Regen Braking for Beginners.... I have questions

[Smithery]

I actually hate Tesla's approach here and think it presents safety issues. Basically, Tesla regen handling is wildly different depending on temperature and state of charge. No regen at 100%, limited regen in the cold, etc.

To be fair...

Tesla did not make the decision to limit regen when the battery is cold: Physics did.

Tesla did not make the decision to limit regen when the battery is full: Physics did.

It seems like a better solution would be to have a fixed deceleration factor that matches "maximum regen," and to introduce mechanical brakes as needed to match that if temperature/SoC factors prevent maximum regen. That way the driver's expectations can be constant.

Cars engine braking with automatic transmissions have had literally the exact same problems we're discussing with regen.

Low engine speed in a high gear when you take your foot off the gas? Very mild engine braking.

High engine speed in a low gear when you take your foot off the gas? *SIGNIFICANT* engine braking.

And everything in between. And these variables come up with almost every driver almost every day, as opposed to the battery conditions we're discussing.

Nowhere in the history of automatic transmissions has anybody suggested "This is way too variable. We should have the car smartly apply the brakes when the driver takes their foot off the accelerator so the experience is always the same."

Because it simply hasn't been an issue.

Another partial mitigation would be to include enough unusable battery buffer to allow regen even at "maximum charge" in more conditions. This has the helpful side effect of maximizing battery life as well. The downside would be locking drivers out of some of the battery they paid for. Tesla's solution of just asking the customer to set a maximum SoC percentage is good on paper, but not particularly elegant and again offloads the complexity to the driver.

Anytime your engineering solution is "a buffer" you haven't solved the problem. You've only made it less likely.

We spend vacation in the mountains then head home to the valley semi-regularly. At least once a year we mismanage the charge before heading home. We'll leave the cabin with battery capacity and therefore with regen, but charge to 100% while still going downhill and have to start using friction brakes.

That's the type of situation an "invisible, inaccessible buffer" brings up.

Not a big deal because we're familiar with the phenomenon day-to-day, but would be a HUGE surprise if it *only* happened once or twice a year while going downhill.

(Another upside to a proper battery buffer would be excluding range that you never/rarely get to use from the EPA calculations - Tesla sells you 300 miles of range and then asks you to subtract 30-60 miles to maintain battery health, which sucks even if the logic pans out.)

Tesla has never - not once - told me to not charge my car to 100%. I've looked before on the website, the manual, and in-car UI. They don't ever say that.

They say not to leave the car charged at 100%.

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So basically, to offer a differing opinion...

I would be livid at Rivian if they decided to not give me full regen most of the time just because it might be variable some of the time.

And I would also be livid if they hid usable battery capacity from me to solve a problem that doesn't exist.

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[Smithery]

Not sure if this is new, but the car tells you right on the main screen when regen is limited.

My 2017 Model X has always told us this, as did our 2014 Rav4EV before that.

 

[kylealden]

Tesla has never - not once - told me to not charge my car to 100%. I've looked before on the website, the manual, and in-car UI. They don't ever say that.

They say not to leave the car charged at 100%.

Plenty of room for healthy disagreement so I won't try to argue your point by point rebuttals, but for this particular comment - both my Model 3 and Model Y have popped in-car UI saying "repeated charging to 100% will damage the battery" or something to that effect when I have set it to 100% charging on road trips. It's a reasonable owner education measure and I'm not particularly upset about it, but as battery capacities increase I think this is the sort of complexity that should be designed out of EV ownership.

It is worth noting that all EVs, including Tesla, have some amount of "invisible" buffer, some of which is accessible at the bottom of the battery ("0" is not really 0), some of which is cordoned off for keep-alive tasks after the drive motors are disabled. I would be extremely surprised if there's not a small amount at the top too, to mitigate potential damage from overcharging. (For that matter, both my Teslas only charge to 99% from day one). So the buffer is really a question of how much, not if.

Anyway, we're well and truly in the weeds now I respect the disagreeing opinions and will back off of further posts here to keep things on topic with regen questions.

 

[cwoodcox]

It is worth noting that all EVs, including Tesla, have some amount of "invisible" buffer, some of which is accessible at the bottom of the battery ("0" is not really 0),

Just like hard drive and SSD manufacturers include a certain amount of extra capacity to compensate for bad sectors (hard drives) that will crop up eventually, and wear-leveling (SSDs) so they can maintain their rated capacity for a longer period of time.

 

[Smithery]

Just like hard drive and SSD manufacturers include a certain amount of extra capacity to compensate for bad sectors (hard drives) that will crop up eventually, and wear-leveling (SSDs) so they can maintain their rated capacity for a longer period of time.

Indeed.

In these cases, they're compensating for permanent failure states dictated by physics (sectors will go bad, current flash technology suffers a write limit) and their warranties agree.

e.g. If a HDD has more sectors go bad than designed and therefore the drive drops below advertised capacity in the warranty period, you get a replacement.

