCds is not actually very
    useful at all. Doing it is beneficial only under fairly specialized
    conditions, and doing it improperly is more likely to damage the
    cells that it is to help.
     
  7. w_tom

    w_tom Guest

    Once some older tech NiCds would suffer a 'memory' problem.
    Solution was to complete discharge and then recharge them.
    Newer technology NiCds no longer have that problem. No reason
    to fully discharge a NiCd other than to put all to a same
    discharge state before recharging.

    And yes, cells from different manufacturers could have
    completely different characteristics. For example, some NiCd
    are for high load, short term use. Others are for long term,
    slow discharge. Some have longer shelf life and therefore
    will discharge faster. So you didn't know so much science
    exists in a silly little battery? Check Panasonic for their
    application notes on NiCds - if they still exist.
     
  8. Dave VanHorn

    Dave VanHorn Guest

    Both Nicad and NIMH have this problem, but it's been way overblown, and
    confused with other issues. I've seen it specifically in Sanyo HR-AUC cells
    where I ran 10-30 cycles to the same discharge voltage at C/2, then
    recharged normally at 1C. The result is a slight voltage droop during the
    subsequent full discharge at the same voltage point.
    Absolutely they do! Nicads work better at low temperatures, and high
    discharge rates, than NIMH's do.

    It makes for easy diagnosis of the battery by plotting the time/voltage
    curve.
    Power out is then easy to calculate, as well as joule output.
     
  9. Dave VanHorn

    Dave VanHorn Guest

    Not a stellar idea. All the cells should not only be of the same exact
    type, but they should also be the same age, both in calendar terms, and
    "experience".

    If they are put back into a pack together, you'll have the same thing
    happen.
    The reversed ones are permanently damaged now, to some degree, and will have
    less capacity than before, making this even more likely.
     
  10. Dave VanHorn

    Dave VanHorn Guest

    0.9 is what the makers reccomend typically, but there's not much run-time
    difference between the two.
    It does erase the "memory effect", but that's hardly worth doing, in most
    cell types.
    All it really does is spend cell life.
     
  11. w_tom

    w_tom Guest

    Just to avoid confusion, my paragraph was discussing NiCds
    only; not MiMHs. Some NiCds are for high power, short term
    operation. Other NiCds are for long term, low power
    operation. Mixing both types in a same stack can cause
    problems such as premature NiCd failure. A stack of NiCds
    (and NiMHs) should be same manufacturer, date, age, etc (as
    another has posted) to avoid problems such as reverse
    charging.
     
  12. Dave VanHorn

    Dave VanHorn Guest

    No confusion here.
     
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