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12 volt 20A battery charger using computer PSU

Discussion in 'Home Power and Microgeneration' started by Bud Weather, Jul 26, 2005.

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  1. Bud Weather

    Bud Weather Guest

    I agree with the theory, and I'd give it a try...
    but my gut sensation is that they'll go up in smoke.

    My 2 cents,

    Bud Weather
  2. Norman Webb

    Norman Webb Guest

    I am trying to recall my electronics 101 education.

    Can I series the 5V supply of three computers power supplies to give a 15V
    20A fast charger?
    Old AT psu's are a dime a dozen and much cheaper than a 20A 12 volt
    This would be powered by one of those el cheapo 500W generators to top up my
    225Ah batteries of my solar setup.
  3. Vaughn

    Vaughn Guest

    The answer may not be as simple as you first think. Remember that these
    power supplies are switchers and are not necessarily transformer isolated. For
    starters, do a simple test to find out if the power supplies are referenced to
    AC ground. If so, it won't work. That done, you are still not home free but at
    least you have a good start.

    Also, I remember that you have to do something to make them start. I seem
    to recall that they need +5 on a certain lead.

    If you could lay your hands on a surplus 24 Volt transformer and a Variac,
    you would just need to add a heavy diode bridge and an ammeter and stick it all
    in a case to build yourself a dumb fast-charger that would probably last you for
    the rest of your life. You might even be able to substitute a lamp dimmer or
    fan control for the Variac.

  4. If you can limit the current it might work, but the supplies will do
    foldback (shutdown on overcurrent) instead of a hard current limit at
    (say) 20A. Look for a surplus 15V power supply with adjustable
    current limit or a real battery charger...
  5. Norman Webb

    Norman Webb Guest

    Vaughn wrote in message ...
    The old AT type just needed a load to start.

    The "new" (last ten years??) ATX supplies have some type of signal between
    the psu and motherboard
  6. rebel

    rebel Guest

  7. Ptaylor

    Ptaylor Guest

    In short,No.

    Because the secondary side ground(5/12V neg wires) is connected to earth
    ground.(non-isolated)If you connect another supply to it in series (+ to
    -) you've just shorted the second supply to ground,through the first
    supply's ground connection-The magic smoke comes out.(possibly from both

    It may work if you left the earth grounds disconnected (and don't let
    the cases touch eachother/anything -they're internally grounded
    also.)..but this may also pose a shock hazard,if something were to go
    wrong and pose a short to the case.
  8. Norman Webb

    Norman Webb Guest

    Ptaylor wrote in message ...

    I haven't got the multimeter out yet because I haven't picked up the psus
    but that makes sense.

    Another option might be to change the 12V regulator to one with a higher

    The only reason I am playing is I have time to spare but not a lot of money.
  9. That maybe true, but maybe not depending on the manufacturer. A UL approved supply's does not have current carrying ground. Grounds
    are for safety. It might be that the secondary (-) is isolated from ground. Or can be easily isolated. Allowing the stacking of

  10. Ptaylor

    Ptaylor Guest

    In almost every PC supply I've come across,the AC earth ground and the
    secondary side ground (DC negative) are connected together through a
    thick trace on the circuit board,and it's metal case.If you know where
    to cut the copper traces with an X-acto knife,you *can* isolate them,but
    this probably a bit tricky for most,unless you have a bit of electronics

    One thing you might be able to do,is to parallel the 12V outputs of
    multiple supplies,but I hold no responsibility if it goes up in smoke. :)
  11. Norman Webb

    Norman Webb Guest

    Ptaylor wrote in message ...
    You would need blocking diodes from each supply I think and there goes
    another 0.4v-0.7v drop. :-(
  12. als

    als Guest

    Possible? Yes.
    Practical? Maybe.

    First item: isolate the cases. Connect each through 1 or 2 uf cap to earth
    ground. An old oil (motor run) cap will work fine. Wouldn't hurt to tie
    one case-to-case, also.
    Second: add a small load (enough to get regulation) to the +5v outputs of
    each supply. Usually, 20 ohms or so, but some may need 10 or less.
    Third: add some sort of current limiter to the output. A couple of auto
    headlamps would work.
    Be aware that most PeeCee supplies will not be able to supply a continuous
    20A load indefinitely. But should be able to charge at least a single
    225AH if it is not drained too badly.

    There are available, on the web, plans for the conversion of such supplies
    to a 13.8v output, but from the question you are asking, you probably do not
    want to tackle that type of project.

  13. Cool, do you have any references? I've found one that involves
    rewinding the transformer, but that seems a bit much, and one at that's not available online...
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