Regenerative Braking Safety Issue

jquint · Aug 22, 2015

I have been trying to get a response from Tesla about this issue since January 2015. I have called my service adviser many times and have written multiple times to ElonMuskOffice.

The issue concerns a safety issue regarding the Model S regenerative braking system performance on ice.

The Model S performance is extraordinary in most conditions. It even drives well in snow, with appropriate snow tires. The traction control system is excellent, and it is a good feature to be able to adjust the regenerative braking to “low” in snowy conditions. The trade-off of efficiency vs control of the vehicle is certainly worth it.

However, in icy conditions the situation is very different. I was caught in an ice storm in New Hampshire on January 18th. I have to tell you that driving the car, even with the regenerative braking set to low, was terrifying. Any time that I lifted my foot even slightly from the accelerator pedal caused the rear end to twitch. There was nothing I could do to prevent it. Even at low the minimal braking application degraded the vehicle’s stability.

That day the storm was supposed to be a rain storm but the air temperature was cold enough that it became an ice storm. I actually made it about 14 miles (in nearly an hour of driving) but just shy of Interstate 93, which I knew would be treated. I take full responsibility for the accident, because I could have (and should have) pulled over and waited out the storm. However, I believe that the regenerative braking system contributed to the accident.

Based on what I have learned from experience, I would like to strongly urge Tesla Motors to add a third option to the regenerative braking setting of “Off”. They should not try to have their engineers come up with a software solution for detecting ice, because on ice once there is any wheel slippage it is already too late to regain control. Tesla should trust the intelligence of its customers and add the ability to disable the regenerative braking completely. That would make the car much safer on ice.

neroden · Aug 22, 2015

What you need to do on ice that bad is to switch into *neutral*. That's what I did the last couple of times I hit the ice. That'll turn off both regen and acceleration, *both* of which will make you skid on ice.

It's right there on the stalk, but I assume you overlooked it since it's not useful for anything but ice.

JOEV1 · Aug 22, 2015

neroden said:
What you need to do on ice that bad is to switch into *neutral*. That's what I did the last couple of times I hit the ice. That'll turn off both regen and acceleration, *both* of which will make you skid on ice.

It's right there on the stalk, but I assume you overlooked it since it's not useful for anything but ice.

Thank You for this thread. It open my eyes about something I really worry.
I live some distance up the mountain in Salzburg. Every winter I encounter different and sometimes difficult driving condition.
Never really a problem going up. And as we use to say "everyone can go up - even with a standard ICE AWD vehicle - the hard thing is to come down in a safe manner." (not just once my car - a VITARA - suddenly was facing 180° after a curve, when the rear came around)

Now learning that regenerative braking can be a disadvantage and becomes even dangerous on slippery or icy roads when going down.
The idea to engage "N" and use just the brakes is not really a good solution. Finding position "N" by moving the stalk might even result to engage "R" by mistake... and then one really has a problem.
So, as "@jquint" suggested to disable regenerative braking might be a first step.

Perhaps I will write to ELON as well.

Spacela · Aug 22, 2015

JOEV1 said:
The idea to engage "N" and use just the brakes is not really a good solution. Finding position "N" by moving the stalk might even result to engage "R" by mistake... and then one really has a problem.

FYI, above certain speeds (~5 mph IIRC) it's impossible to shift into Reverse or Park. If you miss "N" and go all the way to "R" the car won't actually shift into Reverse, it will go to neutral. Similarly, if you try to put the car into Park at speed, a message will display saying "Park not Available." However, the car will activate the emergency brake if you hold the park button for more than a few seconds.

AWDtsla · Aug 22, 2015

That sounds scary. IMHO they should definitely give you the option, also being able to fully disable traction and stability control.

FlasherZ · Aug 22, 2015

The following applies to RWD cars only - I'm unsure how Tesla handles regen with the dual-motor models. I could imagine losing your steering wheels to regenerative braking lock-up to be problematic.

I have a similar experience, although I wouldn't classify it as "scary". If you are not accustomed to regenerative braking it will feel odd to you as the back tires lock up; the car continues on its existing path and the front tires (your steering tires) keep rolling. It feels like your back end is dragging on the ground. Unless you have something wrong with your braking system, the back tires will be consistent in their lock-ups, so it's not like they will easily take you sideways in the car, and stability control will help to correct any problems. Bottom line is that if this occurs, you've lost traction, regardless of the type of car that you're in.

