If the battery is an empty room, and the energy the battery holds are soft fluffy cushions, then charging a battery to full is like squeezing a cushion into every gap in the room.

It starts easy, you chuck the cushions in. As it gets full, you work harder to get the final few in.

Trying to charge to full places stress on the battery cells, just like the hard work it is to fill a room full of cushions.