2013 Tesla Model S Long-Term Road Test

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2013 Tesla Model S: Regenerative Braking

August 20, 2013

2013 Tesla Model S

Last week's two-night adventure in our long-term 2013 Tesla Model S proved fruitful in demonstrating one of the car's better features: adjustable regenerative braking and one way it could be improved.

I began experimenting with the two modes (Standard and Low) on my way home from work and it was immediately clear that the "Standard" setting is far more aggressive. Enough so, in fact, that I began to wonder if the brakes lights were triggered simply by lifting off the throttle. So, I did an experiment to find out.

At home I had my wife drive up and down our street in both modes. We designated two points on the street, one where she was to lift off the throttle and another where she began applying the brakes. In the "Standard" setting the brakes lights came on at the lift-throttle location. In the "Low" setting they came on further down the street at the braking point.

So in "Standard" mode the brake lights probably illuminate every time the driver lifts off the throttle.

Now this isn't a truly scientific test. It could be the case that the brake lights are triggered by a certain level of longitudinal g-load (deceleration) rather than simply by lifting off the throttle, and that the standard setting is more capable of generating that level of deceleration. Either way, though, the result is this: When you lift off the throttle in the "Standard" setting, you'll likely trigger the brake lights.

This is a blessing and a curse. Obviously, if you're slowing down rapidly enough, people behind should know. Tesla and I disagree about that rate.

Here's where the subtlety is lost in the Model S. Any conventional car that allows you to lock the relationship between the engine and drive wheels doesn't trigger the brake lights every time you close the throttle. In those cars you're able to achieve the same or greater deceleration without triggering the brake lights. The Tesla's setup bothers me because in Southern California every time you apply the brakes it affects hundreds, if not thousands, of others behind you.

I simply don't want to trigger the brakes lights every time I lift off the throttle. Let me be clear: I also don't want to be rear-ended. However, I've been using engine braking as a subtle means to slow a car, ease into gaps, and gracefully maneuver without brake lights as long as I've been driving. It's a polite way to do things. And in 24 years of driving this way I haven't been rear ended as a result.

Possibly, I'm the only one on Earth (or at least in Southern California) to whom this has occurred.

Here's my suggested solution for Tesla. Rather than using two modes, the regenerative braking adjustment should be on a rheostat. It could be adjusted using a slider on the touchscreen. Even if the range were calibrated using the existing two settings as extremes, the driver could more precisely control his or her desired decel rate. This would allow Tesla to decouple the brake light trigger from the throttle (assuming that's how it's currently set up).

Keep the brake lights on the brake pedal. Keep traffic moving. Let the driver choose. After all, it's his car.

Josh Jacquot, Senior Editor @ about 7,000 miles

Comments

  • bankerdanny bankerdanny Posts:

    How about you throw a g-meter on and tell us what the rate of deceleration is in each mode then do the same thing with a couple of the sedans in your fleet. I bet that the tesla slows at a much faster rate. I don't think you can cite years of rear-ender free throttle off deceleration as a reason for Tesla to change the brake light application. It would be nice if other drivers always maintained appropriate distances and paid attention while driving, but they don't. Tesla is wisely protecting you (and most certainly themselves) from the risk of a driver following too close getting caught be surprise when you lift off and the regen slows the car at a faster rate than a conventional car. In the end there is no negative impact on you, so I'm not sure why you would want them to change it.

  • jjacquot jjacquot Posts:

    I can think of one reason, Danny: Because I don't only care about myself. I care about you, too. And if I'm causing you to jam on your brakes because of a subtle lift off the throttle then I have every reason to care. Possibly the bigger issue here is that driving with aggressive regenerative braking causes us to change the way we drive. Or maybe it should cause us to change the way we drive. I see another blog post coming. Thanks for the feedback. Josh

  • stovt001_ stovt001_ Posts:

    I'm so glad you thought of the point about it impacting all the other drivers on the road. Our Ford Taurus does not have a cancel button for its cruise control and that absence infuriates me. My wife wondered what was so bad about just tapping the brakes to cancel it and I gave her the same reason: because in LA, the moment someone's brake lights so much as flicker, the entire freeway comes to a screeching, grinding (sometimes crashing) halt on the spot and for miles back. I have this same "engine braking, no brake light dilemma" in my Miata. Engine braking in that car is very, very strong, enough that I worry about being rear-ended. It has never happened, but sometimes when I'm really using the engine braking to slow down, I'll lightly engage the brakes too just to activate the lights.

