why i call Li fussy

I agree that a 1.8A heater is not going to heat a battery very quickly. I also don't think Battleborn are a very good option in general and there heated batteries are poorly implemented, both Renogy, LiFeBlue and Relion all have a better heated battery if you don't want to roll your own.

However the rest of your assumptions are off by an order of magnitude. As I pointed out earlier, the heat capacity of a LiFePO4 battery (it is not Lithium metal) is around 1000 J/kg/c which is about 1/4 that of water. To go from -35 to +5C would require 166Wh (1000J/kg/c*40c*15kg/3600s). At 23W with good insulation that would be 6 hours.

For Relion, the heating time for -20 to 5C is 1 hour which would imply they are using a ~10A heater. Therefor the heating time for -35C to 5C would be about 1.5 hours, which seems pretty reasonable to me.

pianotuna wrote:
1.8 x ~13 =~23 watts. A 100 amp-hour battleborn is 31 lbs. It is going to take one heck of a lot of time to raise the temperature to that level where charging is safe.

The Li is a little more than 53% as dense as water. So it would take about 1.8 watts per pound to raise the temperature 1 degree F.

31 x 1.8 =~ 55 watt-hours.

If we started from -30 F and need to get to -4 F then it would consume 26 x 55 = 1430 watt-hours. That, unfortunately, is more than 100 amp-hours. It would also take 62 hours.

In my case, where I might be starting from -35, it is more or less impossible. Even if there are zero losses outside the case of the battery.

This, without detailed information from the particular maker, is just a guess--but I believe it may be in the ball park for "best case".