@highfigh
When I said "the charge of EV batteries is not linear with time", I was thinking of a sigmoid curve, instead of a straight line. In the example below, imagine the horizontal axis is Time, and the vertical axis is Percent Battery Charge. As charging progresses, a plot of that is best shown as a sigmoid curve, not as a straight line.

For the various reasons described above by

@Trell this is due to the electrochemical nature of charging batteries, and also a deliberate choice by the manufacturers to maximize battery life.

The middle part of the curve (labeled Exponential phase in the example below) is close to a straight line, but above & below that the sigmoid curve deviates from a straight line. The slope of that middle part of the curve (roughly half charged) may resemble the slope of a straight line, but with limits.

So, if you ask "what is the rate of charging", it depends on where on the sigmoid curve you are. (Sorry for the high school math lesson

.)