The small buffers on BEV batteries both at the top (to prevent a literal explosion in the case of overcharging in the heat, for example) and bottom (to prevent the batteries from discharging to the point of breakage) are similar "accepting physics" situations that would also be warrantable if they failed.

But subtracting miles of range from the top end for the purpose of "making sure there's always regen available" is different.

If the manufacturer agrees that this simply isn't a problem with the hundreds of millions of automatic transmission out there, then just ignore it here.

If the manufacturer thinks it *is* a problem that needs to be solved, solve it with blended braking.

I'd wager strongly that every single EV out there that chops 5-10% of battery off the top is due to wanting to prevent leaving the batteries in a high SoC.
And that exactly zero of them have done so to make sure there's always some extra buffer for regen to go into.

 

[BWillR1T]

I don’t know if this topic has been covered on the forum yet but this seemed like the best spot. Does anyone know whether the the brake lights are activated when you take your foot off the “go” or “exhilarator” pedal? I believe there are now a reported 4 levels of regenerative braking (“RB”) from 1 (no RB) to 4 (max RB). From the recent Rivian Stories videos level 4 seems to take you to a complete stop and pretty much allow single pedal driving. I hope the brake lights would at least be triggered at level 4 and maybe level 3.

 

[smiesguy]

Its a rite of passage for the fall when those Tesla dot's appear to show limited regen available... really amps up that seasonal depression!
I would also add in snowy conditions, I turn the regen to low for a more consistent breaking experience. Just my personal preference, but don't like regen when the roads are slick.

 

[BoltEVowner]

I don’t know if this topic has been covered on the forum yet but this seemed like the best spot. Does anyone know whether the the brake lights are activated when you take your foot off the “go” or “exhilarator” pedal? I believe there are now a reported 4 levels of regenerative braking (“RB”) from 1 (no RB) to 4 (max RB). From the recent Rivian Stories videos level 4 seems to take you to a complete stop and pretty much allow single pedal driving. I hope the brake lights would at least be triggered at level 4 and maybe level 3.

Haven't heard about Rivian yet, but the brake lights are definitely activated with regenerative braking in my 2017 BoltEV... not on fire yet.

 

[Jarico75]

It's not good for drivers to ever be surprised by how their car behaves, or to have to intuit it from the weather.

It seems like a better solution would be to have a fixed deceleration factor that matches "maximum regen," and to introduce mechanical brakes as needed to match that if temperature/SoC factors prevent maximum regen. That way the driver's expectations can be constant. There are some disadvantages too - lots of moving parts in all these problems.

Not sure if this is new, but the car tells you right on the main screen when regen is limited.

I echo what Mjhirsch says above. It is clear on the screen when regen is being limited. I think this throttling of regen began because drivers were being surprised by the regen system on slick/icy roads and it was subjecting them to potential slides where the only option was to attempt to feather the peddle to reduce braking and regain control of the vehicle. My opinion is that EV manufacturers would prefer to have maximum regen braking as that improves the overall performance of the vehicle. It is not being reduced "just because".

 

[ajdelange]

Don't know how Rivian manages it but I hope it's the same way that Tesla does. We're just back from three days on the road and I think I touched the brake pedal once the whole time.

If by "blending" you all mean combining friction and regen the only time the Teslas do that is when coming to a complete stop. If your foot is off the skinny pedal regen is full on (provided the battery isn't cold).

Yes, the discs haze up on a Tesla just as they do on an ICE vehicle and it is not uncommon to feel the pads becoming unstuck, accompanied by a loud clunk, when the car first moves after being parked for a while if last driven in wet conditions.

Yes, the brake lights do come on when regen is slowing the car and if you look carefully you will notice that this is reflected in the picture of your car on the screen in front of you (X, S - don't know about 3, Y).

The only problem I have with regen is when it isn't there as in when I drive an ICE vehicle. Coming into a braking situation I take my foot off the gas pedal and nothing happens. Scares the hell out of me for a fraction of a second every time.

 

[ajdelange]

I think this throttling of regen began because drivers were being surprised by the regen system on slick/icy roads and it was subjecting them to potential slides where the only option was to attempt to feather the peddle to reduce braking and regain control of the vehicle.

No, it's done to protect the battery. If the vehicle's BMS tapers during charging it will limit regen when the battery is near full or when cold. If you floor a Tesla on a slippery road torque vectoring comes into play to keep you straight. Similarly if you take your foot completely off the pedal on such a substrate torque vectoring will be employed to keep you straight. And thinking about this I realize that I must correct myself RE my last post. Friction and regen braking are blended to acheive this torque vectoring.

My opinion is that EV manufacturers would prefer to have maximum regen braking as that improves the overall performance of the vehicle. It is not being reduced "just because".

That is correct. Minimizing the use of friction braking minimizes heat lost to friction and thus maximizes efficiency.

 

[DucRider]

If by "blending" you all mean combining friction and regen the only time the Teslas do that is when coming to a complete stop.

