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shorted (?) lead-acid battery

Discussion in 'Electronic Components' started by aurgathor, Jan 27, 2005.

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  1. aurgathor

    aurgathor Guest

    Yesterday I started to charge a car battery that was sitting unused
    for quite a while with a 1A charger. (my other one is broke)
    Initially, the battery was around 6V, that jumped to over
    15 when I connected the charger, but dropped in about 30
    seconds to around 10V after the oxide layer broke down.
    A couple of hour later I measured the battery, and it was around
    10.40V, low enough that I was wondering if it had a shorted cell.
    Then I measured it again before going to bed -- 8.50V!!.
    This morning -- 6.50V, and the charger was too hot to touch,
    and I disconnected the charger at this point.

    I've never seen anything like this. I figured since the charger can
    supply so little current (was 850 mA @ 8.5V) there's no way it
    can overcharge or damage a car battery. I know something isn't
    right since after a day, even with a wimpy charger like that it should
    read more than 6.5V. Any idea what's happening? How did the
    cells become shorted, if that's the case? (the battery wasn't moved
    or jarred since charging began)

    TIA
     
  2. Dave Platt

    Dave Platt Guest

    Yesterday I started to charge a car battery that was sitting unused
    From what I've read of lead-acid battery design, my understanding is
    that one not-uncommon failure mode is that deep discharging (whether
    due to excessive load or due to failure-to-keep-charged-while-in-
    storage) can result in a physical deterioration of the plates in each
    cell. The plates weaken, disintegrate, and the rubble falls
    to the bottom of the cell. You can end up with chunks of the plate
    material forming bridges across/around/through whatever is normally
    used to keep the plates separated, and this shorts out the individual
    cells.

    If this is what happened to yours, then the charger wasn't at fault.
    The battery was fatally rotted even before you hooked up the charger,
    and the electrochemical changes which occurred during the charging
    process (and perhaps converted the rubble back to a conductive form)
    simply made the existing damage apparent.

    As far as I know there's no practical way to recover from this... take
    the battery to a dealer for recycling, buy a new one, and treat the
    new one a bit more gently :)
     
  3. Kim Clay

    Kim Clay Guest

    Three discharged (but not completely) cells = 6V, the other three cells
    are at 0.0V each due to internal shorts.
    The three non-shorted cells are taking a charge (3*2.2V= 6.6V). One
    cell still has a hard short. The other two cells are charging (from
    0.0V) somewhat faster than they were discharging (as they were/are
    shorted internally or they wouldn't have started out at 0.0V).
    Three non-shorted cells are taking a charge (3*2.2V= 6.6V).
    Now two cells have an internal short that your ~1A charger can not
    overcome & other cell is closely behind (trying to internally short your
    ~1A charge).
    Three cells partially/fully charged & the other three cells fairly
    solidly shorted.
    You started off with 3 discharged cells & 3 shorted cells. You ended up
    with 3 charged cells & 3 shorted cells.
    It didn't damage the battery - the battery was defective before you
    started charging it :)
    Because (from your first sentence) "battery that was sitting unused
    for quite a while". Car batteries self discharge just sitting on the
    shelf. A discharged battery that sits is quickly self destructing
    internally, as you have found out.
    A _fully_ charged battery that is in good condition _may_ sit on the
    shelf for 6 months or more & still function OK when put into service. It
    is not good for it!! At least put it on a charger once a month to top
    it up. There are _much_ better ways to store a battery - as on a float
    charger.
    Take a look at the "CAR AND DEEP CYCLE BATTERY FAQ" at
    http://www.uuhome.de/william.darden/index.htm for all the good info :)
     
  4. aurgathor

    aurgathor Guest

    That is not correct. A discharged battery can go down that much,
    or even more without the cells having internal shorts. Case in point,
    a couple daysbefore this one I had a battery that measured 1.7V!
    (in car, with a light left ON). In a couple of days, that battery was
    measuring 14V, and worked just fine. I just checked it today --
    12.85V.

    Based on the voltage measurements after a few hours of charging
    (10.40V -> 8.40V -> 6.40V) some cells must've become shorted
    during the charging process. Alas, I no longer remember the voltage
    after the first 10 - 15 mins of charging -- that may have been close to 12V.
    I expected sulfation, and while that's no good either, it's less
    deadly, and it's easier to reverse than an internal short.

