No brake llights above 136 kph (85 mph) while using regen braking

It's not a problem. The regen brake lights are based on rate of decel. Once the rate of decel is high enough, they will activate. If this was not the case, they would be coming on any time you took your foot off the accelerator. Which would be quite annoying.

Being in California I don't drive over 85mph but I suspect the answer to your question is that at such high speeds, when you lift your foot off the "go" pedal the Model S slows down gradually like an ICE car would (without the driver touching the brake pedal). So no reason for the S brake lights to come on. At speeds under 85 the Model S deceleration is more abrupt when you lift you foot off the go pedal, so the brake lights are activated.
So that's my theory. Not able to test it out here, legally...

I understand the concern, but as others have said, the brake light don't come on on any other car either when you lift off the accelerator at high speed. the rate of deceleration is probably similar.

What people often don't understand is the rate of acceleration and deceleration depends greatly on speed, not just how much power is applied. 40 kw gives you a decent amount of acceleration from a stop, but it does very little once you go 70. the same is true for deceleration. 40 kW of regen at 70 mph isn't much. At 20 mph that would be a very strong force slowing you down. The energy needed to accelerator or decelerate a 10 mph difference is very different depending on your speed. So 60 kW of regen at 90 mph isn't that much, hence the brake light won't come on. At 50 mph the same 60 kW of regen will slow your car down much stronger, and the brake lights come on.

To avoid the effect of having very low deceleration at higher speed and then getting stronger and stronger as you slow down, Tesla modulates the regen power down as the car slows down. The regen power is reduced but the rate of deceleration stays (roughly) the same.

ecarfan said:

Being in California I don't drive over 85mph but I suspect the answer to your question is that at such high speeds, when you lift your foot off the "go" pedal the Model S slows down gradually like an ICE car would (without the driver touching the brake pedal). So no reason for the S brake lights to come on. At speeds under 85 the Model S deceleration is more abrupt when you lift you foot off the go pedal, so the brake lights are activated.
So that's my theory. Not able to test it out here, legally...

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I can assure you the rate of deceleration is much greater as speeds increase.

When letting off the throttle at various speeds with my vbox:

92 MPH = 0.336 g
74 MPH = 0.144 g
68 MPH = 0.133 g
65 MPH = 0.127 g


Since air resistance is proportional to V^2, letting off at 150 MPH would probably be close to 0.7 g.

Maybe there's some regulation that prevents the light from coming on for regen above 85 MPH, but it doesn't seem the safest choice.

Slcuervo, just went out and did some tests. My car does not have the issue. I tested it at various speeds up to approximately 175 km/h (110 mph).
What version of the firmware are you running? I'm now on 2.7.77 and I have an auto-pilot equipped car.
Could it be that this issue is limited to classic versions of the Model S?
I agree that it is an important safety issue.

try it going downhill--i'd guess it will apply greater regen braking.

How can this be an issue??

Who in the world will be following you when you are doing 85, and why would you want your brake lights to go on? Are they tail gating and you want them to back off?

I live in CA, and I DO drive 85. It's pretty common. I have found, actually, that when in cruise and passing someone over 90 mph, the car beeps at you, because cruise does not go over 90. But I don't care what the brake lights are doing. I don't think someone is going to hit me from behind at that speed. My gas car never did have brake lights go on.

You ought to do your deceleration g forces on a gas car and see what they are. With a manual tranny, it slows down pretty fast.

I had a guy get all perturbed because my brake lights didn't go on when I was coasting to a red light. He saw it was a Tesla, followed me into the parking lot and yelled at me. I asked him if his lights went on while he was coasting. People are supposed to slow when cars are in front, with or without lights.

roblab said:

Who in the world will be following you when you are doing 85, and why would you want your brake lights to go on? Are they tail gating and you want them to back off?

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In Texas it doesn't matter what speed you travel at, there will always be someone right on your tail.

slcuervo said:

2.7.77 too, non AP.

I hope it's all non AP cars and not only mine!

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Ok, thanks. I will get it confirmed by some European non AP drivers so that you know if it is specific to your car. Might take 24 hours since it is already late at night over here. Had to do my testing in the dark as well.

