Good idea, I was really just messing with it. The reason it didn't turn on is they never went below the setpoint on the Tstat (50F IIRC) They are sitting on 1" of foam, and the wall is 1.75" of spray foam, but nothing on the living side. (Footnote: I found the battery issue.... In some of the next posts.)If you want to heat your batteries, I suggest insulation them. 1/2" foam is the minimum I think. Otherwise that 65W of heat will get lost to the atmosphere quickly. Another option is a fiber blanket. Thinsulate or similar works fine. Just don't put anything flammable within about 2" of the terminals, as a poor connection could cause a fire.
Also note those batteries are dense. It takes hours to change temp if they are allowed to get cold before turning the heater on.