In the morning I got software 6.20 for my 6.45M. As a precaution,
After the first sync over BT and mobile: FREEZE !!
Restart the clock. Now you can see how the battery power decreases 1% in 3 minutes
Reset to defaults, same issue
Not that a 1% battery decrease in 3 minutes isn't unusual, especially after a firmware update when there is more to sync.