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Fully charged batteries have a SG of 1.26 and freeze at -72 deg F
Half discharged batteries have a SG of 1.19 and freeze at -12 deg F
With no load on the battery, it will drop from full to half in about 250 days at 50 degrees ambient temp
Taken from "Boatowners Illustrated Electrical Handbook" Second Edition by Charlie Wing
You should have at least one good electrical reference book on hand.
Self Discharge is slowed by lower temperatures, that is why it's harder to start your car in the cold...the battery doesn't give up its power as easily.
The bigger question is 'do you have the proper charger for maintenance free batteries?' An old style ferro resonant charger will ruin that battery in a few years. A proper charging setup will enable at least 5 years and probably a lot more life for the battery. It's part of a system.