system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: My phone always tells me to unplug charger when reaches 100%; am I hurting my battery by leaving it plugged in?
gpt
*possibly*

Lithium Ion batteries, which are the most common type in modern phones don't necessarily like being at 100% all the time. This is a stress on the battery.  The general advice is to charge to 100 (or even a little under more like 80) and let it run down to about 30 or 40 - charge it back up again and let it run down a bit - for ideal operating conditions.  

Li-Ion hates extremes so avoiding overcharging (charging beyond 100% - uncommon in modern phones) and deep-cycling (letting the battery drain entirely) is important. 

Lithium Ion batteries also don't like heat, for example which is caused by leaving it on the charger and is generated by having the battery fully charged. 

This said - your phone manufacturer also knows this, and if they're worth anything they don't keep trying to charge your battery once its full. They stop - let it discharge a bit - then send it back to 100.  In fact some manufacturers don't use the full capacity of the battery except when calibrating. 


Realistically, environmental factors like heat are much more damaging than having it at full charge (which is why some devices turn off and lock themselves out if they start overheating) so don't worry about it too much. But yes, you are stressing the battery a little bit.