Another person explained it differently and that sounds correct:
VolkerP said: 08-07-2012, 03:38 AM
Regen is limited to 60kW power fed back into the 85kWh battery pack. We don't know if the smaller packs will have even less regen. The Roadster power gauge goes up to 40kW regenerated power. With Model S at nearly twice the Roadster's weight, there is twice the kinetic energy to deal with but only 50% more regen power. That will limit max regen deceleration to 2/3 of Roadster max regen deceleration.
In addition to that, Tesla smoothed the curve when regen kicks in. In the Roadster, regen jumps to 100% as fast as you lift the foot off the accelerator pedal. Not so in Model S. There is a short delay during which regen is building up. I don't know the length of the delay. But a quarter of a second would be enough to make the transition feel much softer. That was Tesla's design intent and they succeeded.
from
http://www.teslamotorsclub.com/showthread.php/9614-Performance-Regen-vs-85-kWh-Regen
Someone else also suggests that
Tesla may have capped regen at -0.15g, rather than at some specific kW rating.
And it make senses from a driving experience perspective not to design in the full possible regen that the generator and battery are capable of by simply releasing the accel. That would be too abrupt -- too much like a golf cart or go kart. A gradual curve is better -- or even better a cap on the amount gs.
But if you ever want to decel more than the curve or cap allows, your only option is friction braking. It could be that the amount of energy there just isn't worth trying to capture. It is certainly true that if you are very efficient and lucky in not over accelerating then you rarely have to decel more than the accel pedal release will allow. But the point remains that there is by design forgone regen opportunity lost whenever you need more regen than allowed by the accel release.
Again to summarize, what are limiting factors in how much regen to capture? The factors include:
1) what the battery can take. The Superchargers can deliver 90kW? so presumably we aren't close to this limit of the battery.
2) What the motor/generator mode can deliver. It looks like this is capped at 60kW?
3) The braking force on the tires before they lock/lose traction. This is different for using the front only, or rear only, or all four wheels. Certainly you can capture the most force from using all four wheels, next most would be just the fronts. Last would be the rear wheels only.
4) From a driving experience there may be too much "regen surprise" so a limiting factor would be how much regen can the driver experience before they feel like they are in a go cart? Should the car come to a short halt when the driver releases the accel?
