Any trickle charger in the 1 amp range. I like the
Battery Minder Plus; I feel that desulfation works. There is always an on going debate about desulfation among some folks, but it seems to extend the life of batteries I've used it on, and seems to keep them closer to new-capacity longer. Of course, this takes years to measure, but I'm sticking to it.
A fully charged lead acid battery freezes at about -65°F. A discharged battery freezes easily. A frozen battery is junk and cannot be rejuvenated. Batteries self-discharge:
Quote:Lead-Antimony Batteries:
Antimony is used as a stiffener in the grids of lead plates of deep cycle batteries. While antimony makes the plates stronger, it also causes battery cells to self-discharge more rapidly. Self-discharge is a deleterious discharge because it creates a hard lead sulfate that crystallizes and ultimately destroys the battery.
The only way to avoid self-discharge is to keep lead-antimony batteries on a charger when not in use. Self-discharge lessens as temperatures decline, so if it's cold enough, then a full time charge isn't necessary. Be sure to do a full charge at least once a month in very cold weather, and every two weeks if it gets above freezing.
Lead-Calcium Batteries:
Batteries made with lead-calcium plates, such as the so-called maintenance free types, have low self-discharge as long as the weather isn't too warm. These batteries can be left fully charged for several months without experiencing sulfation. It is good to bring them to a full charge a couple of times during the winter.
http://www.amplepower.com/primer/winter/index.html 
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