Battery went from 16% to 11%

Hi, Riad! This is normal. Sometimes iOS displays one percentage and then after restarting shows another. I believe this is because restarting your iPhone causes iOS to re-calibrate its battery readings, and it's not something to be concerned about!

Have a great day. :)

Weird things can happen temporarily when the charge drops below 20%. I would avoid shutting down the phone and then powering back up immediately. Give the phone a few minutes to rest, then power it back on.

You do know that even when you shut down the phone......it is still doing things, correct?

For example, when your phone is "on", go through the process to shut it "off".

Do you see that message just below the slider about "iPhone Findable After Power Off"? The phone is still sending out location information, even when it is "off".

It's actually NOT healthy to let your battery drop below 20%. Try to keep your battery better charged.

You're welcome.

Of course! Thank you. :)