Regen braking and brake lights

[Doug_G]

The kinetic energy isn't linear with speed. It's exponential.

Actually, it's geometric. Not nearly as bad as exponential, but pretty bad.

I've found that at highway speeds the brake lights won't come on if you have regen limits in effect. Also bear in mind that even if the display doesn't show a regen limit, you may still have a 50 kW limit (the yellow line is off a bit on the high side - not as bad as it used to be, but still off).

 

[Brass Guy]

So... it's a matter of kinetic energy rather than rotor rpm... that makes more sense.
Since the max regen is 60kW, does anybody know the speed that the brake lights do not illuminate above (during max regen)? Or do they always at 60kW?

 

[David99]

Actually, it's geometric. Not nearly as bad as exponential, but pretty bad.

thanks for the correction! I think I'm confusing the terms.

kenic energy = 0.5 * mass * speed * speed
Basically double the speed mean 4 times the energy.

 

[wycolo]

Assuming its not allowed in NA to use amber taillights, Tesla could have used PCM on the red stops with the intensity corresponding to the amount of regen, culminating with a final jump from say 80% to full brightness when brake switch is ON. Seems that would offer a logical progression to the tailgater. But I'm guessing TM tried it already and proved it to be too tricky. Besides it is simpler not having PCM, even though the lure of PCM is it only uses one wire to do all: park, brake, 4way. This appears to be only 2 levels but I'm guessing the new cars with PCM do actually modulate between the park level and full ON. Given this opportunity why wouldn't they?
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[mknox]

Assuming its not allowed in NA to use amber taillights, Tesla could have used PCM on the red stops with the intensity corresponding to the amount of regen, culminating with a final jump from say 80% to full brightness when brake switch is ON. Seems that would offer a logical progression to the tailgater. But I'm guessing TM tried it already and proved it to be too tricky. Besides it is simpler not having PCM, even though the lure of PCM is it only uses one wire to do all: park, brake, 4way. This appears to be only 2 levels but I'm guessing the new cars with PCM do actually modulate between the park level and full ON. Given this opportunity why wouldn't they?
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While amber turn signals are allowed in NA, it has been mentioned that Tesla's European implementation lacks a certain US requirement around the surface area of each tail light element. I don't believe this restriction exists in Canada because EU-spec lighting, while not required, is permitted in Canada as far as I know. I suppose Canada is just too small a market to deviate from the more conventional US-spec systems (with minor exceptions like compulsory DRLs).

Modulating the brake light intensity is probably in violation of a bunch of current codes and regulations.

 

[caddieo]

I haven't done a serious study of this but did check things out to satisfy my own (limited) curiosity. The brake lights come on, it seems, at varying regen values depending on the speed (amount?) you are decelerating. So at highway speed and slowing down, I noticed it was around -30. At city road speed (50-70kph or 30-40mph), it was somewhere around -15. Then after that it's a real challenge to get them to light up at all without the brake pedal. Since I often modulate the fun pedal coasting up to a red light, I think that means my brake lights are often NOT displayed even though I'm slowing down somewhat. However I also watch the behaviour behind me and change my deceleration rate depending on what the other driver is doing (i.e., delay any regen braking until closer to the stopped traffic, etc).

I'm still getting used to estimating exactly when to fully release the fun pedal so that a) I come to the near-stop on regen only at the perfect distance behind the stopped traffic or at the red light, and b) I get maximum regen on the car. Not concerned about refilling the battery charge; it's just a fun challenge for me. Yes, I'm easily amused - comes from all those schoolroom times of being told to go sit in the corner. :tongue:

I have similar ideas and impressions. In my case, by doing my observations at night with the rear camera on, I relate the on/off activations more to how abruptly I release the fun pedal than to any specific regen number (although they are probably related). Under the right traffic conditions at night, I sometimes make a practice of seeing how I can feather the pedal without turning the brake lights on as I approach an intersection and then have them come on as I finally touch the brake.

