Well, I got a second occurrence of Sudden Onset Turtle Syndrome. I had parked the car overnight in the street, low of -14F. I went out to move it to the charger before I left town, and got an immediate turtle at -10F. Leafspy screens show 66% SOC, 1 degree F battery temp, but 34mV max cell delta.
Unfortunately, I left town with both
keys in my suitcase, so the leaf sat there, forlorn and unplugged, for 4.25 days. Over that time, ambient temps were (low/high): -14 / -6F , -5/-16F , -13F / +1F, -10F / +12F, +0/+6F
The car was parked with no protection; weather was cloudy; and the car is white. So I was expecting that the 300W battery heater would kick-on quickly, and run for about 34 hours (10kw-hrs... until the SOC drained to 30%), then stop. After that, the pack temp would approach average ambient, which would probably be between 0F and 3F (which should be OK, as my manual says the heater doesn't kick-in until below 0F).
However, when I returned, I was surprised to find that the actual battery temp was 8F... and the traction battery had only depleted by 3 GIDs or "0.2% SOC". That could mean that the battery heater never came on at all (or my 2017 S doesn't have one?). Or that the LeafSpy-reported numbers are just wrong.
Anyways, the car has turtled twice: once 1/3 mile from home at -2F; and once immediately at power-on at -10F. In the second instance, cell balance showed 34mV delta, but after sitting for 4 days (and the pack warming-up to 8 degrees F), the cell imbalance improved to only 18 mV. So I'm suspecting that cold temperatures magnified the measured cell imbalance, triggering a spurious turtle; with a possible contributing factor of a non-working battery heater. I'm going to call it "Frozen Turtle Syndrome" instead.