Roy brings up some good points and since there is no budget mentioned, I'll add in the cheapest solution that I just started using on incidental batteries I have around the house (extra's, unused vehicles, lawn tractor). What I did was take a wall plug=in transformer (the kind you have for small electronics) and replaced the phono 1/8" jack plug with two alligator clips. The wallwart I used is only 300ma and for two 6V's a more appropriate current would be 1.5 amps or so. These transformers can be bought at electronics distributors for less than $10. Alligator clips can be had at an auto parts store or big box store.
If you disconnect a battery cable you could put one of these small tranformers on the batteries and trickle charge the batteries. You would have to check, occasionally, to make sure your water level is good but it is about the cheapest battery maintainer I've found.