 

[wycolo]

> modulate between the park level and full ON

To be clear I meant in a few steps, or gradually based on pedal pressure (or decel). Not some pulsing madness.

Hmm, pulsing madness!
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[mknox]

> modulate between the park level and full ON

To be clear I meant in a few steps, or gradually based on pedal pressure (or decel). Not some pulsing madness.

Hmm, pulsing madness!
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Right. I just think varying levels of brightness would be difficult to discern.

I remember long before the CHMSL was introduced (the "third" brake light) being at a car show and seeing a prototype third brake light that grew horizontally as deceleration increased. It would start out as a 6" or so strip at the top of the rear window and expand outwards with higher deceleration, contract inwards with reduced deceleration and go out completely when not stopping. That wasn't a bad idea, but would probably be hard to implement across so many different vehicle designs.

There was also a proposal for cars to have a front brake light to help reduce collisions at intersections. The idea being you would know at a left turn in an intersection if the car approaching you was actually stopping, or gunning it to beat the light.

 

[wycolo]

CHMSL? Center High Mounted Stop Light?

Didn't the T-Bird once have the entire rear filled horizontally with taillights? And turn sigs sequenced across half the car? That was neat, but could have been an aftermarket mod.

I may be over-thinking this whole subject due to my having never seen an MS randomly in the wild, much less being able to follow one down the highway. THAT would be fun; maybe someday soon!
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[mknox]

CHMSL? Center High Mounted Stop Light?

Yes, that is the official terminology.

Didn't the T-Bird once have the entire rear filled horizontally with taillights? And turn sigs sequenced across half the car? That was neat, but could have been an aftermarket mod.

Yes, I think a number of '60s and '70s Fords had something like this. I remember on one iteration of the Ford LTD, they had a light that went completely across the back and when the brakes were applied a section in the center lit up (much like today's CHMSL) along with the lamps on both ends. This was long before the CHMSL was introduced in the 1986 model year (1987 model year in Canada).

 

[JST]

Right. I just think varying levels of brightness would be difficult to discern.

I remember long before the CHMSL was introduced (the "third" brake light) being at a car show and seeing a prototype third brake light that grew horizontally as deceleration increased. It would start out as a 6" or so strip at the top of the rear window and expand outwards with higher deceleration, contract inwards with reduced deceleration and go out completely when not stopping. That wasn't a bad idea, but would probably be hard to implement across so many different vehicle designs.

There was also a proposal for cars to have a front brake light to help reduce collisions at intersections. The idea being you would know at a left turn in an intersection if the car approaching you was actually stopping, or gunning it to beat the light.

Actually, modern BMWs have this feature, sort of--if you brake lightly, certain segments of the tail lights illuminate. If you brake harder, additional parts of the lights illuminate. You can see it quite clearly at an autocross or track event.

The trick, I think, is that the brake lights have to meet minimum size requirements even when they come on lightly. The Model S may not have enough brake light area to do dual mode. Or, more likely, this is just another of those "nice to have" features that Tesla didn't have the time to engineer for the first model run.

 

[Doogue]

My sense is that the brake lights flash on and off as you re-gen more than what would be typical in a regular car. In fact, the other day I was driving in heavy freeway traffic where the speed would range up and down from about 40 MPH to 70 MPH with the ebb and flow of traffic. There was a guy fairly close behind me and I could see him waving his hands wildly at me in the rear view mirror and I think he thought I was being an ass and tapping the brakes constantly.

If we're not feathering the accelerator, we all look like tools slamming on the brakes all the times. I've commented on this before - there should be a medium regen setting that either delays brake lights by say 1 second, so that if you take your foot off accelerator for more then 1 second, then brake lights come on, but otherwise they don't.

Tesla drivers, myself included, look like NYC cabbies from behind - gas, brake, gas brake.

