So I installed the firmware in the picture above that was the 520 mb version. When that one was installed, the other one I had also downloaded that's about 410 mb said that there was a newer version already installed (presumably the 520 mb file), so it
didn't install the 410 mb version of the firmware. Fine.
I had my SP7 plugged into power the whole morning while I was doing this stuff. So I unplugged at 100% battery and let it sit for 1 minute. No issue. 2 minutes. No issue. 5 minutes. No issue.
I'm thinking "problem solved!"
I just got to a coffee shop to get some work done. I'm not plugged in. I stop engaging with the computer for less than a minute to order my coffee, and it shuts down. I checked the battery and it was at 88% battery when I booted back up.
So there may be some validity to the 80%-89% range being a problem.