I went with Guest brand on-board chargers and keep 'em plugged into the wall anytime the boat is parked at home. I've got a dual 5x5 on the TM batteries and a 6 amp single on the start battery wired parallel on the 115 VAC side so I can hook up one plug with an extension cord. The new 3 stage smart on-board chargers allow you to do this due to their "maintenance" mode and they are fine for Lead Acid, Gel and AGM batteries alike. Many folks claim the life of the battery can depend heavily on the quality of the charger. There's lots of web sites that will educate you on anything you want to know about deep cycle batteries and smart chargers. They mostly recommend that you maintenance charge 'em anytime they aren't being used.