Bill Kearney wrote:
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.

All these little adapters work well on a regulated voltage, but with an
alternator running, the voltage can be far from regulated.