Why don't you use some solar cells to top up the batteries? Lead acid batteries are pretty immune to overcharge, so if you arrange to put about 14.2 v. into then during most of the daylight period, you might keep them charged forever. What you do is use a zener diode at the input from the solar cells so that only a voltage greater than, say 12 v. gets to the batteries. (You might want to do this through a boost regulator which can take any voltage above, say 5 v., and provide a 14.2 v. input.) At night, the zener prevents the battery draining through the solar cells. It's admittedly a crude charger system, but I used this scheme to operate yard lights indefinitely with only the solar charge applied.
Of course, you have to either buy a suitable solar cell array, or gin one up from single cells.