Regenerative braking and ice/snow

[Snow and Rocks]

Curious more than anything - has anyone encountered ice or slippery snow while driving something with strong regen?

Does the traction control work "in reverse" ? I.e. does it limit the braking effect according to the grip available?

[theboss]

I found my e-tron in snow (on winter tyres) to be the most controlled winter driving experience I've ever had and suspect that this is down to the superior precision of torque vectoring when you're talking about microcomputer controlled motors on each wheel. I also suspect this applies as much during regen as when applying power given my experience. I never once got it to slip under regen, only if I jumped on the brake pedal hard enough to engage the friction brakes. It was a few winters ago now but I was absolutely blown away. I was execting a big compromise due to weight.

[raspy]

theboss said:

I found my e-tron in snow (on winter tyres) to be the most controlled winter driving experience I've ever had and suspect that this is down to the superior precision of torque vectoring when you're talking about microcomputer controlled motors on each wheel. I also suspect this applies as much during regen as when applying power given my experience. I never once got it to slip under regen, only if I jumped on the brake pedal hard enough to engage the friction brakes. It was a few winters ago now but I was absolutely blown away. I was execting a big compromise due to weight.

It could well be the winter tyres that made the difference there. I personally would not take chances if I had regular summer tyres from the factory. I will be turning off regen should conditions become icy or snowy.

"However, during the cold winter months it's strongly advised that you turn regen braking to low, or off completely.

In normal conditions, regen braking works by turning the kinetic energy of the car into a chemical energy stored in the car's battery. During the winter months, however, it can lead to dangerous, unexpected handling characteristics. On slippery roads, the brake-like force to the wheels can cause a loss of grip, reducing the tyre's ability to help steer the vehicle. This can result in the vehicle going into an uncontrolled and potentially dangerous slide."

https://www.leaseplan.com/en-gb/blog/electric-vehi...

[Kawasicki]

I‘ve driven some absolute death-traps and done that were amazingly stable.

A lot of it comes down to the quality of the calibration… I have seen the same car go from bad to great, just with calibration.

[Caddyshack]

I can adjust the regen on my Twizy using an app, you can put it so high that it actually does handbrake skids...I do not think it has traction control at all on braking but it does seem to cut power if it wheelspins sometimes although I can do donuts on mud and grass.

[off_again]

Our i3 had pretty aggressive regen braking and I was concerned about that. I hadn’t driven it in snow, but ice and freezing rain and didnt have an issue - which I was worried about given the short wheel base. But surprisingly it was fine. You cant turn off the regen braking either, but found that decent tires made the difference. There was a video online of a Canadian owner struggling, but that was in deep snow on summer tires - so fixable.

[Snow and Rocks]

Our Model Y is having winters fitted tomorrow so will find some ice and experiment. The regen is definitely aggressive enough to cause issues on a slippery surface so will be interesting to see how it reacts.

[theboss]

raspy said:

It could well be the winter tyres that made the difference there. I personally would not take chances if I had regular summer tyres from the factory. I will be turning off regen should conditions become icy or snowy.

"However, during the cold winter months it's strongly advised that you turn regen braking to low, or off completely.

In normal conditions, regen braking works by turning the kinetic energy of the car into a chemical energy stored in the car's battery. During the winter months, however, it can lead to dangerous, unexpected handling characteristics. On slippery roads, the brake-like force to the wheels can cause a loss of grip, reducing the tyre's ability to help steer the vehicle. This can result in the vehicle going into an uncontrolled and potentially dangerous slide."

https://www.leaseplan.com/en-gb/blog/electric-vehi...

I think what the quoted article means, is that its better to disable automatic regen braking so as to avoid coming off the pedal suddenly which would be like braking moderately hard.

The idea on snow and ice is obviously to brake gently.

In cars like the e-tron, any engagement of the brake pedal will result in regen braking up to a certain amount of force, and friction braking beyond. Therefore in a snow and ice situation you should actually be using regen exclusively, if you're having to brake hard enough to engage the friction brakes you've probably already sliding.

Thus, regen itself isn't bad, it's just uncontrolled/sudden regen from lifting off, that could catch somebody out.

I'd also prioritise the tyres above anything else. I wouldn't want to drive any 2.6t SUV on snow, on fat summer tyres.

Edited by theboss on Thursday 8th December 16:04

[The Wookie]

I might be a bit out of date for road cars but with more basic/typical actuation/ESP systems will shut off the regen and hand over to the ABS as soon as a lockup event is detected.

It does need to be implemented properly off brake though so there is a chance on some vehicles in very low grip scenarios that it could lock the wheels in the same way as would happen with a downshift or selecting a lower gear with an IC car without the brakes applied. Make your own guess as to which purveyor of technically capable but hastily programmed electric vehicles would be my highest candidate for this trait.

More advanced and integrated systems with decoupling and torque based control may manage the regen as part of the slip control, basically it won’t let you lock up and it will keep harvesting as much energy as it can.

Slightly OT but I remember testing an early REEV prototype during one of the big freezes about ten years ago, the test track was sheet ice. The car had quite aggressive regen and the motor control was fairly immature, the first time I lifted off with the regen activated it actually spun the wheels backwards! Mildly exciting

[Discombobulate]

They had to tweak the software on the i3 as it did offer unintended trail braking in the snow

[RobbyJ]

Trying not to sound like a typical rude PH responder. Personally as I've been driving an EV for a while now I very quickly learned letting off the accelerator was the same as light to medium braking so modulate it accordingly regardless of conditions. Unless you're a total numpty it's surely a non issue?

