From your other post:
“Same here. Today I charged mine full and unplugged it around 9am and I did hiking, walking and climbing for 3h30m and after 9 hours, my battery went down to 55%. What gives?“
I’m not sure what your expectations are, but that seems pretty reasonable to me. Apple promised 18 hours battery life, you have 55% after 9 hours.
And this doesn’t even take into account what I assume to be continuous activity tracking for 3.5-4 hours of that time.