Keeping wet cell batteries on float forever is not good for them. They will dry up. The two best things you can do for them: either leave them fully charged and disconnected, or get a small solar panel to offset the small amount of self discharge of wet cells.
Read the Ample Power Primer, tech tab, www.amplepowerprimer.com
and the West Marine Advisors, here: The West Advisor: West Advisor Articles
scroll down to electrical
They explain battery charging systems.