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My NiMH cells reversed polarity! Oh no!

Discussion in 'Electronic Design' started by [email protected], Aug 13, 2006.

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  1. Guest

    Is this bad? Or will re-charging them fix the problem?

    The situation: I have 16 of them powering my laptop (18.5VDC input
    required, using my battery pack in lieu of the external switching power
    supply running on mains power). The pack lasts about 2 hours.

    Lately, though, I noticed my battery pack cuts out early. I measured
    the voltage with my AMM, and the needle pegged NEGATIVE.

    Is this bad? Must I throw the batteries away? Or will charging fix
    the problem?

    Michael
     
  2. Yes. Several cells have failed and have been charged in reverse by the
    others.
    Yes; they should be recycled.
    No; it is likely to make matters worse, by causing further cells to
    fail.
     
  3. vasile

    vasile Guest


    You have opened the batery pack and have full access to any of the
    NI-Mh cell terminals ? If yes, charge seprately cell by cell, using a
    fast charging algorithm.
    Then discharge separately every cell using nominal load (the same
    current which is discharging the battery your laptop) and measure
    discharging time. You'll discover there are cells which have a long
    discharge time and cells with very short discharge time. Probably some
    cells does not charge almost at all. So, replace defective cells with
    new ones, charge the whole pack with small current (0.1*C to 0.4*C
    untill the C=1.2*I*t equation is true (I is the charging current, t is
    the charging time, C is the cell capacity measured in amperes*hours and
    written on every cell).
    In 80% from those situations, only one or two cells are deffective, or
    the laptop charging circuit is broken.
    In the other 20%, the battery pack is very old (more than 2-3years of
    usage) and need to be replaced.

    greetings,
    Vasile
     
  4. Phil Allison

    Phil Allison Guest

    "John Woodgate"

    ** Failed ???

    No need for cell *failure* to ocurr for the polarity to go reverse during
    the discharge of a battery. When a cell is fully discharged, the internal
    impedance goes very high so it can easily take up a small reverse voltage.

    While not desirable, it takes a lot more to kill a rechargeable cell than
    this.



    ** Rediculous nonsense !!

    The reverse voltage cells will recover normal polarity when charged as usual
    and there is NO bad effect on the cells that have not reversed.

    The Wood Duck is quacked.



    ........ Phil
     
  5. Phil, he's measuring the TOTAL voltage as negative, so there are more
    reverse-charged cells than good ones. Unless he can charge the cells
    individually, which means breaking open the pack, that situation is
    unrecoverable.
     
  6. Phil Allison

    Phil Allison Guest

    "John Woodgate"

    ** Failed ???

    No need for cell *failure* to occur for the polarity to go reverse during
    the discharge of a battery. When a cell is fully discharged, the internal
    impedance goes very high so it can easily take up a small reverse voltage.

    While not desirable, it takes a lot more to kill a rechargeable cell than
    this.



    ** WRONG - that is simply quite impossible.


    ** Not possible in the scenario as described.

    The PC cuts out the battery load at a positive voltage.

    The Wood Duck is more Quacked than I thought.






    ......... Phil
     
  7. It's what he told us. If it's wrong, it's not my error.
     
  8. Phil Allison

    Phil Allison Guest

    "John Woodgate SNIPPING MANIAC "

    ** Failed ???

    No need for cell *failure* to occur for the polarity to go reverse during
    the discharge of a battery. When a cell is fully discharged, the internal
    impedance goes very high so it can easily take up a small reverse voltage.

    While not desirable, it takes a lot more to kill a rechargeable cell than
    this.



    ** WRONG - that is simply quite impossible.


    ** Not possible in the scenario as described.

    The PC cuts out the battery load at a positive voltage.

    The Wood Duck is more Quacked than I thought.


    ** The OP's heading say "cells" .

    The OP's text alone is ambiguous.

    You failed to exclude the impossible.

    That is a massive blunder.


    ** Believing impossible things is your error.

    Your claims about cell failure are in error.


    Quack, quack, quack ......





    ......... Phil
     
  9. John - KD5YI

    John - KD5YI Guest


    Make sure your "AMM" test leads are connected properly. Put the red lead
    into the AMM's + (plus) hole. Put the black lead into the AMM's - (minus)
    hole. Set the AMM for 20 or more volts. Touch the red probe to the battery +
    terminal and touch the black probe to the battery - terminal.

    Is "the needle pegged NEGATIVE"?

    John
     
  10. Phil Allison

    Phil Allison Guest

    "John - KD5YI"


    ** It won't be - as that is simply impossible.

    The get whole pack reversal, you would have to charge it in reverse.

    The OP may have one or two reversed cells - or else he is an utter idiot.




    ........ Phil
     
  11. Go on. You just want me to call you Phyllis Alison again.
     
  12. Guest


    Oh, sorry, meant to say that 4 batteries out of the 16 had negative
    voltage. Sorry I didn't clarify this. (Whoa... if the *whole*
    battery pack came out as negative, that would be weird!)

    I tried this: I put one bad one in a flashlight (in series with the
    "wrong" polarity with another good battery, to ensure the batteries
    drained), then charged. Curious, I took the bad one out, and it
    registered a positive voltage of about 0.3V.

    So, is it fixed, or is it ruined forever?

    thanks,

    Michael
     
  13. It may be OK. Charge it slowly by itself, being careful to control the
    current to C/20 or so, and stop when it shows 1.2 V. It won't be fully
    charged, but if it holds that 1.2 V above 1 V overnight you stand a
    chance. Do that with each one that shows a negative voltage. Then charge
    the whole string.
     
  14. John - KD5YI

    John - KD5YI Guest

    Yeah. I just saw his most recent post. Sorry to bother.

    Thanks, Phil.

    John
     

  15. Go ahead. It makes her day.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  16. Face it, Michael. You're in love with Phalluson.
     
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