[Kevin Sevcik]

The single biggest killer of lead-acid batteries is leaving them sitting around uncharged. They sulfate and likely will lose a lot of their capacity.

One solution is to always leave the batteries on a trickle chargers, as was suggested to you. If you have a quality charger, it can handle trickle charging without overcharging and exploding your battery. You other option is to pull the batteries out once a month or so to charge them back up.