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Thread: smart guys needed: why is there a voltage drop in this situation?

  1. #1
    Variable Bitrate
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    Surrey, BC, Canada
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    smart guys needed: why is there a voltage drop in this situation?

    Quick background: I'm running a fairly high power system, microATX board with Athlon XP 2000+ plus 4 USB peripherals and a desktop harddrive. Power is supplied through an ATX power supply hooked up to an inverter. I also have a tank setup with a 12V 4.5Ah backup battery and a MBR3045 diode. Circuit would be mainbattery->diode->backupbattery->shutdowncontroller->inverter->powersupply->mainboard

    Problem: System doesn't like to run without the engine running. Also, it doesn't survive a crank.

    Tests performed: Using a multimeter I tested voltage at the inverter's +12V input point. Without the engine running, without computer running 12.5V. Without engine running, with computer running 11 to 11.5V (inverter beeps and usually cuts out after 30 sec. or so). With engine running, computer not running 13.5V. With engine running, computer running 12.5V.


    As you can notice, there's a 1V drop when I turn the computer on. WTF is going on? Is the computer using that much juice? HELP!

  2. #2
    Raw Wave rando's Avatar
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    With no load, your tank battery supplies the 12.5v. Once you fire up the inverter and computer, the tank battery is unable to supply the required current and the voltage drops until the diode is forward biased and turns on. At that point main battery is supplying the bulk of the current but the voltage is now too low to make your inverter happy.

    Since you're running an athlon and deskto HD, your system is probably pulling around 100W. Add in an inefficient inverter and your total power demand at the battery might be 150W or more. That's more than enough to make the voltage on your tank battery drop.

    Then again, I have no idea what I'm talking about.

  3. #3
    Variable Bitrate
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    I also tried to run the system off just my backup battery, but when I press the power button on the computer the system turns on for a second and I see the voltage reading drops to 10V then the system turns off and voltage goes back up to normal (12.5V).

    Running the system solely off the main battery has a similar effect but the system will run for about 30 seconds with the voltage dropping to about 11V and then it will shut off and voltage returns to normal (12.5V).

    Do I have to have a full size battery as my backup? This freakin sux...

  4. #4
    Variable Bitrate
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    Well I just tried using a fatter wire between batteries and guess what, it improved the voltage under load... the system actually runs now without the engine running. What a stupid minor thing and yet it ****ed me off so much. Tomorrow I'm upgrading all my wire (I was using 18 guage throughout).

  5. #5
    Raw Wave rando's Avatar
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    lol, yah 18G is not going to do it for your main power wires. I use 4G for my main power lead. The power runs to a distribution block. From there I have 10G to the computer and 8G to my amplifier. 18G is more appropriate for you turn-on wire and short power leads running from your computer PSU out to devices. If your going to run long leads, a larger guage is preferred.

  6. #6
    Constant Bitrate
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    May 2004
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    18 gauge? You're lucky there was enough voltage drop for the inverter to shut itself off. You have no idea how much of a fire hazard you can create if you use the wrong gauge wiring.

  7. #7
    Newbie
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    Jun 2004
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    using a stiffing cap would be a good idea aswell getting rid of those crapy inverters, but using a 1Fared or 1/2 cap will help with your cranking problems, after a while your battery is gonna drop to 10v on its crank cycle adding a cap will keep 14v to your system untill the system starts charging. dc/ac inverters make noise and steel power, more so the cheap ones, invest in a low wattage MB aND hd add a good dc/dc inverter makes life much better

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