  • stovt001_ stovt001_ Posts:

    Josh - another good point about the differences in the way we drive an electric car vs a conventional ICE car (you're full of good points today). I don't have a lot of experience in electric cars (unless you count indoor karts, or what K1 passes off as indoor karts) but it does seem like electric cars respond more to changes in throttle position to slow down, vs coasting like conventional, automatic-equipped ICE cars. Just as you adapt to not having idle creep in many cars with automated manuals, maybe you just get used to not coasting in an electric car.

  • owaishaq owaishaq Posts:

    If you don't lift your foot completely then the lights don't turn on. I have noticed if i am going at 70 and only lift my foot enough so that i am getting back less then 50kwh of energy back then the lights don't turn on. If i lift my foot completely then i get 60kwh back and light turn on. Try not to lift your foot completely when you need to slow down. So far i have no issues with this mechanism.

  • tokyorush tokyorush Posts:

    The car does not turn on the brakes whenever you lift off of the throttle, it is based on deceleration. If you want to test without having to have someone out of the car, just turn on the picture of the car by pressing the T at the top of the screen. You will note that this picture of the car (actually any picture on the screen) actually shows what the car is doing - what lights are on, what doors are open, and, importantly, when the brakes are on. Now, you can have someone sit in the passenger seat and see when the lights go on or not. Note - they don't go on unless you are decelerating pretty rapidly - at least on my car. I'd want the lights on.

  • cobryson cobryson Posts:

    @stovt001 Little secret, try hitting the "Resume" button when you want to Cancel. In my Mustang, it's labeled only "Resume", but functions as a "Resume/Cancel" button. I'd bet it works the same way in your Taurus.

  • greenpony greenpony Posts:

    I think you're all missing the point here. 24 years of driving and not one single rear-ending? You are luckiest driver I know of.

  • greenpony greenpony Posts:

    Cobryson, that's interesting. I've been driving Fords for years and never realized that. You can bet I'll try it out at the next opportunity. What MY is your Mustang, btw?

  • cobryson cobryson Posts:

    @greenpony It's a 2011, but as I recall my old '98 worked the same way...however a bit of googling seems to imply I may be crazy. 99.9% sure my 2011 works that way though...

  • The Tesla just uses an accelerometer to determine when the brake lights should come on. This means that it sees no difference between heavy regen and heavy manual braking. Your concerns about the brake lights is honestly more of an issue with how all cars handle their brake lights rather than the Tesla specifically. Coming from the video game world, I've wondered why brake lights aren't analog so as to match the analog input of the brake pedal itself. My idea is to have the main tail lights start very dim and get brighter the harder the brake is pressed. At the same time, the center "light bar" in the rear would only be lit up (at full brightness though) on the outer edges with light braking, and then the farther you push the brake pedal the farther inwards from the sides the "light bar" would move inwards, but would be fully light up with the brake pedal at "only" around maybe 75% or so. For example, it could look like this with light braking @________@, with semi-moderate braking it would look like this @@@___@@@, and would be fully lit up with heavier braking. Considering most "light bars" are made up of individual lights anyway, I can't see this being that difficult/expensive to implement...

  • bankerdanny bankerdanny Posts:

    I understand your point Josh, but your response doesn't address my question. Is the rate of deceleration materially faster on the S in normal mode than a regular car with no regen? If it is then the lights are a good warning measure to make sure drivers behind you are paying attention to the shrinking gap between them and you. It doesn't take much pressure on the brake pedal to activate the lights, that's why I (and I am sure you) don't "jam on the brakes" every time you see brake lights in front of you on the highway. Why should I react any differently to the lights coming on when you activate regen on the S in front of me? I jam on the brakes based on how fast I am closing on you, not the mere presence of the lights.

  • i think that further study is needed before suggesting that the tesla should not activate the brake lights upon roll off deceleration. take the tesla and an 'equivalent' rival, drive them side by side on a closed course at ,say, 60mph, then at an agreed upon time roll of the throttle. if the standard car needs brake application, hence triggering it's brake lights, to decelerate at the same rate as the tesla then i think there is nothing wrong with the tesla's brake light activation.

