Fix iPhone battery problem

A couple of days ago my iPhone 6s suddenly started loosing power. Rapidly it went from 80% to 1% (but 1% could last very long time).
It was fast to charge up, but again it rapidly lost power and went to 1%.
Since I could use the phone for a long time when it stated 1% I didn't think it was the battery that was the problem.
I did some (a lot) search and found this advice that could help, and it was much easier than resetting the phone and much cheaper than a new battery.
The article I found, was on a Danish part of "ON X" and it was started in 2012 but has comments right up till now from greatfull readers that has been saved by the tip:


Shutdown the iPhone
Remove SIM card
Power On iPhone and accept that there are no sim card inserted
Shutdown the iPhone
Insert SIM card
Power On iPhone


And - voila - problem is gone (ok, some had do to do it twice)