Normally I can slip my running laptop into a backpack, go somewhere, and if it's no more than about half an hour later, it'll still be running. At the destination I plug in the AC power unit and all is well. I run it off of the AC unit before and after the trip, have the screen at less than full backlight brightness, and don't have any peripherals that burn power.

Sometimes the wireless switch accidentally slides in the backpack, and that causes extra power to be used and the laptop dies before I reach the destination. Sad, but so be it.

But sometimes the wireless switch is off, I've reached the destination in less than 30 minutes (typically 10-20 min), and I know the battery was fully charged, yet the machine is off. Is there a way to determine, after the fact, why the machine shut itself off?

I'm running Linux on a fairly powerful Gateway with 4GB RAM, fancy nvidia graphics, dual core cpu, chosen more for number crunching power than battery life, but should last easily for half an hour if not an hour.