Actually Andrew, I thought the same as you about constant charging but we both appear to be wrong! According to Battery University, constant charging may in deed be stressing the battery. In my case, my Wife and I got iPhone 6s Plus models at the same time. I am mostly at home and have kept my phone pretty much on the charger all the time. My Wife uses hers normally, charging overnight. My battery seems to be failing while hers is still working normally.

https://batteryuniversity.com/index.php/...