I returned my 1st one thought it was defective.
Apparently design issue, my 2nd one does the same thing. When it detects a Dynamic Work Out it uses up the battery in 2 hours.
In Settings, Workout Detection I turned off Alerts and all Activities to Detect. Battery no longer gets killed playing tennis, bowling, etc.
I haven't experimented with turning any part of it back on to see which part or parts were killing the battery