 

[jerry33]

If we're not feathering the accelerator, we all look like tools slamming on the brakes all the times. I've commented on this before - there should be a medium regen setting that either delays brake lights by say 1 second, so that if you take your foot off accelerator for more then 1 second, then brake lights come on, but otherwise they don't.

Tesla drivers, myself included, look like NYC cabbies from behind - gas, brake, gas brake.

That seems odd. I've watched and it seems as if the brake lights on my Tesla activate far less than other cars.

 

[mknox]

That seems odd. I've watched and it seems as if the brake lights on my Tesla activate far less than other cars.

Indeed. This is an old thread and I think subsequent firmware updates since I originally posted my concerns a couple of years ago have improved things.I don't notice my brake lights flashing on/off nearly as much as I used to.

 

[Naonak]

If we're not feathering the accelerator, we all look like tools slamming on the brakes all the times. I've commented on this before - there should be a medium regen setting that either delays brake lights by say 1 second, so that if you take your foot off accelerator for more then 1 second, then brake lights come on, but otherwise they don't.

Tesla drivers, myself included, look like NYC cabbies from behind - gas, brake, gas brake.

They still come on way too early and often. I find myself having to look at the little toy car and feather the pedal to keep them from going on when they shouldn't. The brake lights are really unsafe, especially at higher speeds, as it is barely slowing down the car but it looks like you are slamming on the brakes and it freaks other drivers out. Letting completely off the accelerator on the highway for a brief period should not trigger the brake lights, ever.

 

[scottm]

Maybe it's a calibration issue for your particular car. I wonder if there's a "standard test" or what the rule of thumb would be ... when driving on level ground to determine if the car regen braking indicator is calibrated within an acceptable G-force range?

In my car, while traveling at highway speeds (50+ MPH, 90+ km/h) it takes several seconds of active full regen (after lifting completely off the go pedal) to cause enough deceleration for brake light to come on. Even at Normal regen setting and no regen limiter showing on warm battery that has room to take on charge, and a hard full green bar of gen happening. If I'm really hoofing it at 130 km/h the car hardly slows for several seconds after foot lift-off and I'm getting full-on solid green bottomed-out maxi-regen action and no brake light... because, I'm not really slowing enough to warrant it. SOoooo much inertia in these beasts.

So I think it's pretty safe to say lift off for brief periods will not trigger regen brake light. By the time the car has slowed enough and is now slowing harder and harder the brake light does come on, as I would expect and want it to because the car is very obviously reducing its speed enough to need to warn those behind.

 

[mknox]

Maybe it's a calibration issue for your particular car. ...

In my car, while traveling at highway speeds (50+ MPH, 90+ km/h) it takes several seconds of active full regen (after lifting completely off the go pedal) to cause enough deceleration for brake light to come on.

Same here, but it didn't used to be when I posted about this a couple of years ago up-thread. I think newer firmware has improved it quite a bit.

Could be a bad accelerometer in the car. I would ask Service about it if the brake lights are too twitchy.

 

[CzTom]

My experience is that at freeway speeds, gradually lifting off the accelerator tends to keep the brake lights off, while lifting quickly makes them light up. I don't know if this is software driven or because the car has an accelerometer and keeps them off below some threshold...or if it's all in my head. YMMV

 

[Naonak]

I'll ask service about it next time I'm in.

CzTom, that's the same experience I have with the accelerator.

 

[CanuckS#69]

Coming from decades of manual transmissions, I can't stand the regen brake lights. If I want the driver behind me to know that I'm stopping, I can feather the brake, I don't need said driver to think that I'm braking when I'm just slowing gradually. A downshift and in some cases simply letting off the go pedal in a manual is more rapid stopping power than regen and I've never been rear ended in several million kms of driving. To me, the brake lights come on when I'm saying to the person behind me, "I'm stopping now" not when I am varying my speed due to smaller changes in traffic speed. I find the separate law for regen and manual transmissions to be hypocritical and would rather be able to turn that "feature" off, especially as it is NOT a legal requirement in Canada.