system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why shouldn't we let our smartphones go to 0% battery and neither charge it to 100%?

My past smartphones with a lithium battery were always great in the beginning but with time they lost battery life so charging it twice a day became regular. With a new phone I would like to prolong the battery lifespan. I've read that the phone should not be fully discharged neither have the battery 100%. Why's that? How does that affect the battery lifespan?
gpt
First, the percentages your phone shows you are not the absolute percentages for the battery.  There's a safety margin on both ends to keep it in the happier zone.

Lithium Ion batteries suffer at ultra low charge levels.  It relies on a reversible chemical reaction that builds up material on the ends when its discharging and puts it back into the liquid when its charging.  Every charge cycle some of the material builds up on the ends permanently and doesn't return to solution which cuts into your battery life, if you bring the battery charge too low it builds up more.  If you bring it too low it can drop below a critical voltage where it can never be recharged again, this is why your phone doesn't let your battery get to within 10% of its *real* charge level, the reported 0% charge isn't 0% real charge.

Overcharging is another risk because the battery will begin to heat up because there's no chemicals left to store the power in so it just turns it into heat.  Battery powered devices have charge controllers on board that will cut off the charging so you don't have to worry about this.

For the most part, people with wayyyy better information about the specific quirks of your battery have already designed a good battery management system for it so you don't need to worry as much.  Your phone is actually happiest when the battery is at 100% and on the charger because it cuts power to the battery and just runs off the charger power so hours spent like this are hours that aren't going up and down using up charge cycles.