  • agentorange agentorange Posts:

    I understand why Tesla have done this. Watch in traffic those that get baffled by a car slowing down without brake lights showing because it is a manual and it has a lot of engine braking. The motorcyclists here are always complaining about near misses because a bike slows down really quickly when you roll off the throttle. Sad to say much of this is a sad reflection of the poor driving standards in the US where following way too close is the norm. As a result a flash of brake lights is bound to result in more anxiety than if everybody drove with more sensible spacing.

  • stovt001_ stovt001_ Posts:

    @cobryson: Our '04 Taurus has four steering wheel control buttons for the cruise: one for on, one for off, one for accelerate/set and the last for decelerate/resume. So if I hit the resume button while cruise control is active, it just bumps the set speed

  • stovt001_ stovt001_ Posts:

    Speaking of cruise control on our Ford Taurus, that really illustrates the absence or presence of engine braking. In the Taurus, if you use the decel button to reduce the set speed by 2 MPH increments, it will be a very long time until you actually see a drop in speed unless you're going up hill. In the Miata, one flick of toggle in the decel direction and your head is snapping forward.

  • cobryson cobryson Posts:

    @stovt001 fair enough, wasn't sure what the layout was in your Taurus. Confirmed on my drive home today that my '11 Mustang has a hidden Cancel feature on the Resume button.

  • bassrockerx bassrockerx Posts:

    i dont see the big deal. if i am behind someone i would rather see brake lights and get a warning. if i am at a propor following distance matching speed then i would not notice you was slowing down unless i have closed a lot of distance and then the speed difference could be great. if i am closer then it is reaciton time and with no lights to react to it makes it harder for me (requires a lot of attention) if the car is aplying the brakes at all whether manually or automatically it should be required to signal that brakes are being applied.

  • As @owaishaq and @tokyorush point out, the brake lights do not always come on. When I am coming to a stop sign/light or slowing traffic ahead, I typically do not lift completely off the accelerator pedal. This way I have a more gradual stop than if I lift off completely (which sounds like you did in your test). In this case when you feather the accelerator pedal, the slowing isn't as sudden, and the break lights often do not turn on. If you want to test at different slowing speeds, as @tokyorush states, you can simply tap the "T" at the top of the 17" display, and the picture of the car on the screen will show when the break lights are illuminated.

  • jalger jalger Posts:

    It's called THE LAW. The Volt does it too. This is a safety issue and the notion that if I hit the brakes, or slow my vehicle down rapidly by any means and choose not to alert the fool riding my rear end is ludicrous. Seems like some people just have to find something to complain about but it's a safe bet Tesla was forced to do this due to Federal Safety Standards.

  • plazman plazman Posts:

    I'm frequently surprised how little the Edmunds staff understand this car. Maybe it's because they spend so much time driving ICEs and only take these out for a weekend. For anyone that buys them, they research them and they understand a lot more about them. Case in point, #1, you didn't need to have your wife drive up and down the street. You can tap the Tesla logo (or the Controls button) and see a live representation of your car, including when the brake like are on. #2, when you drive the Model S, you very rarely use the brakes, and when you do, it's at very low speeds just to come to a complete stop. In every driving, you modulate your right foot. Let up gently, the car slows smoothly, lightly regenerating, no brake lights. Let up aggressively, full regen and brake lights. My friend also has trouble learning to drive it correctly, because he's in the habit of completely and abruptly lifting the accelerator pedal. But I expect more from people that review cars professionally. Thank you though, I'm enjoying reading your comments and agree with most of them!

  • skintrade skintrade Posts:

    Another way to assess the braking is to turn on your backup camera and do this experiment at night. You can see when your brake lights come on in relation to your foot coming off the throttle.

  • jeffhre jeffhre Posts:

    My understanding is the brake lights don't come on simply because you are modulating the pedals, but only if there is enough regen braking involved to slow the car enough to warrant an "I am braking now" signal. In any case that's what manuals are for right. This information has been published before. The car has an accelerometer, so no, it is not a random oops no pedal pressure and regen is in normal better fire up the brake lights algorithm.

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