Using a panel to charge your Lead Acid is a more common approach that airplane and heli guys use for field boxes to make their complete setup lighter. A 18AH Lead acid battery weighs about 13# and the panel only about 3#, This saves about 10~15# in extra weight from using a larger battery. Unless these guys like flying in the rain, it almost ensures that their battery never goes dead during a day of flight. A
I skip all the mess of batteries and just built a car to run with no batteries or gas, just bright clean sunshine.