Why does fast charging only works until about 50%-60% of battery capacity and charge rate slows down after that?

Question

Why does fast charging only works until about 50%-60% of battery capacity and charge rate slows down after that?

In: Engineering

Answer 1

Imagine stuffing small soft foam balls into a box. Initially it is easy to stuff the balls in as there is a lot of space in the box and there is no need to compress the foam balls much. But as the box fills up and you want more foam balls in it, you have to press harder and squeeze the balls a bit in order to fit. This slow you down.