I also noticed that current draw in shutdown or standby remain the same unless you really cut power somewhere between your pc power supply and the battery. This is really simple to explain:
ATX specs that a power supply should always have the 5Vsb (stand by) ready for the mobo. This is done for stuff like suspend-to-ram (S3), Wake-On-Lan, or Wake-On-Whatever-you-can-set-in-your-BIOS.
Since the system has to monitor for example the network card's input to detect a WOL package ('magic packet'), or the keyboard to power up on space bar, or the mouse or modem or so, even after a shutdown all ATX systems that support waking-up need that 5Vsb and will use it.
Funny thing is that Supend-to-RAM needs virtually no power extra as compared to a complete shutdown. So the current draw on the RAM chips and virtual circuits presents the same load on the 5Vsb, as a shut-downed ready-to-wake-up system would do.
Meaning if you don't hard-cut power to an ATX system every shutdown, you might as well use Suspend-To-Ram (power state S3) since you will be using the 5Vsb anyways!
My system for the moment draws about 150mA when in Stand-By or Shut down mode. It's a EPIA 10000, rest doesn't really matter unless you use power-on USB devices lika a mouse or keyb or so. I have a battery that has 70Ah rating. I guess I could leave my system 'suspended' about 5 days with no problems.
Morale : If you don't hard-cut power to the system every shutdown/hibernate (same effect - just different boot up times) you might as well use Suspend-To-Ram since you're wasting the energy anyways (2 sec. boot up anyone
).
Personally I think a car system has enough power to support a DC-DC PC solution with Suspend-To-Ram. After all your car radio draws current to save it's settings the same way ! You get a truckload of nice features like instant-on; instant-off; use while engine off; combined with smart shutdowns without manual intervention ever.
That way when someone comes in your car and sees the system they'll ask : Hey, where did u get this awsome stereo
Just my 5€cents
