Brake lights or not…

[eNate]

Somebody somewhere wrote that some EV has a brake light status indicator on the dashboard. I don't remember specifics but it sounds handy to have, or at least informative for the curious.

On the residential hill that I referenced, I routinely drive that at night and can see the glow of my brake lights reflecting off of other cars and street signs.

 

[alohart]

The steep residential hill I referenced earlier I drove today then measured. It's a 14% slope, losing 100 ft over a 700 ft run. The i3 descends it at a constant 21 MPH with no accelerator nor brake inputs from me, just "natural" regen.

Despite the constant speed, the brake lamps come on and remain on the entire 23 seconds of this descent.

You've provided strong evidence that the brake lights are illuminated when the regen power exceeds a certain level even when the deceleration rate is 0.

I could argue that brake lights should not be tied to regen power but only to deceleration rate. They should illuminate only when the deceleration rate exceeds a certain level to warn a driver behind to decelerate as well. Illuminating brake lights to maintain a constant speed, a 0 deceleration rate, is unnecessary because the driver behind would not need to decelerate. This brake light behavior occurs in ICE vehicles because the brake lights are illuminated by a switch on the brake pedal that doesn't know whether deceleration is occurring.

A good counterargument might be that on a descent, the driver behind might not be paying attention such that his vehicle is accelerating down the descent (e.g., @Bat66's motorcycle experience). By illuminating the brake lights while descending, even when not decelerating, the driver behind would be alerted to control the speed of his vehicle.

I can't think of a situation in which brake light illumination being tied to regen power alone wouldn't also work correctly when decelerating. Whenever an i3's brake pedal is pressed, regen also begins, even when in 'N', so a certain regen power level would function light a brake pedal switch. No need for an accelerometer to control brake light illumination..

 

[eNate]

In this same steady speed scenario, with just the slightest touch of throttle applied, the brake lamps stay dark.

So I don't think it's based on regen levels. I think it's based on (lazy) logic that the brake lights should come on if:
* brake pedal is applied
* neither pedal is applied but car is in motion
* preset g-force is exceeded

I could similarly make an argument (I'm not, just hypothetically) that if an EV is accelerating from zero, maximum rate of acceleration, then reaches desired speed (let's just say 65) and suddenly stops accelerating and begins traveling at that steady speed, that the brake lights ought to illuminate to warn the trailing (also accelerating driver or rider) that the EV is no longer accelerating, to prevent a rear end collision.

Perhaps the time has come for "coast lamps." I've seen them deployed on trucks here and there – amber center-mounted lamp that briefly illuminates when the truck is coasting, but not braking (as best I can tell). Or perhaps any change in the rate of acceleration. I'm not 100% sure what the activation threshold is.

 

[alohart]

In this same steady speed scenario, with just the slightest touch of throttle applied, the brake lamps stay dark.

Did the brake lights turn off when lightly pressing the power pedal after they had been on while descending? The power pedal must be depressed a bit to maintain a constant speed while descending certain grades even though the regen braking power might be equivalent to light friction braking that would illuminate brake lights. Would pressing the power pedal turn off the brake lights in all situations?

So I don't think it's based on regen levels. I think it's based on (lazy) logic that the brake lights should come on if:
* brake pedal is applied
* neither pedal is applied but car is in motion
* preset g-force is exceeded

For this to work, the brake light controller would need to illuminate the lights when the horizontal or vertical but not sideways g-force exceeded certain levels. The brake light illumination g-force level when descending would be much smaller than when decelerating. However, my watch can detect falling, so inexpensive accelerometers must be pretty sensitive.

 

[eNate]

Did the brake lights turn off when lightly pressing the power pedal after they had been on while descending?

Going back to this particular residential hill that I've been describing, yes. It's touchy because it doesn't take anything much on the accelerator to begin speeding up, but yes, just a baby's breath on that pedal and the brake lights cut off.

I would equate this to the "dumb" logic of a brake pedal operated switch, where any brake application = lights ON even if a car isn't slowing (or could even be accelerating).

I think it was a similarly easy decision for the engineers to posit that in most circumstances if neither pedal is being operated, the car would be slowing (regen) and therefore the lights should come ON.