Winterizing

If the batteries are fully charged, you can just disconnect them and leave them be for the winter; they will be fine in the cold (finer than in moderate temperatures, in fact). If they're stored inside, they should be on a maintenance charger of some sort, at least sporadically, to keep them charged.

Of course, if you're concerned about theft, removal is not a bad idea. It's hard for a miscreant to steal something that isn't there.