My TEG hasn't been able to communicate with my PowerWalls for several days. In Northern Virginia we've had temperatures in the teens and single digits (Fahrenheit). I'd noticed that they were charging more slowly, which is to be expected, but then I noticed that they were neither charging nor discharging at all, and my reserve (I leave 15% for power outages in Self Powered mode) dwindled to zero. I then noticed that my Tesla app showed 0x PowerWalls.
Called Tesla support a week ago, they said they'd push a new update the next day & I should call back if it still wasn't working. They updated the TEG on Friday December 29th. The app now sees one of the PowerWalls. It charged a little bit on the 30th, but I haven't seen any charging or discharge since (in fact I've been sending a bit of solar back out to the grid at certain times of the day).
So I called Tesla support again this week, and they confirmed that it's almost certainly due to the extreme cold. I was told that engineering is working on an update that'll keep the batteries warmed/preconditioned in colder temperatures, and that the system should just start working again on its own once the temperatures warm up. The rep also speculated that pushing the update to the TEG might not have caused it to recognize the batteries due to cold weather that day. We're supposed to go above freezing next week, so hopefully they're right and the TEG will again recognize the PowerWalls on its own.
I'll keep the board posted. As the second rep said, it's Tesla's first deployed winter for PowerWall 2, so hopefully they're learning and will have a software solution soon. Outside of this incident I've had zero problems since last summer.