Model S regen

[Doug_G]

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.

[doug]

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.

[TEG]

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.

[TEG]

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...

[rabar10]

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?

[doug]

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).

Good point.

[rabar10]

(thanks for splitting the thread)

Wanted to ask this again: has anyone produced a regen / negative torque graph from their Roadster log data? It might provide insight on what to expect from Model S...

[neroden]

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.

[JRP3]

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.

[Thumper]

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?