wrote

I have a friend with a PA-28-161, although that's not relevant to the
solution. He made himself a portable pack with a couple of gel-cell
batteries and a cigaret lighter type plug. Gives him an hour or so of
backup power for the radio and GPS if the plane's system goes down.
Easy to carry, charges externally and compatable with any 12V system,
with a plug. I don't know what the FAA would say, but if I lost power
in the clouds, I'd call it an emergency and take PIC responsibility.

I assume he charges it at home, and not from the plane's electrical system?

The voltage put out by a plane or car's electrical system is too high, and
will reduce the capacity and life of gel cells. A schottie diode put in a
charging line will reduce the voltage from an alternator, to make it safe
for charging the gel cells.
--
Jim in NC