I strongly recommend an external regulator for deep cycle batteries. It
helps them to charge at the rates they are designed to accept. You can, if
you want, "dial in" a higher or lower amperage for faster or slower
charging, or just set it on auto. If you're just running out to the market,
crank up the regulator and get the most you can out of your short trips. You
can find these in a variety of flavors (and prices) on some of the boating
and marine equipment sites.
Jim C
2 boats and 2 vans
Mexico