Bill, the optimum battery temp is 25°C or 77°F... by the time the battery temperture drops to 50°F you will have lost half of your batteries capacity. This is why I use battery warmer blankets and monitor the temperatures. I bought them at Advance auto parts... 60W or 80W are available.... it takes 4 minutes to raise the temp of you group 31 battery 1 degree... an hour to raise the temp 15 degrees.... and if you started at 50°F after an hour you'd still only be at 65°
I use my excess PVsolar charging during the day to keep the temps as close to 77° as possible.... only to watch their temps drop all night.
Excess heat for composting, or even electricity for a fan just won't be there most of the year,.
Then, there is more bad news... as the batteries get hotter than 77°F their efficiency also drops. (Hint... read the literature on your battery... look for the test temperature at the point that they give you the specification.... yep, 25°C or 77°F
Hope you give this one a lot of thought
Sleepy