How do your Automatic brake lights work?


brake light indicator

How to find out when the brake lights come on.

In single pedal drive cars the brake lights come on when you lift off the accelerator pedal, but I wondered at what point that happens.

Initially a simple optic sensor installed in the brake light cluster and amplified to a LED on the dashboard would be able to show you.

Although a simple circuit is fairly easy to build it comes with numerous problems.

1) It requires a power source - where to get it or use batteries

2) It requires opening the brake light cluster to install it - may affect warranty

3) It requires running wires inside the car  - can be a real nuisance.

4) It turns out the brake light cluster also houses the running lights so there is always light in there and the absence/presence of light is an issue.

5) The circuit has to detect the light intensity change or simply relay the light intensity to the dashboard, and is a bit more complex and could be annoying with a light on the dashboard changing intensity

In the end I discarded that as a solution and went for a more simple approach - a piece of kitchen aluminium foil !

Carefully positioned in the rear window near the stop light it reflects back the brake lights on state which can be seen in the rear view mirror! [As shown in the photo]

Why did I want to know?

There are various reasons why this unknown is important, and it is strange that it is not shown on the dashboard by default in electric cars which display the power being used [in blue] or the charging under slowing down [in green].

An option to extend the scale in red would be easy to add when the brake lights are on [or brakes are being applied].

First reason is for cars following you: are your brake lights constantly coming on when you slow down and if not at what point do they actually come on.

Next reason is to maximise charging to extend the distance you can drive by minimising any physical braking.

Does this change when the battery is completely full, as there is nowhere for recharging to go.


The bigger picture

Brake light monitor
    





This rather fun photo shows the simple solution operating.

It may take a while to work out what is being shown due to the front facing, inside and outside and rear facing inside and outside and numerous reflection components.

Essentially the red light in the rear view mirror is what is seen when the brake lights come on with the original installation.

The installation took a few minutes to install but absolutely did the job..

What was discovered?

The car that was being used is a Fiat Abarth 500e.

In this model the brake lights come on when the accelerator pedal reaches a certain fixed point as you lift off.

This gives a consistent mode of operation and unless you lift off to that point you can accelerate and de-accelerate as much as you like without the brake lights coming on, and presumably without physical braking.

The mode of operation may vary for different cars so it is useful to know for each car.

What this means, as long as the aluminium foil is not red, all energy recuperated is presumably going to the battery.

So what if the battery is fully charged ?

The point the brake lights come on seems to stay the same but the slowing down effect is slightly less as there is nowhere for the electricity to slow you down to go. What is not known yet is if partial braking is applied without the brake lights coming on.

Note: So if you want to maximise recharging, it can be a good idea to not fully charge the car.

I was asked does the recharging increase when you brake. Well it does, at least the rate does increase in this car but as energy is being wasted using the brakes, less energy is being stored in the battery, the extra recharging is simply a means to help slow the car down for a very short period.

What I also discovered is if you stop on the flat, once stopped, the brakes lights may go out, this can be an accident hazard if cars are coming up fast behind you and not realise you are stopped.

So this simple solution is providing a lot of useful information that may have never been considered important.


Is it legal?

Probably not, but as the stop light in the window is fully visible, it would be hard to justify why it is not legal.

Was it fun to come up with  - Yes. An amusing solution; I share here.

Will I keep it - Yes, I have got rather used to it being there, and replaced the original piece of aluminium foil I stuffed in there for a test with a more elegant version, properly positioned and held in place.

Of course, none of this really matters — unless, like me, you find joy in noticing the unnoticed and finding out how things really work.

Who knows some other condition may occur unexpectedly. At least I know how my car is performing and how it can potentially affect other road users and how to get the most from the car.

This is an individual experiment, not official manufacturer guidance. Use at your own risk. Actual Vehicle safety/regulations may vary by country.