Do brake lights come on when regen braking is activated?

Do brake lights come on when regen braking is activated?

My insurance agent is asking.

jboyd | May 23, 2017

Yes they do. Also, I found out in last upgrade when battery is fully charge regen doesn't function until there is capacity available in the battery for charging.

jboyd | May 23, 2017

Also, just look at your vehicle icon on dash, it shows light bar and lights coming on.

shurst | May 23, 2017

yes, once you start slowing down significantly, if you just let off the accelerator a touch it does not, but if you let off enough that is similar to deceleration of braking, the brake lights come on. Your little car on the dash shows when the brake lights turn on.

Tarla's Driver | May 23, 2017


The long answer is that it checks how much the regen is slowing the car and decides if the brake lights need to come on, so sometimes they don't come on immediately, or they go off as the car slows down to where the regen starts to slack off.

But from a safety standpoint, the brake lights go on if the car is behaving like a traditional car with the brakes applied.

Tarla's Driver | May 23, 2017

@jboyd, also be aware the regen is limited if the battery is cold. It always takes us a bit to get used to the change in driving in October or November. If you live in warmer areas and take a trip to someplace cold, you'll get a surprise.

roadkill | May 23, 2017

Thank you

Bighorn | May 23, 2017

Someone should write this stuff down somewhere.

tes-s | May 23, 2017

No. Brake lights come on based on decelleration, whether caused by regen, brakes, or any other means.

Regen can occur without brake lights being activated. Brake lights can be activated without regen.

patrick40363 | May 23, 2017

you might want to read the owners manual.

jboyd | May 23, 2017

Thanks Tesla Driver, didn't know that. Living in SoCal, weather/termperature is never a consideration.

Bighorn | May 23, 2017

You can see limited regen with temps in the low 50s.

CEYA GAS | May 23, 2017

I always learn so many new things everyday on these forums!

rschulze | May 23, 2017

My experience is the brake lights come on when regen is approximately 40 kWh as shown on the round dashboard icon for power consumption.

PBEndo | May 23, 2017

It might be a little more complicated. Several videos of cars on dyno's show brake light activation during regen without the driver apply the brake pedal.

Bighorn | May 23, 2017

There are no absolutes related to kWs of regen. You can have no brake lights with 60+kWh of regen--all depends on the speed and terrain. Or they can come on much earlier than 40kW in city traffic. Folks with the original, pre-AP cars and a central speedometer have much more granularity in observing this.

mscott | May 23, 2017

Everybody who thinks this is regen-related can easily test Bighorn's (as always, correct) assertion to the contrary. Go out to your nearest (not-too-congested) highway, get up to about 70MPH. Making sure nobody is behind you (an infrequently used exit works great), let off the accelerator completely. Your regen will max out (60+kW) but the brake lights (which you can see on the IP) won't come on until you've slow down to around 50-55MPH. This is because the amount of kW generated by regen braking is a function of the car's speed. You can very easily hit max regen at 70MPH but not actually be slowing very quickly. It's not until you slow down to about 50MPH that max regen is slowing the car enough to turn on the brake lights.

Similarly, you can be going 20MPH and let completely off the accelerator. Your brake lights will come on almost immediately, even though you're only getting <20kW of regen. That's because the deceleration is significant, even though the amount of regen is not.

tes-s | May 23, 2017


I believe the answer to the OP question "Do brake lights come on when regen braking is activated?" is "No".

Regen can be active without brake lights coming on.
Brake lights can come on with regen and no braking.
Brake lights can come on without regen.

My observation is brake lights come on when regen is causing sufficient deceleration, and they also come on when the brake pedal is pressed regardless of deceleration or regen.

UnshodBob | May 23, 2017

@tes-s - I agree with you except I'd say the answer to the OPs question is "sometimes." That is, it's not a cause and effect thing. You make an excellent point that brake lights are not directly activated by regen at all. It is probably an accelerometer and the brake pedal switch working in tandem which control the brake light activation. There is no regen to brake light connection, but sometimes the brake lights will come on when regen is activated.

tes-s | May 23, 2017

My wife told me sometimes the brake lights come on when I pick my nose. I'm pretty sure it is not a cause and effect thing. :)

GHammer | May 23, 2017
Bluesday Afternoon | May 23, 2017

Not a problem, tes-s.

Run4Waffles | May 23, 2017

"Bighorn | May 23, 2017
Someone should write this stuff down somewhere."


