$25 to $50 gets you an electronic charge regulator (solar) or AC powered
battery maintainer that will not overcharge your batteries.  I have two
batteries, left in my winter stored Westfalia (-35C at its coldest) charged
by a 15watt solar panel and controller.  Six years and counting on both, no
leakage.  Leaving a non-regulated charger on a battery will kill it much
quicker than sulphation due to low voltage.