system
Given the following post, respond as if you're commenting on Reddit:
human
eli5 Why does the phone seem to lose battery faster when it's below 20%?

Is it a psychological thing like how time seems to move faster when you're late? Or is my phone lying to me?
gpt
It very much is lying to you, yes.

Phone battery display is an estimation, and its accuracy depends on the state and age of the battery. The phone knows roughly whether the battery is full, half-empty or empty, but knowing that it's exactly 19% or 20% is a good deal harder. You sometimes see a recommendation to change the phone fully and then let it discharge -- this is not for the battery's wellbeing. It allows the phone to gather better stats on the battery's health and calculate the percentage better, because if the phone has spent 2 years going between 60% and 100%, it might not have noticed that the effective capacity of the battery got much lower meanwhile.

Also, yeah, the percentage display isn't necessarily correct at all times. They know that people would complain if their phone was stuck on 98% (you can see that sort of thing happen to old devices), so the phone will often fudge things for display.