I don't have a Stromer, but I wonder about insulating the battery too in the winter. I took a ride the other day before the snow (upstate NY, about 25 degrees) and even though the battery was room temperature when I took it out and snapped it on the bike, the charge just fell off a cliff after only a few miles. The main indicator still showed four bars (max) but the voltmeter or whatever it is that shows current demand was bottoming out under any significant load, climbing a hill or going to a higher assist level. I don't think I could have made 10 miles, a far cry from the usual warm weather behavior.
Pretty sure there are heated vests ( powered by seperate battery) used in motorcycling , not very nice looking , but this should buffer the wind wrapped around ouside of battery . Good hand warmer on stops too.
I wish there was one.
But, it's not very difficult to make one using 1/8" or 1/4" neoprene sheet.
I have a Bosch battery sleeve on my bike and it really helps. It's -12'C here in Chicago and with sleeve I am getting 30 miles of range.