ev brake lights, when do they come on?

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jlittle

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#322750 19-Sep-2025 11:08

How do EVs, and other vehicles with automatic adaptive cruise, manage the break lights?

There are situations where a car may decelerate sharply without the driver touching the brake pedal. Or, one might be using one-pedal mode. Are the brake lights turned on then?
Or even gentle regen going down a hill?

Regards, John Little

Behodar

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#3416197 19-Sep-2025 11:10

On my Toyota they turn on when the car slows down "enough". Simply following a car with radar cruise typically won't trigger the lights, but it will if the front car slows down sufficiently. Basically it seems to emulate real-world braking fairly well.

It's easiest to tell when driving at night, because you can see the red glow in the rear mirror.

Edit: But a while ago I saw a video where the guy was lamenting his car - I think it was a Hyundai - for only turning on the lights when he physically presses the brake. Apparently they wouldn't turn on when following, even if coming to a stop. I don't know the specific model.

Edit 2: It was probably this video.

SteveXNZ

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#3416202 19-Sep-2025 11:18

Pretty much all modern EVs have sensors (mostly accelerometers) that measure deceleration and activate the brake lights even if you don't touch the brake pedal. I can easily see this at night when lifting of the accelerator in my BYD Atto 3 - there's a red brake light glow coming from the back, easily visible as I've removed the parcel tray.

crowmaster

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#3416204 19-Sep-2025 11:29

Without touching the brake, manual driving - Kia EV6

Brake regn levels - 0 is coasting and then levels 1,2,3 and max increasing in braking effect.

If brake regen mode is at level 2,3,max - the brake lights light.
If the brake regen is mode is at level 0 or 1, no brake lights.

Radar Cruise Control

Basically follows the above, light slowing down, no brake light, essentially if about level 2 or more, brake lights.

Levels regen level compared to ICE cars

0 = cruise - if you put your ICE car in neutral, but even then level 0 has less "drag" because there's no running gear, etc and it just coasts and coasts...

1 = light - closer to an ICE car with the drivers foot off the gas and car in gear

2,3 = braking, with increasing intensity - an ICE car on the brakes, except we don't apply any actual brakes, at all

max = max reg gen, slowing up briskly (but not an emergency stop, not "harsh braking) plus applying actual brakes at the end and coming to a complete stop

Hope that helps!

But every EV will be slightly different.

jlittle

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#3416372 19-Sep-2025 15:21

Thanks all. I'll look out for the "red glow" the next time I'm driving the Leaf at night.

Regards, John Little

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#3416378 19-Sep-2025 15:53

Yeah as said above - with the Atto 3, there seems to be a lot of brake light action, whenever the car is "slowing".

So with adaptive cruise, it'll look like I am constantly dabbing my brakes, or possibly even riding the brakes at times.

I don't notice it in the daytime, but at night the glow on the rear window is quite noticeable and frankly makes me wonder what it's like to follow me.

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RunningMan

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#3416426 19-Sep-2025 15:57

There's some EVs that seem to activate the brake lights at any level of deceleration - it's kind of like the boy who cried wolf if you happen to drive behind one as you really don't know if they are braking or not.

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Mehrts

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#3416427 19-Sep-2025 16:05

RunningMan:

There's some EVs that seem to activate the brake lights at any level of deceleration - it's kind of like the boy who cried wolf if you happen to drive behind one as you really don't know if they are braking or not.

I've noticed this quite a bit too, I wonder if the driver is just a bit too agressive with the regen, or whether the car is just a bit too sensitive with the illumination of the brake lights under very light regen braking.

My Model 3 has to be slowing down at a relatively decent rate before the brake lights come on when regen braking. I can see my high brake light in the rear-view mirror.

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#3416438 19-Sep-2025 16:50

See 5.2.1.30.2 of https://unece.org/sites/default/files/2023-05/GRVA-16-49e.pdf which EU adopted (1.3 m/s^2 = slowing 4.7 km/h per second):

If I'm not mistaken, this deceleration is only relevant to Leaf from 2018 (older Leaf didn't have such strong regen).

robjg63

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#3416443 19-Sep-2025 18:27

Mehrts:

RunningMan:

There's some EVs that seem to activate the brake lights at any level of deceleration - it's kind of like the boy who cried wolf if you happen to drive behind one as you really don't know if they are braking or not.

I've noticed this quite a bit too, I wonder if the driver is just a bit too agressive with the regen, or whether the car is just a bit too sensitive with the illumination of the brake lights under very light regen braking.

My Model 3 has to be slowing down at a relatively decent rate before the brake lights come on when regen braking. I can see my high brake light in the rear-view mirror.

On a Tesla, you can also see when the brake lights come on in the screen. The animation of the car shows the brake light when it is on. I gather it is a sensor that detects deceleration and works out when it is appropriate. Seems pretty spot on when I decided to take notice of this around a year ago.

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Wheelbarrow01

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#3416515 19-Sep-2025 22:42

Not an EV, but my wife's 2024 Toyota C-HR hybrid has a depiction of the brake lights on the dashboard which lets you know when the brake lights are on (both when applied by the driver or the car itself).

Batman

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#3416546 20-Sep-2025 09:27

jlittle: How do EVs, and other vehicles with automatic adaptive cruise, manage the break lights?

There are situations where a car may decelerate sharply without the driver touching the brake pedal. Or, one might be using one-pedal mode. Are the brake lights turned on then?
Or even gentle regen going down a hill?

depends on the EV. my Kia will light up at a certain threshold of kW of regen (which is not very much, so people think i'm riding on my brakes all the time)

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#3417300 22-Sep-2025 09:11

jlittle: Thanks all. I'll look out for the "red glow" the next time I'm driving the Leaf at night.

paulchinnz: ... If I'm not mistaken, this deceleration is only relevant to Leaf from 2018 (older Leaf didn't have such strong regen).

Agreed, my 2016 Leaf does not engage the brake lights on regen, EVER. Only when the brake pedal is depressed.

Please keep this GZ community vibrant by contributing in a constructive & respectful manner.

paulchinnz

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#3417306 22-Sep-2025 09:28

Yeah the 2018 Leaf was quite an improvement towards single pedal driving.

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#3417308 22-Sep-2025 09:52

My MG4 has a 'power' indicator in the bottom right of the dash display - smash the accelerator and it goes high, go down a hill with regen on, it goes negative.

I've noticed that if the negative number is -10 or below when doing over 50km/h, brake lights will be on. Slower than 50km/h, they'll come on at -8 and below. May be some grey area between.

Edit, it's very rare I ever touch the brake pedal in the MG4. I have the One-Pedal turned on (annoyingly, i have to turn it on every time I start the car) and it's rare I need to help it.