Most people don’t realize that to get the maximum life out of of their battery in their iPhone (or other phone) you should not just charge it all the way to 100%. Actually you should not charge it past the 80% mark. Oh and try to never let it run down all the way either; When it hits 15-20% you should charge it. Running out of battery is bad for it. So to sum up you should keep the battery level somewhere between 20% and 80 % to maximize the battery’s life span.
Why? Batteries have limited full “cycles” and you want to try to stay within the cycle.
This is explained in details in this iFixit article
https://www.ifixit.com/News/90983/why-charging-your-gadgets-over-80-is-such-a-bad-idea
