Posted for "Brent S. LNC IV-P" <viejo@attbi.com>:

I think that using any quality 12 volt brand portable drill battery would
work. It could be mounted in the manufacturer supplied charging unit and
kept fully charged by the airplanes charging system. Then when needed it
could be wired to automatically take over the electrical duties till
landing.
Brent S.
Lancair IV-P