    I guess material fell from the plates, created a highly conductive
    sludge in the bottom, and that's what shorted the cells out.

    One of my friend got a 100A or so charger, so I wonder if
    I can blow the shorts with that. ;-) Of course, I'd need to
    find a good enough container first incase the whole battery
    blows.
     
  5. Kim Clay

    Kim Clay Guest

    It is correct! Your original question was about a car battery that had
    been "sitting unused for quite a while" which you measured at "around
    6V".
    Yes, one can deeply discharge a "good" battery down to 0.0V & after
    the load is removed the cells _may_ recover. A new battery discharged
    accidently to 0.0V & promptly recharged will probably be OK. A old
    battery discharged in the same way may not make it.
    The one key point is that the battery is not left in the discharged
    state for long. Once the load is removed the cells start to recover. A
    non-defective battery will bounce back (even without any charging
    applied) to around 11-12V within 24 hours & probably less. I have never
    timed it :)
    Ran the battery down & it got promptly recharged.
    The cells were shorted from the beginning. These are not 0.0 ohm
    internal shorts but some higher resistance that have bridged the plates.
    It seems your ~1A charger was enough to overcome the internal cell
    short (or bridging resistance) of two of the cells but not the third.
    This resulted in the 10.40V reading early in the charging process.
    During the rest of the charging process the internal shorts within the
    other two cells slowly decreased in value resulting in your 8.4V reading
    & the ending 6.4V.
    Indeed it may have been "close to 12V" near the start. If the charger
    can overcome the internal short a cell will attempt to recharge. The
    plates just fell apart & created a harder short. It took a while for all
    three cells to end up the same way - hard short internally.
    If you have a lead acid (12V) battery that is at ~10.5V or better it
    has a chance of becoming useful no matter how long it has set. If it is
    <10.5V it may (probably) have a shorted cell. These are measurements
    taken of an unknown battery that has been sitting for >24 hours.
    You were attempting to recharge a battery in which 3 cells already had
    internal shorts (or the battery would have been closer to 12V to start
    with). These cells had been in this condition for an unknown length of
    time. The shorts just became more solid as the charging progressed.
    Even if you could magically make the shorts disappear the basic
    trouble is still there. The cells are falling apart internally.

    Kim
     
  6. Art

    Art Guest

    "Good Container" ?? Ever see one of these batteries blow up tossing
    residue, poison, acid, etc all over the place when someone attempts to apply
    a high charge rate to "Blow Off the slight shorts"??
    Not a nice sight exp when it comes in contact with someone's face or
    exposed skin. Not a right pleasant experience in the least. Personally know
    at least one person who lost one eye and has facial burns as a result of
    trying to do what others are implying may work. Buy a NEW BATTERY!
     
  7. What's a new battery cost versus the risk of battery explosion?
     
  8. John Doe

    John Doe Guest

    Cost doesn't really come into this equation.... ;-)

    And as I've said, I won't try this without a container that can
    safely hold an exploding battery.
     
  9. That you believe will, probably, as far as you know, maybe not cause
    anyone any bodily injury, you hope. Have fun...
     
  10. Ross Herbert

    Ross Herbert Guest

    The most likely scenario is that 3 of the cells were shorted and the
    remaining 3 were not shorted BEFORE you commenced charging. The 3
    un-shorted cells would however have been high resistance thus causing
    the initial voltage reading to be high because the charging current
    low. As the 3 un-shorted cells commenced to absorb charge their
    internal resistance reduced thus the charging current increased. As
    the charge on these cells reached 6.6V (approx) they would still have
    taken nearly the maximum available current from your 1A charger and
    this item would have been very hot.
    No... these cells were already shorted before charging commenced.
    Not true. A fully sulfated battery is almost impossible to rectify
    even partially.
    Stupid idea. You just can't BLOW away a build up of sludge at the
    bottom of the cells by pumping in excessively high current. For a
    start it would destroy any good cells in the process even if we
    disregard the hazardous nature of such an action. Even assuming that
    you could disturb the sludge and get it to distribute itself
    throughout the electrolyte where it no longer shorted the cells, it
    would still have to settle somewhere eventually. Guess where it would
    settle?
     
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