Blended braking refers to the brake pedal applying ~100% regen for the first xx% of pressure/travel and then adding in friction braking when needed. Friction braking kicks in mostly during hard stops or when the vehicle slows to the point that one pedal driving requires taking energy from the battery instead of recapturing it. I believe most (all?) FWD/RWD EVs ass in some friction braking on the non drive wheels - less efficient but probably safer.

Tesla is the only manufacturer I know of that does not initiate any regen thru the brake pedal itself. Since most people don't apply both the brake and throttle at the same time during daily driving, the net result is close to the same, but requires that the car be driven with full regen during braking all of the time - even in slippery conditions (they took away the user setting a while ago and AFAIK have not brought it back).

For many drivers, feathering the throttle to control braking does not come naturally, and some do not like heavy regen/one pedal driving at all (my wife and father are both in this camp). Blended braking allows for the vehicle to behave more like an ICE or for a wide adjustment range for off throttle regen.

Since the current Rivian offerings are four motor all wheel drive, it offers them an unprecedented level of control over regen with full control over each wheel independent of all the others. If done right, stability and safety when at the traction limits of the tires should be greatly improved.

 

[EarlyAdptr]

Hey @DuckTruck , not sure if you've been following this thread. You're the only one I know that's actually had the amazing opportunity to sit at the wheel of an R1T.

What was your experience / impression with how Rivian is handling the breaking system? One Pedal or more Blending Braking? Did you happen to notice / experience different levels of regen braking that the driver can adjust?

Thanks again for all your time in sharing your experience with all of us ??

Scott
PNW R1S Oct'18 res holder

 

[ajdelange]

...the only time the Teslas do that is when coming to a complete stop. If your foot is off the skinny pedal regen is full on (provided the battery isn't cold).

Tesla is the only manufacturer I know of that does not initiate any regen thru the brake pedal itself. Since most people don't apply both the brake and throttle at the same time during daily driving, the net result is close to the same, but requires that the car be driven with full regen during braking all of the time - even in slippery conditions

Note that I corrected myself on that in #57. In doing so I pointed out that Tesla does blend friction and regen in slippery conditions. It is done completely by the computer though.

For many drivers, feathering the throttle to control braking does not come naturally, and some do not like heavy regen/one pedal driving at all (my wife and father are both in this camp). Blended braking allows for the vehicle to behave more like an ICE or for a wide adjustment range for off throttle regen.

I understand but is that a good thing? Can't think of a better example of the leading a horse to water thing. I don't see much difference between pushing a different pedal in to increase breaking effort or letting the same pedal come back to do the same thing except that using the same pedal is definitely easier and more intuitive - forward to go faster - backwards to go slower but that's me.

Tesla is the only manufacturer...

That's really the gist of it. They have much, much more experience in this than anyone else. I think we have to recognize that they probably have a pretty good handle on how to manage this. And it certainly is a pleasure to drive it the way Tesla has implemented it. Even my wife, who cannot by any stretch of the immagination, be less technically inclined that your wife or father, likes it. But then she has been "made to drink".

Since the current Rivian offerings are four motor all wheel drive, it offers them an unprecedented level of control over regen with full control over each wheel independent of all the others. If done right, stability and safety when at the traction limits of the tires should be greatly improved.

True enough. And they have infinite flexibility in how they implement the interface between the driver and regen without compromising any of the advantages of this the ultimate torque vectoring configuration. They can put anything in there they like to satisfy/comfort luddites as far as I am concerned as long as the one pedal option is available to the rest of us.

 

[DucRider]

Hey @DuckTruck , not sure if you've been following this thread. You're the only one I know that's actually had the amazing opportunity to sit at the wheel of an R1T.

What was your experience / impression with how Rivian is handling the breaking system? One Pedal or more Blending Braking? Did you happen to notice / experience different levels of regen braking that the driver can adjust?

Thanks again for all your time in sharing your experience with all of us ??

Scott
PNW R1S Oct'18 res holder

In one of the test drive videos, adjustable regen was mentioned. That almost certainly means blended braking.

If regen is solely controlled thru the throttle, the last portion of the pedal travel as you decrease the throttle position has to transition from acceleration to braking, then gradually apply more regen braking until the throttle is "off" and you have 100% regen slowing the vehicle.

When adjustable regen is used, the last portion of the throttle release is "mapped" to the brake pedal. Applying the brakes adds in additional regen over and above what is available by being 100% off of the throttle.

It was inferred in the video clips that regen is reduced in some off road modes, and that is something that was strongly requested by Emma after the Rebelle Rally. I'm assuming that regen level is customizable in all of the modes. If reducing the level doesn't take it away, it gets moved to the brake pedal (blended braking). Tesla has no mechanism for that - the throttle pedal controls only the motors, the brake pedal only the friction brakes (sometimes the computer will intervene on either). When Tesla had adjustable levels, regen potential was lost because it could not be mapped to the brake pedal. I suspect that adjustable regen went away on Teslas as part of the effort to maximize EPA ratings.

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