Typical rated power to the AC adapter for a laptop is close to 150W, but
output to computer is in order of 65W. Explains why the damn things get
hot.

So don't bother with DC from the batteries to AC and then back again. Get a
laptop power supply that has a 12V input. The iJuice (aka iGo) adapters
work well. Then it's just a matter of using a cigarette lighter outlet.