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Can you fix a lead-acid battery that's charged backwards?

Discussion in 'Electronic Repair' started by mike, Jul 16, 2005.

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  1. mike

    mike Guest

    I bought a pair of electric scooters. They'd "fixed" the charger
    and managed to get the batteries charged backwards. Can I discharge
    them and charge 'em up forwards? Any special techniques? Fast/slow/
    charging?
    May be a moot point as I expect the speed controller box is fried too.
    mike

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  2. James Sweet

    James Sweet Guest

    Generally no, since you can't access the individual cells inside the battery
    separately you'll normally end up with some cells charging ok and others
    reversed, it's easier to just buy new batteries.

    The speed controller is probably pretty simple, crack it open and have a
    look.
     
  3. Can I discharge them and charge 'em up forwards?

    ROTFPML!!!!! Is this post serious?
     
  4. I actually have some experience with backwards batteries. I grew up in my
    fathers auto repair shop in the sixties. Back then if you hooked up the
    jumper cables to a generator equipped car you could start the car and the
    generator would reverse polarity and you could go on your merry way with a
    positive ground American car. We would just leave the lights on over night
    and then charge the battery the correct way and polarize the generator (jump
    the positive to the generator output lead for a second) in the morning. If
    the battery was good to start with it would recover fine.

    Jim.........
     
  5. I don't see how, as the cutout wouldn't pull in. That only happened when
    the generator started producing more than battery volts which it couldn't
    when reverse connected.

    One way to instantly polarize the dynamo was to flick the cutout in
    manually.
     
  6. Jim Adney

    Jim Adney Guest

    I think you should have a good shot at this. Try just discharging them
    first, use a light bulb that lights normally across the battery to
    discharge them, and let it go a couple days past the time when the
    bulb is no longer glowing.

    Then connect the charger properly and try to charge them, slowly. If
    they take too much current, insert the light bulb again as a current
    limiter, but you should be able to take the bulb out of the circuit
    after a day or 2.

    Let us know how this progresses.

    -
     
  7. Jim Adney

    Jim Adney Guest

    I guess I'll have to think about the WHY of this, but I've seen it
    happen. I agree that it's odd. The time I saw it, the owner had
    installed the battery backwards, then realized his mistake and put it
    in correctly. When I got to it the battery was correct, but I
    eventually determined that the generator was polarized backwards.

    Now I have to admit that it never occurred to me to wonder why putting
    the battery in backwards would reverse polarize the generator while
    putting the generator in forwards wouldn't correct it.

    -
     
  8. Bill Janssen

    Bill Janssen Guest

    I think the key to an explanation is that the cut out relay senses
    current direction as determined
    by referencing the magnetic field of the current to the magnetic field
    of a voltage fed electromagnet.

    Since both were reversed the cut out relay didn't have a clue as to the
    polarity.

    I hope I explained my thinking clear enough.

    Bill K7NOM
     
  9. In the UK, when I were a lad, most cars were positive ground. This of
    course made no sense to the average punter, and it was very common for the
    clueless to fit a new battery the wrong way round. And the symptoms were
    the battery didn't charge.
    As electronics became more popular, the change was made back to negative
    ground for new vehicles. And if the owner of an older vehicle wanted to
    fit a modern stereo, the easy answer was to convert it to negative earth,
    and I've done dozens. And always had to re-polarise the dynamo before it
    would cut in and charge. If the cut in had operated with the battery the
    'wrong' way round, that would have re-polarised the dynamo and made this
    unnecessary.

    Of course, this was with the infamous Lucas electrics. Could be that other
    makes work differently.
     
  10. jakdedert

    jakdedert Guest

    ....the Prince of Darkness....

    jak
     
  11. Jonathan

    Jonathan Guest

    I had this happen (by accident) with a NiCad pack. I just flattened it and
    re-charged it the correct way no preoblems.

    JD
     
  12. Jim DeClercq

    Jim DeClercq Guest

    In technical terms, you probably have a "sulfated" battery.
    In car sizes, 5 amps for 24 hours should get it charging in the
    right direction. Smaller battery, less current. It will take a
    while. You will be forming the battery, giving it its initial
    charge.

    Go for it.

    Jim



    : I bought a pair of electric scooters. They'd "fixed" the charger
    : and managed to get the batteries charged backwards. Can I discharge
    : them and charge 'em up forwards? Any special techniques? Fast/slow/
    : charging?
    : May be a moot point as I expect the speed controller box is fried too.
    : mike

    : --
    : Return address is VALID but some sites block emails
    : with links. Delete this sig when replying.
    : .
    : Wanted, PCMCIA SCSI Card for HP m820 CDRW.
    : FS 500MHz Tek DSOscilloscope TDS540 Make Offer
    : Wanted 12" LCD for Compaq Armada 7770MT.
    : Bunch of stuff For Sale and Wanted at the link below.
    : MAKE THE OBVIOUS CHANGES TO THE LINK
    : ht<removethis>tp://www.geocities.com/SiliconValley/Monitor/4710/


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  13. mike

    mike Guest

    Thanks for the advice.
    I fixed the charger and re-reversed the batteries. One scooter seems to
    be working fine. The batteries seem to be ok on the other one, but
    somebody ran it with insufficient air in the rear tire and hurt the
    inner tube. Won't hold air long enough to give it a good test.
    mike

    --
    Return address is VALID but some sites block emails
    with links. Delete this sig when replying.
    ..
    Wanted, PCMCIA SCSI Card for HP m820 CDRW.
    FS 500MHz Tek DSOscilloscope TDS540 Make Offer
    Wanted 12" LCD for Compaq Armada 7770MT.
    Bunch of stuff For Sale and Wanted at the link below.
    MAKE THE OBVIOUS CHANGES TO THE LINK
    ht<removethis>tp://www.geocities.com/SiliconValley/Monitor/4710/
     
  14. If the battery is sulphated, most modern chargers won't deliver enough
    volts to get anywhere near 5 amps due to the high cell resistance. Many
    are voltage limited to perhaps 14 volts or so, and might need several days
    to get any result - if ever.
     
  15. Jim Adney

    Jim Adney Guest

    That's probably for the best anyway. A sulfated battery should never
    be charged at a high rate. That will just hydrolize water and can
    damage the plates. IF recovery is possible, and it sometimes is, then
    your best bet is with a very small charging current over a number of
    days. I find that a week is typical for a car battery, but I had one
    that took 2 weeks.

    If you insist on instant gratification, you should just buy a new
    battery.

    -
     
  16. Yup. I was just worried someone might try using a variable bench supply to
    push through the 'required' 5 amps if their charger didn't do this.
    Think you'll end up doing this anyway. ;-)
     
  17. Jim Adney

    Jim Adney Guest

    It's generally pretty easy to tell whether you have a good chance or
    not. If you just measure the voltage while trickle charging it, the
    voltage should rather quickly come up to 2 V or more per cell. If your
    battery seems to be low by some multiple of 2V, then you probably have
    a shorted cell, which generally can't be fixed. If you can get to the
    2V per cell threshold, on a trickle charge, then you have a pretty
    good chance of recovery.

    Make sure the water is covering the plates.

    If there is a broken connection in the battery the charging voltage
    will be high, but it will drop quickly as soon as you stop charging
    and put a very small load on it. You won't be able to fix a broken
    connection, either.

    -
     
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