I work in a boatyard where boats are stored for a 5 month winter. We have customers who leave batteries on the boat(that's what I do with mine), take them home, and have us store them.
The batteries we store are charged at 2-4 amps once a month for 6 hours. I have no idea what people do at home or when they leave batteries on the boat. I will plug in my shore power and charge with the boat's charger once a month for a day.
Bottom line, The most important factor in keeping a battery over the winter is to put it into storage with a full charge and the electrolyte levels topped up. From there you can throw a charge at it whenever it is convenient. Indoor, outdoor, doesn't really matter. As long as it has a full charge it won't freeze.
The biggest killer is storing a battery with less than a full charge. That means 12.8volts after a 1 amp load has been connected for 10 seconds. Checking a freshly "charged" battery tells nothing until the "surface" charge is knocked off.
Trickle chargers are ok because they don't heat the battery enough to boil off the electrolyte. Big chargers are ok, but you can't leave them on for more than an hour or so. If the battery is stored with a full charge and has nothing connected to it, maintenance charging shouldn't require more than 2-3 amp hours to be restored. Since most trickle chargers put out about 1/2amp or less, it will take about 6-8 hours to restore a full charge with one.
