If 50% of your capacity disappeared, your rated range remaining would have been appx. 140, not 257. Energy consumption goes up in colder weather due both to energy inefficiencies and denser air. Also, were you using the A/C to heat the car? The butt warmers? Did you warm up the car? Any or all of these would also have consumed capacity.
Your battery is perfect, right? 279miles @ 90% is 310 miles. You're using 22 miles of EPA-rated range for 12 miles driven. That's because it's now cold out, and you're using heat.
Mine was 258 before the update. One thing I have noticed is that last week when I still had 263, I changed as usual to 90, but in the morning it showed 91%. This was not the first time it happened and now that I think about it, I think every time that happened, than is when the range drops.
Keep in mind that the ambient temp has changed since you bought your Stealth P. When you were leaving it unplugged, it was warm outside. Later, when you changed up your routine, it certainly has gotten colder outside. I think temp change leads to BMS drift.
The thing is, there are likely multiple things happening to cause lower Rated Range numbers. For some, like myself, it could be temperature related. For others, software updates seem to cause a BMS drop, or it could be buffer size changes, etc. And for the unfortunate few, it could be real battery degradation.
