Thanks for your reply.
My experience with other products with batteries are that batteries tend to decline slowly over time. Not suddenly in a couple of weeks.
I noticed about 2 months ago that the battery of the Fairphone did started to decline somewhat. Three weeks ago it did drain within a day. Two weeks ago the battery did go down from 100% to 15% in 2.5 hours. (After that it did go down to 4% in minutes as always.) Because I know someone that did get a bloated battery I looked at my battery carefully I noticed that it was not straight anymore. If I did not knew about the issue I would have bought a new battery myself outside the warranty. Two days later the battery did drain in an one half hour! The defect is now clearly visible. I think several people that are buying a new battery do that without knowing that the battery has a defect, just thinking the battery is from a poor quality.
Off course others are buying a battery as spare in case they cannot reload the battery when it is empty.
I planned to replace my Fairphone somewhere beginning next year because I have several issues with the Fairphone. Original I did plan to do several years with this phone not knowing about the issues and thinking Android will be updated for one or two years. Before the Fairphone I did use second hand phones till they where 4 years old.
I thinking about a Windows phone because the (security) update police of Microsoft.