You're right, it's unsettling because it's a different feeling, and I do think they should have a setting to turn regenerative braking off to ease that. I would stop short of saying it had a liability role to play in your accident, though.

David99 · Aug 22, 2015

I agree there should be an off switch for regenerative braking. There are situations where you just want it off instead of having to feather the pedal all the time. I think the early version of regen didn't have any stability control and it was added later. I believe as soon as the car notices that regen causes the wheels to slip it will reduce it. Just as it reduced power when you accelerate and lose grip. It always causes a slight 'twitch' caused by the wheels losing grip and then regaining grip. This could be avoided by no regen whatsoever, but then if that tiny twitching causes the car to lose control totally, you have been going too fast for the road condition. At this point it's not the regen's fault or the stability control trying to the best possible in the situation. It's the driver going too fast for the conditions. In that situation, the slightest touch of the brakes would cause the same behavior (ABS kicking on).
I also agree, that switching to N(eutral) is very easy without risk of getting into reverse. If you feel any sign of ice and you feel any sign of the car losing grip, switch to N and pull over as careful as you can. The laws of physics apply no mater how high tech your car is.

mikeash · Aug 22, 2015

David99 said:
I also agree, that switching to N(eutral) is very easy without risk of getting into reverse. If you feel any sign of ice and you feel any sign of the car losing grip, switch to N and pull over as careful as you can. The laws of physics apply no mater how high tech your car is.

Note that this applies to a normal car too. Normal cars have "regen" too! Obviously it doesn't actually regen, but releasing the accelerator entirely creates drag if you're above a certain speed, and if you have RWD then you'll encounter the same trouble as described here. And the solution is the same: shift into neutral.

The ability to disable regen entirely sounds like an interesting one, but as it stands the problem and solution are nothing unique to a Model S.

JOEV1 · Aug 22, 2015

mikeash said:
Note that this applies to a normal car too. Normal cars have "regen" too! Obviously it doesn't actually regen, but releasing the accelerator entirely creates drag if you're above a certain speed, and if you have RWD then you'll encounter the same trouble as described here. And the solution is the same: shift into neutral.

The ability to disable regen entirely sounds like an interesting one, but as it stands the problem and solution are nothing unique to a Model S.

Basically, when handling any car in icy down-hill conditions one would be sensible to drive at speeds below 5 MPH.
Its true, in a "normal car", slowing down by easing-off the accellerating dedal (using engine as a brake) is to some degree much the same. And when that is too much one usually shifts to neutral or hits the clutch. We all know that the steering wheels should not lose traction.
Interesting to read here that traction control works in both ways. (positive and negative acceleration). Well, that might just be the solution in our AWD.... if this really is what we have.

The next winter comes for sure and I will be able to tell more...

AWDtsla · Aug 22, 2015

JOEV1 said:
Basically, when handling any car in icy down-hill conditions one would be sensible to drive at speeds below 5 MPH.

No. There is no absolute "sensible" speed. That's just your gut feeling based on cars you have driven in the past combined with whatever your driving ability is.

EVSteve · Aug 22, 2015

For perspective in a Volt an even more serious issue exists thanks to GM's programming. As with the S applying the brakes first engages the regen braking then progresses towards friction braking. The difference with GM is they've elected to disengage all regen braking if the system detects a skid or low traction. The gap between the loss of regen and application of the friction brakes is MASSIVE. The stopping distance in snow/ice is severely compromised and unpredictable. The only way to avoid this is to disable the regenerative system entirely by shifting to neutral and relying on the friction brakes.

That said in the D models I would expect, as long as the regen braking is even, that this issue would be less pronounced but still possible. I've suggested a snow/ice mode which disables regenerative braking. Obviously there will be a significant range impact but I feel it's far better than winding up in the ditch or smacking a curb.

jerry33 · Aug 23, 2015

EVSteve said:
For perspective in a Volt an even more serious issue exists thanks to GM's programming. As with the S applying the brakes first engages the regen braking then progresses towards friction braking. The difference with GM is they've elected to disengage all regen braking if the system detects a skid or low traction. The gap between the loss of regen and application of the friction brakes is MASSIVE. The stopping distance in snow/ice is severely compromised and unpredictable. The only way to avoid this is to disable the regenerative system entirely by shifting to neutral and relying on the friction brakes.

This happens in every car with regen on the brake pedal. Tesla's way is far better.

Matias · Aug 23, 2015

With regen braking setting "low" regen is comparable to ICE motor braking. So you would have the same issue with ICE? But of course disabling regen braking totally should be an option.

