Powering ATX Motherboard on 12V Battery

Discussion in 'Power Supplies and UPS's' started by Drycola, Jul 14, 2010.

  1. Drycola

    Drycola Geek Trainee

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    Hello everyone,

    I need to run an old ATX motherboard on 12V DC batteries instead of the powersupply. I need that for a robotics project.
    I know that power supply provides +12V, -12V, +5V, & -5V DC to the motherboard and other components, but I'm little confused about the +12 and -12 issue, I mean can these be directly connected to the +ve and -ve output of the 12V battery??

    My question in simple words: How can I run motherboard on 12V battery?
     
  2. RHochstenbach

    RHochstenbach Administrator Staff Member

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    Welcome to HWF :)

    A small 12v battery won't have enough juice (amps) to power a computer. You could however try it with a car battery (which are also 12v).
    The -12v is the ground wire for the +12v. You'll also need to power the 5v rail, and if you system has a floppy drive, you'll also need to power the 3.3v rail.
     
  3. Drycola

    Drycola Geek Trainee

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    Thanks for your answer, RHochstenbach.
    Well, my battery has 7.4mAH, will that be enough?
    and my motherboard doesn't have a floppy, so I will only need 5V supply, will I have to provide this 5V from another source? or it is possible to convert my 12V to 5V and supply it from the same battery??

    Thanks again.
     
  4. RHochstenbach

    RHochstenbach Administrator Staff Member

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    No unfortunately not. 7.4mA = 0,0074A, which won't even provide 1W of power (0,8), and you'll need around 300W (25A) as a bare minimum.
    You could add another wire to the 12V output of a battery and put a resistor between it. Let's say you have a car battery providing 35A. You would then need a 7Ω resistor (35A/5V=7Ω, color scheme black-violet-black) to get 5V on that wire.
    However I do have to add that you could damage your motherboard when doing it wrong.
     
  5. Drycola

    Drycola Geek Trainee

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    Oops! I meant 7.4Ah, not mAh !!! :D :p
    Oh, and I wouldn't need a CD drive too...
    So, I'll need 25A? Maybe 3 Batteries on parallel would be enough??
     
  6. RHochstenbach

    RHochstenbach Administrator Staff Member

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    In that case it'll provide 88W of power, which is too low. But if you are using more batteries, the voltage will increase. If it was AC current, you could just use a converter to change volts into amps, but unfortunately it's DC so that's not gonna work unfortunately. Besides if it did work, your batteries would ran out of juice within minutes. So the best solution is a car battery.
     
  7. henry222

    henry222 Geek

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    - - - - From what I'm reading the pair of you are likley to result in a smoking ruin. - - - - -
    You must get some expert help for your robotic project, pref from someone that can see what you're doing.
    You probably need a car battery to provide enough Umph at start-up and to keep the supply reasonably stable. Car batteries are quite cheap, robust and easily charged. The 7.4AH battery sound like one of those Alarm stand-by batteries which can be tricky.
    =A wild alternative could-be a portable mains generator (eg petrol), if it really is a remote operation this will g'tee some length of operation. A 500W one should do, and available in camping/caravan shops etc. providnig useful power with little noise. It's also useful on holiday, trimming hedges etc.
    Only then does the issue of converting the voltages to suit the motherboard. I wonder why you don't use a Notebook PC...these may be run directly from a car-socket (using a fancy lead, no doubt!), - and with little difficulty from a spare car battery. But note that you need to monitor the battery....things are never as easy as they seem.
    You haven't said, but where is the Robotics getting its power . . . ?

    For these reasons, I suggest you find someone Local to provide the Supply-tech.
    Without this "sorted" the robotics will forever be a nightmare - er, IMHO.

    Goodluck.

    PS re-read earlier.... Do not parallel batteries, even if it may seem to do the trick - don't.
     

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