Why does EV charging slow down at 80%?

Because a lithium-ion battery cannot safely take high current when it is nearly full. Battery University says a high current fills a battery to about 70% quickly, then the charger must cut back. That is why fast charging times are usually quoted from 10% or 20% up to 80%.

Lithium-ion charges in two stages. First the charger pushes a steady current while the voltage rises. Once the voltage hits its limit, it holds there and the current tapers. Battery University says the stress builds in the second half of the charge, toward full, and that EVs use step charging to lower current as the battery fills.

Cold batteries charge slower too. The guide to the EV charging curve covers what this means for road trip stops, and the state of charge entry explains the percentages.