Add a back up light bulb in series to slow the charge rate and it will work
just fine, as long as he wont leave it on all the time. A much smaller ( dash
) bulb most probably could be used as a trickle charger.

A smaller battery ( Big lawn more ) with a DPDT, switch and a light bulb in its
charging circuit would be a good bet.

Mike

what would happen if he put 2 12v batterys in series & made 24 volt charger?