Ive posted this in the schematic section also, Hope no one minds, Took me a while to figure out this one, as there was three things i wanted to acheive.
Didnt want to cut into USB cables,
Didnt want to loose the ability of the PC shutting itself down.
Didnt want to have to make do with a simple power switch.
Also had to have a tank battery, so it can make do without rebooting when you restart the engine.
And when its off, its OFF.
This circiut will prevent battery drain from USB devices, allows the PC to shut itself down, and has a tank battery, and also, a feature that allows you to run the PC directly off the battery, during slightly lower than perfect voltages while the engine is not running. (get the most from your battery's charge)
In this circiut, when the PC is off, its OFF. NO drain on the battery whatsoever. If you wanted to leave the System on for a length of time, with a USB device attached, powered, with the pc off, then a simple bypass switch wired around the contacts of R2 would solve that. Just leave the switch on and the USB devices will have their standby power.
All wiring from IGN, BATT, To the tank battery, and ITX power supply should be heavy cabling. The earth to the tank battery, and ITX power supply should also be heavy.
All other wiring can be light.