ST70 | May 23, 2017

@Bighorn- Someone should write this stuff down somewhere.....ha!

PBEndo | May 23, 2017

@tes-s - just saying that there is no deceleration of the car when on a dyno

tes-s | May 23, 2017

Now I understand - no deceleration on a dyno. Duh!

I don't think there is an accelerometer - I think it is an algorithm that calculates deceleration and determines when to put on the brake lights.

p.c.mcavoy | May 23, 2017

@tes-s - You don't need an accelerometer for the car to determine rate of deceleration. That can be determined from vehicle speed. Getting all techie - it's the first derivative of vehicle speed.

UnshodBob | May 23, 2017

@p.c.mcavoy - if I wrote the software routine to do the math, I'd call it accelerometer() :)

DLebryk | May 23, 2017

It was a Tesla Engineer or the owner's manual that said they used an accelerometer, not vehicle speed.

JayInJapan | May 23, 2017

I saw this question asked and answered on a Japanese mailing list a couple of years ago. Here, the brake lights come on based on the rate of deceleration per (I hate that word) Japanese regulations. I'm not sure if the regen rates/brake lights' relationship is different based on country, but it wouldn't surprise me.

J.T. | May 24, 2017

>>>> It is probably an accelerometer

I defer to TeslaTap for any information on whether the regen/braking/brake lights is different on my Classic 2014 85 from what has come after, but at an event at Citifield, Jerome Guillen, the former VP of Gobal sales and Service was asked by me: "What contraols the brake lights during regen deceleration?"

He said: "An accelerometer."

He wasn't drunk, he didn't seem confused and he answered in a way that it sugested that he knew the answer.

Now, there will be those of you who will cling to their theories even after authoritative sources have exploded them, but when you continue to expound on them you'll look pretty foolish.

tes-s | May 24, 2017

If brake lights come on with just regen on a dyno, then perhaps it is a software accelerometer.

Run4Waffles | May 24, 2017

+1 @J.T.

Captain_Zap | May 24, 2017

Thanks JT. I was starting to question myself while reading this thread. I feel better knowing that my temporarily questionable accelerometer is still real.

jordanrichard | May 24, 2017

A question about the OP's question. Is this question from your agent out of curiosity or he/she needs to know for the policy?

As for when the brake lights come on, it seems to me that they will come on when the re-gen amount is equal to or greater than the speed you are traveling. That is why the brake lights will actually go out at some point as you approach a light/stop sign etc. At that point your speed is so slow that it doesn't create enough re-gen.

Run4Waffles | May 24, 2017

@roadkill - what did you relay to the agent and what was the response?

I would be "on my toes" with that question. It would seem to me that the industry documentation should have that classified for them. And this is the first Tesla that they have insured?

Did they come back with a good rate?

roadkill | May 24, 2017

jordanrichard: Q: Is this question from your agent out of curiosity or he/she needs to know for the policy?

A: Pretty sure it was a curiosity question cause he seemed pretty interested in the car and asked
other questions along the curiosity lines. Also, he gave me a quote without knowing the answer.

Run4Waffles: Q: what did you relay to the agent and what was the response?

A: I told him I dunno know but would find out. He seemed fine with that

Q: And this is the first Tesla that they have insured?

A: Honestly, I don't know but doubt it. Although he couldn't find the exact one I was looking at, a
75 RWD. Could only find a 90D and said it was to expensive, over 75K and company wouldn't
insure it. Had to tell him it was cheaper, about 70K. So maybe they don't have a lot of
experience with Tesla's. Came back with $78/mo. I will do some research on that rate and
get at least 3 other quotes though as I know rates can be quite different between companies.

Thank you to everyone for the responses. Very detailed and informative!

tes-s | May 24, 2017

$78/mo sounds pretty good! What company?

kevin | May 24, 2017

My wife asks me this question every few months.

kevin | May 24, 2017
p.c.mcavoy | May 24, 2017

RE hardware accelerometer vs. a software based determination ...

My comment above in response to @tes-s was not intended to claim that I believed the MS did not have a physical sensor. My comment was intended purely to point out that there could be multiple ways to determine rate of vehicle deceleration such as physical sensor or a software based approach off of other inputs.

SbMD | May 24, 2017

@JT - thanks for the confirmation, and wit :)

@p.c. - you are right that one could use other velocity cues to determine the triggering threshold for the brake lights, but the configuration of an accelerometer generally gives it a more accurate response than using derivatives of velocity, or even using a velometer.

