There are heating pads available for batteries. They are often used in areas that are subject to extreme cold, as in -30 degrees F. or lower. AFAIK, there are two types; a pad that goes in the bottom of the tray, and a wrapper that goes around the battery like a coat.
As a battery gets "cold soaked", it loses capacity. These heaters will keep the battery warm enough to give full capacity.
It has been cold enough already around here that I am going to put a battery blanket on the Jeep. It may not be completely necessary, but it can't hurt!