[SWoll]

I've found regen works very well in bad weather and feels more akin to driving in a lower gear in an ICE car and using engine braking rather than the brakes themselves, especially on particularly slippery, downhill stretches.

[JonnyVTEC]

My IPACE with snow and ice mode activated is awesome in this weather, more progressive torque build up and 50:50 bias from the off.

The spikey nature of ICE where acceleration or decel for each combustion or pumping event it quite different to EV when you think what’s happening at the tread blocks, plus far better control for intervention and less rotating inertia on the actual powertrain.

All good.

[Snow and Rocks]

RobbyJ said:

Unless you're a total numpty it's surely a non issue?

I didn't suggest it was an issue - I've quite happily driven around Northern Scotland all winter in various cars without ABS over the years. I was just curious as to how the car was likely to react (if at all) to the lack of grip.

[RobbyJ]

Snow and Rocks said:

I didn't suggest it was an issue - I've quite happily driven around Northern Scotland all winter in various cars without ABS over the years. I was just curious as to how the car was likely to react (if at all) to the lack of grip.

Personally never noticed any issue with regen on snow/ice or given it the slightest thought. I run winters (fitted yesterday) but totally fine.

There's also the point that if it's snowy your battery is likely to be cold and regen less too.

[Kawasicki]

RobbyJ said:

Trying not to sound like a typical rude PH responder. Personally as I've been driving an EV for a while now I very quickly learned letting off the accelerator was the same as light to medium braking so modulate it accordingly regardless of conditions. Unless you're a total numpty it's surely a non issue?

Not all cars are the same, different regen strategy, different tyres, different wheelbase, etc.
If it’s done wrong, it can be a wee bit tricky… even on a wet road.

[TheDeuce]

JonnyVTEC said:

My IPACE with snow and ice mode activated is awesome in this weather, more progressive torque build up and 50:50 bias from the off.

The spikey nature of ICE where acceleration or decel for each combustion or pumping event it quite different to EV when you think what’s happening at the tread blocks, plus far better control for intervention and less rotating inertia on the actual powertrain.

All good.

Indeed, I was amazed by mine last winter - even though I've already had the nature of instantaneous torque control hammered into me by some JLR engineers - seriously impressive stuff.

I don't see any reason to worry about regen levels in any EV on icy roads tbh. Regen absolutely cannot lock up the wheels... And even in it's highest setting the driver is controlling the actual amount of retardation via their right foot, in the exact same way they would traditionally be controlling braking force with the same right foot on the brake pedal. It makes no difference whatsoever if the retarding force you're controlling comes from regen or mechanical braking, you're still controlling it.

What does definitely make a difference is the near light speed torque control inherent to all EV's. At an algorithmic level it's the same principle as traction control on any car, but so much higher fidelity with a powertrain that can respond far faster than ICE ever could.

If you drive an EV this winter, rejoice, if you're a good driver the improved tech will reduce the chances of you washing wide in a turn and finding a tree/ditch. But as we all know... the chances of a good driver doing that in any car are actually tiny. It's not the car that causes the danger, it's the hapless simpletons that drive them and don't give proper thought to the physics at play as they make brake/throttle adjustments.

[The Wookie]

TheDeuce said:

Regen absolutely cannot lock up the wheels...

Being pedantic it can, but it shouldn’t unless it’s been done wrong. Even if it’s done right it can under-rotate the wheels virtually to a stop in a very low grip scenario if it’s not been done well

If the powertrain isn’t checking or doesn’t know that the wheels with regen applied are rotating slower than the car is travelling then it will carry on applying its preset torque until the wheels are rotating too slowly to apply it

That said any half decent road car with a correctly conducted validation process will have all this covered

[TheDeuce]

The Wookie said:

TheDeuce said:

Regen absolutely cannot lock up the wheels...

Being pedantic it can, but it shouldn’t unless it’s been done wrong. Even if it’s done right it can under-rotate the wheels virtually to a stop in a very low grip scenario if it’s not been done well

If the powertrain isn’t checking or doesn’t know that the wheels with regen applied are rotating slower than the car is travelling then it will carry on applying its preset torque until the wheels are rotating too slowly to apply it

That said any half decent road car with a correctly conducted validation process will have all this covered

But also... Regen cannot ever lock up the wheels.

As the wheels rotation reduces the magnetic fields generated become to weak to have any braking effect, let alone a holding effect.

Regen can only retard rotation, it can never stop it.

[Zero Fuchs]

off_again said:

Our i3 had pretty aggressive regen braking and I was concerned about that. I hadn’t driven it in snow, but ice and freezing rain and didnt have an issue - which I was worried about given the short wheel base. But surprisingly it was fine. You cant turn off the regen braking either, but found that decent tires made the difference. There was a video online of a Canadian owner struggling, but that was in deep snow on summer tires - so fixable.

I wouldn't worry too much. I've driven mine in the snow and generally the same rules apply i.e. don't make any sudden moves including lifting off. Reading the road well ahead is enough and lightly lifting off. However, the tyres are so thin that contact pressure leads to ok grip, even with summer tyres.

As with most of my cars, I tend to test cars in empty car parks so will give full regen a go if it snows this year (although I doubt the regen will illicit a big slide).