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Discussion Starter · #1 ·
There are several threads that mention the question of when the Gen 2 brake lamps light. I'll link from them, but I think the definitive answer needs a thread of its own.

Here's the precision testing apparatus: :)



A strip of aluminum foil bent into a cupped shape and blue taped from the center brake light to the rear window, just diagonally enough that part of it can be seen from the rear view mirror.

Here's what I observed:

  • Any depression of the brake pedal at all turned the light on, even if it didn't do any braking.

  • In D, I could not get the light to turn on with my foot on the accelerator or off of both pedals.

  • In L, letting up slowly on the accelerator, the light turned on at some point between the start of deceleration and all the way up. It's off during light deceleration. I always saw it on well before the pedal came all the way up.

  • In both D and L, there can be a considerable amount of regen braking while the light still stays off. Going downhill on the freeway at about 55, I saw -9 kw in D with my foot off both pedals and the light, of course, off. In L, slowly letting up on the accelerator, I saw as much as -15 kw before the light came on.

  • The regen paddle always turned the light on, but there was about a one second delay. It feels like the regen comes on gradually.
Conclusions:

  • If you definitely want the brake light on, use the brake pedal.
  • In L, with your foot on the accelerator, the brake light will turn on and off reasonably, but you won't know exactly when (unless you're dragging tin foil).
  • If you're using the regen paddle, a car behind won't know for an extra second.
 

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Great testing and writeup
 

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Yes, thanks. Was wondering about brake lighting.
 

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Thanks for the testing and results.

Don't know if your testing confirms this, but the salesman that I purchased my Volt from told me that at a recent Volt training course, GM indicated that the brake lights will illuminate with 0.09 g or greater deceleration.
 

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I can just see a cop pulling a volt driver over and chewing them out for riding the brakes down a steep mountain when they're actually using the regen paddle.
 

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I can just see a cop pulling a volt driver over and chewing them out for riding the brakes down a steep mountain when they're actually using the regen paddle.
Or just driving in Low and feathering the accelerator to control speed and driving the driver behind nuts with the blinking brake lights. It could actually inure the driver behind to the point that they ignore the lights when the brake is being used in earnest.
 

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I can just see a cop pulling a volt driver over and chewing them out for riding the brakes down a steep mountain when they're actually using the regen paddle.
Everything I had read indicated that regen use turning on the brake light was based on an accelerometer reading, so in other words, if you are maintaining your speed going down a steep hill by using regen one way or another, I don't think the brake light will come on at all.
 

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Discussion Starter · #12 · (Edited)
Don't know if your testing confirms this, but the salesman that I purchased my Volt from told me that at a recent Volt training course, GM indicated that the brake lights will illuminate with 0.09 g or greater deceleration.
Makes sense. I suppose I could check it using Torque with my OBD-II, but I'm willing to take it on faith.

They also always light when the brake pedal is even slightly depressed, even without deceleration.

Or just driving in Low and feathering the accelerator to control speed and driving the driver behind nuts with the blinking brake lights. It could actually inure the driver behind to the point that they ignore the lights when the brake is being used in earnest.
The same pattern is commonly seen from a driver controlling downhill speed using brakes rather than shifting into the ICE version of L (or 2). Nothing unusual, although it might get the Volt driver unfairly disrespected.

Everything I had read indicated that regen use turning on the brake light was based on an accelerometer reading, so in other words, if you are maintaining your speed going down a steep hill by using regen one way or another, I don't think the brake light will come on at all.
Edit: On reading this, it makes sense. As I said above, I saw considerable regen (negative kW) going downhill without the light coming on. Although I didn't specifically look for it, this is consistent with maintaining speed, no decel, no brake light, no disrespect and no lecture from a cop. :)

Genius, sir... Thanks.
Thankee, kind sir. The tinfoil also prevents the brake light from being controlled by aliens. ;)
 

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Thanks for the testing and results.

Don't know if your testing confirms this, but the salesman that I purchased my Volt from told me that at a recent Volt training course, GM indicated that the brake lights will illuminate with 0.09 g or greater deceleration.
So... about 2MPH per second then, I think...
 

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When I worked on solar cars in the late 90's/early aughts, federal highway laws prohibited cars from using brake lights unless the brake pedal was depressed to engage mechanical brakes, no matter how much regen was being used (or engine braking, for that matter). This was clearly a safety concern and many home built EV's ignored the restriction though the governing organization for solar car racing required compliance with the law. Only relatively recently was that changed to not only allow non-mechanical beaking to engage the brake lights based on a minimum threshold of "negative g's," but actually require it in certain cases. This was adopted in the Gen 2 Volt, but would have been illegal for GM to do for much of the Gen 1 production run. Or so is my understanding... I only briefly looked at the updated rules when they were published, and recall there being a minimum deceleration rate where illuminating the lights is permitted, but not required, and now a threshold deceleration rate where the brake lights must be illuminated. Long story short, baesed on my understanding, it's based on g's, not how far the pedal is depressed, etc.
 

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Or just driving in Low and feathering the accelerator to control speed and driving the driver behind nuts with the blinking brake lights. It could actually inure the driver behind to the point that they ignore the lights when the brake is being used in earnest.
You obviously have never driven in New York City behind any taxi or limo driver!! They all drive with two feet and constantly ride the brake even as they are accelerating!!!
 

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Discussion Starter · #18 ·
You obviously have never driven in New York City behind any taxi or limo driver!! They all drive with two feet and constantly ride the brake even as they are accelerating!!!
That's because they follow so closely and their safety margin is so small that they can't afford the time to move a foot between brake and accelerator. They would probably like aggressive one-pedal braking/accelerating, as in the Tesla, but maybe not find enough braking in the Volt even in L.

Except one-pedal wouldn't display a brake light all the time, even while accelerating, which they probably like in order to force the vehicle behind them to keep more distance.

(In my experience this applies more to taxis (yellow and black), who get paid by the mile. The big limos are paid by the hour and usually drive with minimal speed change to give their VIP passengers a comfortable ride.)
 

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Here is a decent article about brake lights (officially called "stop lamps") during regen conditions. There are links in it to the U.S. regulations and the international (UN) regs.
 

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Great post...glad somebody did the test so I don't have to :)

I was driving my mom around in my new Volt and she was concerned that driving in "L" was going to damage it. When I showed her that it was 100% regenerative braking and not "downshifting", she then switched to "but that won't turn on your brake lights, will it?"

And now I have an answer :)
 
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