Just because ICE cars don't light up their brake lights when engine and air braking doesn't mean they shouldn't. Tesla is one of the first cars to turn on brake lights in response to something other than a brake pedal being pressed past a certain point.

I think cars should light up their brake lights when they exceed a certain level of deceleration whether it be from physical brakes, regen braking, engine braking, air braking or any combination that exceeds a certain rate of deceleration.

Indeed others have been designing solutions around just that:

Motorcycle brake lighting system doesnre slowing down
GearBrake - Smart Brake Light Moduleâ„¢
Adaptive Brake Light (ABL) - The 3rd Eye

sorka said:

I can assure you the rate of deceleration is much greater as speeds increase.

When letting off the throttle at various speeds with my vbox:

92 MPH = 0.336 g
74 MPH = 0.144 g
68 MPH = 0.133 g
65 MPH = 0.127 g


Since air resistance is proportional to V^2, letting off at 150 MPH would probably be close to 0.7 g.

Maybe there's some regulation that prevents the light from coming on for regen above 85 MPH, but it doesn't seem the safest choice.

Click to expand...

But clearly the proportion of deceleration due to regen becomes a smaller amount of the total deceleration.

IMHO I'd rather not have my brake lights come on when coming down from extra-legal speeds. Flashing brake lights give away your former speed on the highway. This happens often if you drive a manual, with no brake lights.

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roblab said:

I had a guy get all perturbed because my brake lights didn't go on when I was coasting to a red light. He saw it was a Tesla, followed me into the parking lot and yelled at me. I asked him if his lights went on while he was coasting. People are supposed to slow when cars are in front, with or without lights.

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Brake lights are just a hint, you are supposed to be in control of your speed and observe what's in front of you at all times. "His brake lights weren't on" would never pass as a defense, if you rear end a car you are responsible. Which is also why in Germany tail-gating is the ultimate roadway sin.

AWDtsla said:

Brake lights are just a hint, you are supposed to be in control of your speed and observe what's in front of you at all times. "His brake lights weren't on" would never pass as a defense, if you rear end a car you are responsible. Which is also why in Germany tail-gating is the ultimate roadway sin.

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Most of my cars have been either manual or semi-automatic. No brake lights when gearing down in those. Of the three times I was rear ended, two were when the car was parked, and the other was in a car that was an automatic. Never had any problems with no brake lights in a standard transmission car while actually driving.

To slcuervo: have you been able to ascertain your claim by having another tail you and let you know yea/nay wrt your brake lights? I would wonder if any problem that might exist COULD be solely in the dashboard display.

To the overall point, however: I would suggest that any responsible driver who is interested in decelerating a vehicle going 135kph/85mph....should very most definitely make use of the brake pedal.

sorka said:

I can assure you the rate of deceleration is much greater as speeds increase.

When letting off the throttle at various speeds with my vbox:

92 MPH = 0.336 g
74 MPH = 0.144 g
68 MPH = 0.133 g
65 MPH = 0.127 g


Since air resistance is proportional to V^2, letting off at 150 MPH would probably be close to 0.7 g.

Maybe there's some regulation that prevents the light from coming on for regen above 85 MPH, but it doesn't seem the safest choice.

Click to expand...

Sorka -- interesting data on these deceleration rates.

As you say, the force and hence the contribution to deceleration (in Gs) of air drag goes as v^2. On the other hand, the contribution to deceleration (in Gs) of the maximum regen level in kW goes as 1/v. The point is, the higher the speed, the lower the contribution of regen as a % of total deceleration when you lift. I suspect the brake lights don't go on unless either the total Gs due to regen is over some threshold, or the percentage of the total deceleration due to regen is over some threshold, neither of which condition can be met at very high speed. Why is this okay? Because when you lift your foot in the limiting case of extremely high speed, a Tesla with regen won't decelerate at a significantly higher rate than an ICE car, even though the absolute deceleration of both is high due to air drag. Since it's okay for ICE cars not put on the brake lights upon lifting at high speed, Tesla's thought must be that it's also okay for a MS, which would only be decelerating at a slightly higher rate.