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Sealed led acid battery wont charge?

Discussion in 'Hobby Electronics' started by Darren, May 15, 2004.

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  1. Darren

    Darren Guest

    I have pulled an old 7ah 12v sealed led acid battery out of a UPS
    When I put it on the charger the volts jump up to 40v, why is this? is the
    battery stuffed?

  2. Wild Wizard

    Wild Wizard Guest

    yep and so is your charger (shouldn't be giving 40v to charge a 12v battery)
  3. Arpit

    Arpit Guest

    Well maybe the charger isn't stuffed. Its probably a current limited
    supply or equivelant, relying on a proper battery to bring the voltage
    down to a sane level without sinking too many amps.
  4. Well, you probably need a better charger.
    If you want to try and recover the battery, you will need to leave a
    charger attached for a a few weeks. Some batteries occcissionally come
    back to "life" but quality varies.
  5. L Chung

    L Chung Guest

    One of more of the series connected 2-V cell(s) have developed a high
    impedance. No charging can return the battery to normal.

    Before you bin it, see what I have done to such a battery to get some
    of its life back.


  6. Hello Mr Chung,
    you are a man after my own heart :)

    on some of my old sealed batteries, sonnenshien brand,
    like the ones shown in Mr Chung's web page, you can
    see the outline of the connecting link between the cells.
    It is very easy to drill and separate the link between the
    cells. Turning a 12V battery into a larger capacity 6 V
    battery was easy. I have successfully hack sawed away
    one complete side where there were two dud cells to make
    a slim 6volt battery.

    about charging. Bypass the dud cells first as Mr Chung
    described. Drilling and soldering is easy.
    Try your charger with some series resistors to limit the

    I like the constant current method. Fixed current for 12 to
    14 hours or whenever I remember days later. :)

    Here is a simple circuit for a constant current charger.
    Look at figure 7 on page 5.
    Resistor R1 sets the maximum current.
    For 1 amp, find a resistor around 1.2 ohms 2W or slightly more
    in your junk box. Parrallel up whatever you have in your junk.

    If you want to make your charger even nicer, you can add
    a normal voltage regulator after the constant current regulator.
    Build up Figure 5 on page 5.
    That way your charger would be set and forget ( almost.)
    Constant current of around say 1 amp when the battery
    is dead flat and then the current drops to a trickle later on. So
    you can float charge at whatever voltage is recommended.

    You will need two LM317s one for the constant current section
    and one for the constant voltage section. You could probably
    use your existing charger as the supply to the two additional
    current and voltage regulators.

    Have Fun,
    John Crighton
  7. Darren

    Darren Guest

    Thanks for your replies

    The charger I have is fine (super nova 250s) I guess it thought there was a
    bigger battery installed so it put the volts up.

    Thanks Chung I will take a look at your site.

  8. Russ

    Russ Guest

    The Super Nova is a constant-current charger, so in an attempt to get the
    current up to a target value (I don't know what that charger does in Pb
    mode - 1 amp or something?), it pumped up the volts - what you should have
    noted was the charge current at 40 volts. As others have mentioned, it is
    probably stuffed (or one cell is), but letting the Super Nova have a go at
    it for a day or two certainly won't hurt it, as it will automatically drop
    the voltage once the battery starts to charge.

  9. Darren

    Darren Guest

    You can set the nova to charge at any current.

    It wont charge the battery at all, it just jumps up to 48V and says "battery
    voltage to high" or somthing like that.

  10. Russ

    Russ Guest

    Sure, that's kind of what I meant - you specify a fixed charging current,
    and following the I=V/R thing, the charger ramps up the voltage to the load
    (battery) until it sees the target current being drawn.
    Which means, as others have said, that the battery, or at least one of the
    cells, is buggered. I was assuming the Super Nova might keep trying to
    charge the battery at that maximum voltage until something happens, but it
    sounds like the charger is deciding to give up straight away.

    As I mentioned before, one method I've heard of is to try a high voltage for
    a sustained period in the hope that the battery will come good, but
    obviously you need to be around to drop the voltage once it starts charging,
    or else it will draw excessive current and something will probably blow up.

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