I installed 2 apps to monitor system resources and understand which processes / apps are using up my battery.
Surprisingly the CPU is mostly idle and nearly no system time is used. I did not find an app which allows me to monitor system kernel threads, but I assume that the battery is not used up by kernel resources as the monitoring apps show that no system time is used (processor time used by system).
That matches what I see in battery usage (system settings). The battery usage statistics list applications not running and is not changing.
Nevertheless, my battery is still draining very quickly.
I compared the power my battery is delivering with the iPhone 6s of a colleague.
We both had no apps running. I had Bluetooth and WLAN turned off whereas my colleague had both turned on.
Nevertheless, my battery was delivering 2 times the power than the iPhone of my colleague.
Decreasing display brightness to "zero" does not help to reduce power consumption.
Any idea how I can find out which "part" of the iPhone is using all that battery power if no app is running?