Ben W
Chess Grandmaster (Supervised)
I believe the reason that regen cannot get the car all the way to a stop is simple physics. The kinetic energy of the car (which regen braking is harvesting) is computed as:
Ek = m * v**2
So kinetic energy is proportional to the square of velocity. That means it decays non-linearly as the car slows down. At a walking pace, the car does not have enough kinetic energy to drive the drivetrain running as a generator, and the car keeps rolling.
Better physical scientists or engineers than me (I am a neuroscientist by training) will, no doubt, refine my explanation!
I'm not sure this quite explains it. Suppose you want to use regen to decelerate at a constant rate. The kinetic energy of the car at 5mph is only 1% of what it is at 50mph, but the required "regen" (in kW) at 5mph is also 1% of what it is at 50mph, so they should balance out. In my experience the regen applies a roughly constant deceleration (perhaps 0.1g) down to about 5mph, then tapers off below that. This is probably a comfort issue; the sudden stop at 0mph might feel rather abrupt if there were no tapering.
In any case, if regen is consistently applied from 50mph down to 5mph, that means the regen is capturing 99% of the available kinetic energy. (with ~30% losses converting to electricity of course.) The benefit of regenerating the last 5mph is negligible. But I'd still be curious to hear Tesla's official answer/reasoning about this.