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!