I am using a 2 1/2 amp hr gel cell. It keeps up the lights at idle well enough (not prolonged idling). I have a magneto. I may go to a LED replacement for the tail/brake light, so that should help.
I fabricated an aluminum holder for the battery. It has room for Styrofoam cushions for the battery, and holds the battery via velcro strap. PM me if you want a sketch of it.
1963 Atlas (Original Owner)
The Second Law (of thermodynamics) rules.