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Model S regen

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Doug_G

Lead Moderator
Global Moderator
Apr 2, 2010
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Ottawa, Canada
If I understood you right, yes, we did discuss this (sort of); was told that, unlike in the Roadster (where one could practically do one-pedal driving given the aggressive regen and no-creep?!), the Model S will feel 'more mainstream' (my words). For instance, during the beta rides at the factory, to slow down from the 70+ mph that the drivers hit on the straight stretch, they couldn't rely on the more-subtle regen alone and were using the hydraulic brakes heavily.

I don't think this is by intent; rather, it is a limitation of how fast the batteries can swallow regen energy. The car is significantly heavier.
 
I don't think this is by intent; rather, it is a limitation of how fast the batteries can swallow regen energy. The car is significantly heavier.
Hmm.. well how fast can the batteries take energy? We know they should at least be able to do 90 kW.

Back of the envelope, if the car weighs 4000 lbs, 60 to 0 mph purely on 90 kW regen would take 7.3 seconds.

Tesla says out of the motor they get 360 hp =~ 270 kW, so if you got that full regen, you'd get 2.4 seconds. (seems plenty quick)

The gauge on the Roadster suggested its regen was limited to 40kW.
 
I suspect that there are complexities to the way regen works that limit it for other reasons.

A quickie search turned up this possibly relevant example? (although maybe not since it describes DC motor regen) :
http://www.cafeelectric.com/curtis/regen/index.html
...Older designs had to switch the batteries into a parallel configuration so that battery pack voltage was lower than the generator voltage. Without such complexities, current wouldn't flow...

Anyways, having the motor run in regen mode may not be as simple as reversing the capability of forward-drive mode. There may be some additional circuits or constraints that limit the max regen power as compared to the max power the inverter & motor can make when accelerating.


 
The Australian Electric Vehicle Asn: cntrolling regen
...Some drives (e.g. AC propulsion's, limit regen to 40% of forward power).
One reason is vehicle control, the other is the power and heat issue if the vehicle were to regen down a long steep hill.
Regen returns power to the batteries at the torque and current limits set up in the controller. Regen max voltage is often a preset max for the controller and battery pack voltage may need to be set to match that...
 
Another question is at what point regen becomes torque-limited (i.e. motor-current-limited) as opposed to power-limited (i.e battery-current-limited).

The current Model S torque graph shows flat torque up to ~57mph, where the peak power point is, and then power gradually falls off to ~80% of peak up to ~120mph.

Under regen, I don't know what the maximum negative torque would be and at what MPH that would be reached, but I know regen in the Roadster gets weaker the slower the car gets, so it has to hit that negative torque ceiling somewhere. (Part of the weakening Roadster regen at very slow speeds would also be due to allowing creep, i.e. it won't slow the vehicle to a full stop so it approaches the creep speed.)

Separate question, but one that could enlighten this conversation on Model S regen strength: has anyone produced a regen / negative torque graph from their Roadster?
 
I don't think this is by intent; rather, it is a limitation of how fast the batteries can swallow regen energy. The car is significantly heavier.

Hmm. I hope it's not by intent; I like the "minimum-brake-pedal driving" thing and used to do it all the time in manual-transmission cars at low speed.

But I guess a heavier car would definitely have more momentum, so you'd have to feed more energy as regen in order to get the same level of "motor braking". But anyway I would hope the settings include the maximum viable level of regen.
 
I suspect that there are complexities to the way regen works that limit it for other reasons.

A quickie search turned up this possibly relevant example? (although maybe not since it describes DC motor regen) :
http://www.cafeelectric.com/curtis/regen/index.html

Anyways, having the motor run in regen mode may not be as simple as reversing the capability of forward-drive mode. There may be some additional circuits or constraints that limit the max regen power as compared to the max power the inverter & motor can make when accelerating.

No such complexities exist in AC induction, the limits are all heat and battery related, probably mostly battery. Most Li cells have higher discharge than charge rates, and putting too much current into them too quickly drives the voltage above the breakdown voltage of the electrolyte, which is around 4.5V I think.
 
I don't think Tesla uses the motor windings and inverter as part of the charger the way ACP used to, so no there should be no connection between regen capability and charging capability. Even so charging is limited to power from the wall where regen is not.
Theoretically the motor should be able to regen back as much power as it can take going into it for acceleration.
 
The limit Tesla has put on the regen may be artificial. The driving experience for model S needs to mimic a sport sedan not a roadster. From what I've read, the roadster decelerates very fast, akin to moderate/hard braking when in full regen mode. Fun in a roadster, but not the sensation one expects in a luxury sport sedan. As an example, there are times when I drive that I take my foot off of the "accelerator" just to find a more comfortable position; to have the car suddenly decelerate like the roadster would be very disconcerting. Tesla has to take into account the driving expectations, and not just how much regen is possible for the battery size, weight of car, available traction, etc.
 
The limit Tesla has put on the regen may be artificial. The driving experience for model S needs to mimic a sport sedan not a roadster. From what I've read, the roadster decelerates very fast, akin to moderate/hard braking when in full regen mode. Fun in a roadster, but not the sensation one expects in a luxury sport sedan. As an example, there are times when I drive that I take my foot off of the "accelerator" just to find a more comfortable position; to have the car suddenly decelerate like the roadster would be very disconcerting. Tesla has to take into account the driving expectations, and not just how much regen is possible for the battery size, weight of car, available traction, etc.
All the more reason to make it a user preference. Leave it stock at a "light" level for the Bimmer and Merc drivers and as they come to like the one-pedal driving and for those of us who are used to driving EVs, let us crank it up. My wife only drives the Roadster a couple times per month but she adapts within a couple stop signs every time. When I let friends drive my car they love the regen.
 
Have I conceptualized this wrong? Isn't regen limited to exactly the charger capacity. I think I have seen 10KW on the boards. It is the same problem for the car to turn AC into DC whether from the AC motor or the wall. Nicht wahr?

It depends if everything is being put back into the battery. There have been a number of designs shown over the years, both for Electric and Hybrid, where regen is stored short term in Ultracapacitors. They have an almost unlimited charge rate. Honda pushed this for a while. I'm not sure if it ever made it into production. All the EV companies know how to do this - but there are cost/complexity issues. Not clear if this is part of the Model S or not.
 
According to reports the Regen will be user-selectable via the touchscreen:

When I pleaded for driver-adjustable regen on the Model S, he said this was under consideration. (Two days after the event, while writing up this report, on a whim I e-mailed Tesla boss Elon Musk and repeated my plea for driver-adjustable regen. Three hours later, I got this reply: “I totally agree that regen should be driver-adjustable and it will be on Model S.” The message was Cced to JB Straubel, Tesla CTO.)

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