Thank you @Lawrence! I don't like iCloud Photos because since I have over 50Gb of photos/video I don' want to pay the monthly fee for that. I take care of my backups...
The good thing is that over night the battery drain was just 4% in 7 hours. I did 2 things:
1. Turn wifi off: so that any major sync (like photos would stop). I made sure that any major cellular updates are turned off.
2. Turn off Siri & Search indexing in Photos - to minimize the "risk" of lots of background activity that might drain the battery.
I will debrief in a couple of days if it is ok, but as you said we need to identify the process/app(s) that are causing the drain (if the hardware is fully ok).
One more question: after a major update (ios 10 to 11) how long does ios take to reindex the files? Is it hours or days?
Thx!