Fenix 7: Why do activities use so much more battery?

I would guess because The watch is checking the Sensors way more offen in Order to correctly track your Fitness. 

I did not enable any sensors. My guess would be that the cpu clock is or something like this is raised in an activity. But I did not find anything to lower the power usage of an activity. My app would only need to check the sensors every minute.

4390130 said:

I did not enable any sensors

So, you've disabled the HRM, GNSS, and ANT+/BT sensor connections for all activities?

4390130 said:

My guess would be that the cpu clock is or something like this

My guess is that watchfaces are running in a different chip area to save power. Here the structure of a 7x.

It's because when you're just in smartwatch mode (aka, just viewing the watchface), the watch goes into a sort of low power "standby" mode where the CPU is clocked down and doing only the bare minimum amount of work. As soon as you start an app or activity, it wakes everything back up and clocks the CPU to full power, even if the app itself isn't really doing anything. Perhaps that isn't the most efficient way for the Garmin OS to do things, but that's just always how it's been.

If the power saving mode is activated the battery usage of the activities are exactly the same. So i also think that activities must be running on a different core or with a higher cpu clock.

So there is no way to reduce the power usage of an activity yet?

Take a look at the Power Manager.

Do you mean the power saving mode in the Power Manager? Enabling the power saving mode does not reduce the battery usage in my case. It still uses 3 to 4 percent of the battery per hour with power saving mode enabled.

I don't have a Fenix, but I did battery endurance experiments with my FR255.  With wrist HR and GPS disabled, I did not notice significant battery consumption during a one hour walking activity, where it was just using accelerometer and altimeter to log estimated pace and climb info.

See https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-255-series/329907/one-for-fellow-battery-nerds if you're interested..