Zythryn · Aug 23, 2015

I've driven Model Ss through three winters. I have always left regen on "Standard" and have never had an issue as long as I was driving at an appropriate speed for the conditions.

I do know a number of people who state they switch regen to "low". However, I have found that unnecessary.
We drive daily in the winter and have winter tires on one, while leaving all seasons on the other car.

I have had to turn off traction control more often than switching to low regen.

I'm not saying others haven't had issues. I just wanted to make sure that any readers are aware that the OP is not a universal experience.

dsm363 · Aug 23, 2015

Not much of a reference but through one mild winter here with heavy snow only once. No issues on standard regen.

jquint · Aug 23, 2015

Zythryn said:
I've driven Model Ss through three winters. I have always left regen on "Standard" and have never had an issue as long as I was driving at an appropriate speed for the conditions.

I've driven through two New England winters and generally agree. However, the point I am making concerns freezing rain, not snow. There is a significant difference between icy snow conditions and solid ice.

Several people have made the point that in such conditions there may not be much one can do other than not drive. I agree with that. But if you have to drive, I'd rather take my chances with all-wheel intelligent braking where I control via the brake pedal vs. regenerative braking that kicks in automatically when you lift your foot from the accelerator.

svp6 · Aug 23, 2015

Drove last winter through several Minnesota blizzards and on iced roads. No problems at all - but on winter tires and AWD. Car is on par with all previous AWD I used (BMW/AUDI/Infiniti models).

Skotty · Aug 23, 2015

I think the correct solution is to have the anti-lock breaking system also have control over regen. I would assume they already do in most cases. Hybrids have been using regenerative braking for many years, and should be subject to the same theoretical problem. However, it's possible some regen capable cars don't do this, or pehaps they just don't do it very well.

Pollux · Aug 23, 2015

Hi, @jquint,

I think it adds to your credibility that you are distinguishing the pieces of the problem for which you were responsible from those for which you think the car could have been programmed to behave differently.

What kind of tires were you using? OEM all-seasons or dedicated winter tires? I find that the Nokiian Hakkapolitta R2s have made a significant difference in my 2013 P85+'s winter driving performance. Although anecdotal, I have confirmed this distinction by driving the same vehicle with my summer tires in December (unplanned, knew it would be bad, and it was) and very similar loaners with all seasons (unplanned, turned out to be a great opportunity, the all seasons are much better than summer tires, no surprise there, but eye-openingly not as good as the winter tires). For the first time in my driving life, I understand why people use winter tires. I've always ever previously used all-seasons. I am such a believer now in winter tires that I've gone back to my wife's Prius and now rotate her all-seasons for winter tires (Nov-Apr). But only because I care about her safety; were that Prius to get totaled, I would have to view it as an opportunity rather than a tragedy.

All this said, I can see a real use for a "no regen" option. In general, I think there should be ways for drivers who care to be able to switch off the obvious nanny features of the car -- whether that's Autopilot, TACC, ABS, traction and stability control. Sometimes, the driver really does know best. Or, dammit, just wants to throw some awesome donuts around a parking lot! And all of these nanny features can have safety implications. (I realize regen isn't a nanny feature per se.)

Alan

jquint said:
I've driven through two New England winters and generally agree. However, the point I am making concerns freezing rain, not snow. There is a significant difference between icy snow conditions and solid ice.

Several people have made the point that in such conditions there may not be much one can do other than not drive. I agree with that. But if you have to drive, I'd rather take my chances with all-wheel intelligent braking where I control via the brake pedal vs. regenerative braking that kicks in automatically when you lift your foot from the accelerator.

jerry33 · Aug 23, 2015

Skotty said:
I think the correct solution is to have the anti-lock breaking system also have control over regen. I would assume they already do in most cases. Hybrids have been using regenerative braking for many years, and should be subject to the same theoretical problem. However, it's possible some regen capable cars don't do this, or pehaps they just don't do it very well.

Hybrids turn region off whenever any of the braking safety systems engage (TC, VSC, ABS, etc.). I believe the Leaf does this too, but I don't have enough miles on it to say with certainty.

- - - Updated - - -

Pollux said:
All this said, I can see a real use for a "no regen" option.

I have no problem with there being a choice, but having driven the Model S on ice days (same thing as freezing rain in New England), I've not had a problem with standard regen and creep off. As with any new type of system, a bit of practice before critical use goes a long way.

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