I agree with Stephen about the voltages, and even have a car voltmeter
that plugs into the "cigarette lighter" as I've found that a good tool
to know if power is OK in car i.e., around 13.5 volts on the system when
driving. Over a number of years I've twice had charging problems and
nearly had car die on the highway.
The point is more that external power would avoid the myriads of
"special" batteries that are atrociously expensive in most cases. It
should be possible to provide some protection/regulation to permit
external supplies. I'm sort of surprised we haven't seen such an offering.
JN
Be careful using 12V batteries. The Asus may be expecting a regulated
(stable) 12V input. Batteries are unregulated power supplies. In
addition most 12V aren't exactly 12V. A 12V lead-acid battery is 12.6V
fully charged, under 12V when discharged, and up to 15V (or more) when
charging.