[RobertSmalls]

By mass, your battery is mostly lead, which has a very low specific heat: 0.031 times that of water.

A large, deep-cycle battery holds about 6qt of acid, so perhaps yours holds a gallon / 3.8kg. Add in 0.4kg of water to represent the lead, SO4, and PP, and you have 4.2kg. So 80Wh would raise its temperature about 16°C / 30°F.

That should bring you above freezing. How hot do you need?

How much heat is generated in the battery during charging?

Edit: I just realized there are THREE reasons you need insulation. One, since there's a thermal insulator between your heater and battery (the PP battery case), you'll need a much better insulator between the heater and the environment. Two, if heat is generated during charging and discharging, you'll need to retain that. And three, a fully discharged battery has H2O instead of H2SO4, and can freeze slightly below 0°C. Perhaps that third point was obvious to you, but not to me.