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Thread: Car power supply (battery charger) from old pc supply

  1. #1
    Variable Bitrate stimps's Avatar
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    Thumbs up Car power supply (battery charger) from old pc supply

    This is great for those times when you have to spend large amounts of time downloading large files to your car pc or configuring stuff etc. with the car parked in the garage and no engine running, and you dont want a flat battery. Sometimes a normal battery charger attached to your car battery hasnt got enough grunt to keep up with the car battery when its got a PC and everything else running off it. Most cheap crap type battery chargers fail at 2amps and the battery voltage starts to get dangerously low and then your car pc drops out/shuts down what ever

    Ive done this to two PC power supplys so far, taken out of old computers, first one was a old AT type. I removed all the hdd fdd motherboard power plugs etc and opened up the power supply and looked at the board. Now im an electrician so i know what im doing, but if you dont feel comfortable doing this and you really dont know what your doing, maybe its time to close it! but if you feel ok about handling elecrtricals which have live, dangerous voltages then continue

    The 12v output on some of these things can supply 4 to even 8 amps. One ive seen (but it was no good it was blown up) was capable of supplying 12amps!

    But they dont work too well as a battery charger as the open circiut voltage on these switch mode supplies is just that, 12v. But you can change that

    if you look at the yellow wire to the circiut board, somewhere along that track it has to lead to two resistors, in a T config. One goes to ground, one to the 12v. and the connection between goes to the regulator chip, (power supply controller)
    If you change one of these resistances values, you can set your output voltage higher. Typicaly you would set it to 13.8v

    I have done this on two supplies. First I lifted one leg on both these resistors and put in series with them a pot each. Usually 5k pots, set them to zero, and then with a voltmeter on the output, turn it on. It should still show 12v. When you slowly increase one of these pots, the voltage should go up. THe trick is to figure out which one needs the pot and work out how you can control the voltage externally with a permanently mounted 5k pot on the case.
    I now have two switch mode battery chargers, both controllable from 10.8 volts to 14.5 volts, with pots on them so i can set the voltage.

    The ATX type is the same, however it needs the low signal to the green wire to turn, so you need to put a switch in, however ive had problems with the control circiutry, shutting down the power supply when you connect a load. it seems it doenst like sudden changes in demand. There is no problem with a AT style like this.

    and the performance? well my AT one, managed a max of 4.5 amps into a 50watt head lights bulb, at 13.5volts. I didnt have a higher load that would not cause the power supply to shut down.
    And the ATX one, well when i managed to get it to stay on without shutting down due to its sensitivity to load, i managed a whopping 10amps at 14volts, before my meter complained!!

    Even with my AT supply, i can leave it connected to the car battery, and have my car pc on, and the usb power supply for the external usb sound card, the current block, (a current and voltmeter for the amp) and the 600 watt amplifier running! With low sound level of course.

    I left it running like this for several hours with music at a low level, doing other work to the pc and after i finished, the battery voltage was actually higher, proving the power supply had enough grunt to even charge the battery while everything was running.

    Ill post a picture up here soon.

  2. #2
    Raw Wave Confused's Avatar
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    Wow, good post and LOTS of information

    This will really help those with a garage to store their cars in!


    Garry
    Co-Developer of A.I.M.E.E
    www.aimee.cc

  3. #3
    Variable Bitrate stimps's Avatar
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    some photos

    Heres the AT and ATX style pc power supplies, converted to adjustable voltage battery chargers. Normally you would set the open circiut voltage to 13.8v, and then attach to the battery. Will suupply enough power to power the car with the pc on and even the amp, and still have enough reserve to charge the battery. You can see i've removed all the PC wiring and put on a terminal to attach the external wires to the car battery. You can also see the external mounted Pot (havnt put a knob on it yet)
    Attached Images Attached Images   

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    Maximum Bitrate binary.h4x's Avatar
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    awesome, nice and clean
    2007 Honda Fit Sport 1.5L SOHC-VTEC

  5. #5
    Clover Grayscale's Avatar
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    very sweet. i'll have to give it a go sometime even tho i have one of those 12v regulated power supplies from radioshack that goes up to 3 amps. Enough for me. I havent actually used it yet, it was from an old project, but im sure it works fine, and it was cheap. I don't need a 13.8v input either since i use no ITPS Not using an 12v devices...nothing runs off of 12v

    that a bad thing to do even tho nothing runs off of 12v?
    CarPC install is starting to come along again...

  6. #6
    Variable Bitrate stimps's Avatar
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    Grayscale im not sure you understand what this is for. This is a stand alone battery charger, capable of higher currents than normal units. For keeping a car battery charged up even with high loads running off them.
    Not sure what your questions means either. sorry dude.

  7. #7
    FLAC john1701's Avatar
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    im interested. any specs, diagrams care to share?
    The MC of Florida Car PC Meets
    CAR:2001 Ford Crown Victoria
    PC: Acer EEE 900HA with Win7 with CF 2.0.
    Memory:1G
    Drives:160gig
    WI-FI:Cingular
    GPS:IG2k9
    Screen:Xenarc 700TSV
    The Florida Meets Thread

  8. #8
    Variable Bitrate stimps's Avatar
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    each power supply is different, so you just gotta open one up and see if you can figure it out. All you have to do, is find the resistors that set the output voltage, (sense voltage for dc to dc switch mode chip, regulator whatever) and change it with a pot.

  9. #9
    FLAC john1701's Avatar
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    sweet
    The MC of Florida Car PC Meets
    CAR:2001 Ford Crown Victoria
    PC: Acer EEE 900HA with Win7 with CF 2.0.
    Memory:1G
    Drives:160gig
    WI-FI:Cingular
    GPS:IG2k9
    Screen:Xenarc 700TSV
    The Florida Meets Thread

  10. #10
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    very interesting. great idea. this would work great with newer computer power supplies too. mine, for example, has three 12v rails. each one of them can supply a max of 19 amps. 19 amperes x 3 rails = 57 amperes total. another decent idea is to keep your old battery charged however you can and hook that up to the car with jumper cables. that might not work too well though. my personal car is also a "police car." i found that one twelve volt battery just doesn't cut it. in doing research for some police equipment, i somehow stumbled upon 6 volt batteries with the same CCA and amp hours as 12v batteries. i bought four. hooked two up in series, hooked the other two up in series, then phased the two sets (hooked the two sets of two up in parallel). i've got two of my 6 volt batteries under the hood and the other two in my trunk, all connected with 0 gauge wire. i can run the onboard computer and all the equipment off the batteries for quite some time and still start up with no problem. i also have the lights and siren tied into the car alarm. having had so many dead batteries from the alarm tripping, and needing the extra power anyway, led me to go ahead with the configuration i just described. i recommend a 50-100 amp battery charger if you don't want to chop up a power supply though...much easier and less fiddling (and probably costs less. computer psu's are a bit pricey). in all honesty, though, that power supply idea was brilliant. still is. it's just not practical in my opinion. i do applaud you for the idea though...it was pretty good.

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