PBEndo | May 25, 2017

FWIW, I think there is a hardware accelerometer. But several dyno videos show the brake lights come on after regen has kicked in. It is possible that in each case the operator hit the brake pedal, but I am told you are not supposed to do that when on a dyno. Perhaps, the majority of the brake light activation is based on the accelerometer readings but there may be an additional software activation at times. Just speculation on my part.

tes-s | May 25, 2017

Yep - I don't know how it works, just observations. If the brake lights come on with regen-only on a dyno, that suggests there is more at play than just an accelerometer. I cannot verify that is the case since I have not observed that or even gotten it from a first-hand source.

I do know from personal observations:
1. Regen can occur without the brake lights coming on.
2. The brake lights can come on with only regen - no brakes applied.
3. The brake lights come on when the brakes are applied, with or without regen.

That is for my 2013 S85. It could be different for other cars.

UnshodBob | May 25, 2017

@tes-s - your observations agree with mine for my 2015 S P90D. I'd expect all the cars to behave the same, with respect to method of activation of brake lights.

Haggy | May 25, 2017

The bottom line is that the brake lights come on when they should, which is when people would expect them to and it would be bad for other drivers if they did not.

Regen won't be there with a 100% charge, but it should be rare that you charge to 100%. Even if you do, you will get some of the regen back after a few miles. In the mean time, the brakes will work the way they do on any other car, and the brake lights will go on based on that.

UnshodBob | May 25, 2017

The car feels funny when it's fully charged. I have a driveway that slopes down to the road about 100 feet, and it feels out of control to roll down it with no regen. Gotta use the brakes! Around the corner is a stop sign, and it's like being in neutral approaching it. I have to use the brakes again there. A few miles away, it's back to normal. I've pretty much stopped charging to 100% before big trips. 90% seems good enough usually. :)

tes-s | May 25, 2017

The times I have seen no regen is when the battery is 100% charged or the battery is very cold.

+1 @UnshodBob! Feels strange....

p.c.mcavoy | May 25, 2017

@PBEndo - "It is possible that in each case the operator hit the brake pedal, but I am told you are not supposed to do that when on a dyno."

Running official dyno test cycles for purposes like EPA emissions of fuel economy certification require that the vehicle be driven to a very set vehicle speed profile. For the cycle to be considered valid, there is a defined tolerance window on speed that must be obtained. This includes at points of common test cycle coming to a complete stop and varying different deceleration events. How much the brakes get used depends at times on how familiar the driver is with the specific vehicle, as well as individual driver behavior. For dyno testing I've been involved with in my professional life there actually can be quite a bit of variation between different drivers, ranging from some that are very smooth, likely seldom use much brake, to those that at times might be referred to as "digital" in their driving behavior, meaning it's full throttle/hard on the brakes to hold within the speed tolerance.

I agree that if someone were trying to achieve the "best" test result from a minimum energy use/maximum range perspective, driving the cycle using the absolute minimum of brake pedal would be best, and I'd expect some drivers can achieve that. But I also expect many might not.

Just my experience.

PBEndo | May 25, 2017

The dyno tests were purely horsepower demonstrations. Efficiency was not a concern.

gjgerhard | September 21, 2019

I am always conscious of any signals I am sending to the drivers around me. That is especially relevant to brake lights. I want to know when the brake lights are activated. Obviously, they will come on when I press the brake pedal, but now we have a car that turns on the brake lights by itself (during hard regen, for instance). I want to know when that happens.

OK, so the brake lights on the little icon come on when the real ones do. That's very cool, I like that. However, for me, and probably every other driver who uses reading glasses, that feature is utterly useless. My car is red. The icon car is red. The brake lights are red. It really is impossible for me to tell whether the icon brake lights are activated without either putting on my reading glasses (not a good idea while driving), or else getting my face about 10 inches from the display (an even worse idea when driving).

One situation where this makes a difference is controlling the cruising speed with the thumb wheel. That can cause a very brief deceleration. Is that activating the brake lights? I have no idea. Do the drivers around me think I must be an idiot because I keep flashing the brake lights on and off? I have no idea.

So, hey, Elon: either make the icon car considerably bigger (or at least give me the option to do that), or put a large white border around the brake lights, or make the brake lights much bigger (yes, way out of scale, that's ok), or let me paint my icon car white. I want to be able to see at a glance whether my brake lights are on or not. Thanks.