First data point from a couple years ago:
Originally Posted by @Michael
I have one Battle Born battery wrapped in the OEM 15 watt battery heater. The heater is essentially an Ultraheat tank heater sized to wrap three sides of the battery, with two 7 watt heating areas. Space is very tight, so the thickest insulation I can fit is 1/2" of XPS on three sides, 1/4" of EPS + a reflective felt pad on the other three sides. I didn't spend too much time fitting the XPS and EPS, so it has gaps.
With the relatively poor insulation and 15 watts of heat, I measured the following:
Ambient temp in the camper: -15F
Temp inside electrical cabinet: -8F
Least-insulated corner of the battery: +22F
Top of battery, under felt pad: +38F
Where the insulation is poor I see +30F temperature difference. Where the insulation is not-so-poor I see at least +45F above ambient.
My conclusion is that with better insulation (I.E. 1" of well-fitting XPS) the 15 watts of heat is more than adequate to keep the battery above freezing under most circumstances and above the low limit of -4F under nearly any condition.
Even with as little as 1/2" of well-fitting XPS I believe that the 15 watts of heat would keep the battery 40-50F warmer than ambient, adequate to prevent damage under discharge pretty much anywhere in North America, and above freezing in most of the continent.
Second data point:
I now have a larger battery that barely fits in the compartment, making it more difficult to fit insulation. It's a lithium battery slightly larger than a group 31, insulated with 1/2" of neoprene on four sides and top (but not bottom), and 80 watts of built-in heating via OEM internal heating pads.
With ambient temps inside the camper at -5F, the 80 watts of heating pads run with about a 50% duty cycle, so there is plenty of headroom for either colder temperatures or lower wattage heating pads.