Well, the engine compartment isn’t airtight, but it’s true that windchill will be greatly decreased inside. I just brought it up because the heat dissipation of even moderate windchills in extreme cold is 3 orders of magnitude higher than a couple watts of heating. Even with no windchill whatsoever, I expect that the equilibrium temperature of a battery heating itself with ~5 watts is only a couple degrees above ambient when we’re talking -30 to -40 sorts of ranges. That’s just not going to help you much.