I do start an exercise, yes. Again, may be worth powering off then powering on the watch. In my case, again, something was clearly thrashing in the background

I do start an exercise, yes. Again, may be worth powering off then powering on the watch. In my case, again, something was clearly thrashing in the background and eating nearly an extra 40% of the maximum battery capacity over 16 hours. It's only happened once, but restarting the watch (in which presumably the process was killed then restarted) resolved the excessive power usage. So far today I've been up since 6AM. It's 11 hours later. I've done a 7.2 mile walk (as an exercise). I have 69% of the battery remaining with another 6 to 7 waking hours left in my day.