Last year, we bought a special battery charger from Coast Energy 818 361-6461, called a BatteryMINDer. It can be left permanently hooked up to 1 to 4 batteries, and it claims to keep them in good shape.
According to the documentation that came with it, as batteries discharge, they suffer from “sulfination” which is the formation of crystals of a sulfur compound that reduces a battery’s capacity. The BatteryMINDer is said to prevent this, and it even has a button on it which causes it to send out pulses that break up the crystals, if needed. (I have no idea how long it takes for sulfination to occur.)
I kept 4 of our batteries from last year plugged into it for the whole year, and they seem to be in great shape. Basically I just plugged the batteries together in pairs using the connectors that FIRST gave us, and then used a jumper to attach the first pair to the second pair. Then I attached the BatteryMINDer to one of the batteries and it kept all 4 fully charged.
This is not a fast charger. It is only 1 amp. But if you want to preserve your batteries, I recommend it.