Old Green, hey? I recently found myself living in near-Canadian-like weather conditions, minus 30 degrees F. Three year old battery must have been frozen, won't turn the starter! So, you apparently have a block heater? In your opinion, for -30 degrees F. temperatures, which would be the best application, a block heater or a battery heater?
My experience, limited, of course, has been that if the Jeep engine starts, it will eventually warm the engine coolant and engine oil. Fortunately, I had a 750 CCA Orbital battery to replace the original 600 CCA Jeep battery, and got to work although the Jeep was mighty cold even though I warmed the engine for about 10 minutes before heading out.