Why Do Electric Cars Brake So Often?

The other day my wife was behind a Tesla and commented they used their brakes a lot more than other drivers.  Last summer I’d thought somewhat the same thing when driving behind a BMW i3. Was this our imagination or is there something to this?

Image of Tesla vehicle with bike on roof rack

Tesla electric vehicle with bicycle (Photo: Tesla Motors)

Regenerative braking

The answer, I found, lay in the regenerative braking system in both of these cars. For the most part, an electric motor and a generator are the same. If you apply electricity to a motor, it will turn. Likewise, if you turn a motor, it will generate electricity. Electric cars take advantage of this duality with a regenerative braking system. When the driver lifts a foot off the accelerator or go pedal (since it is not, after all, a gas pedal in an electric car) the motor works in reverse; instead of using electricity from the batteries to turn the wheels, the kinetic energy from the car’s movement is used to generate electricity that recharges the batteries.

Some electric cars (EVs) such as Tesla and BMW allow full regeneration. When a driver lifts their foot from the go pedal, the regenerative system causes the car to decelerate quite quickly, similar to pressing the brake pedal. People who own these will often talk about one pedal driving as they rarely need to use their brakes. Each time they lift their foot from the go pedal they’ll likely also activate their brake lights. They’re not really braking. But they are. Sort of.

Other EVs limit regenerative braking so their cars will respond more like a familiar internal combustion engine (ICE) with an automatic transmission. Rather than significant slowing, they coast a bit more and brake lights are not as necessary.

There are improvements coming to both. As drivers become more accustomed to electric, all EVs will likely allow full regeneration. As well, Tesla and BMW have both been faulted for brake light activation being too sensitive and are expected to make some adjustments.

KERS in Formula One

Spark-Renault all electric Formula E race car. (Photo: SBNation.com)

Spark-Renault all-electric Formula E race car. (Photo: SBNation.com)

Formula One vehicles have utilized this technology since 2009. Though not really an electric car they are a hybrid with a KERS or Kinetic Energy Recovery System. The driver can utilize the KERS motor as a brake that charges a capacitor or battery (typically about 60kW). When they need a bit of extra power, they can engage the KERS motor for an additional 80 or so horsepower.

Likewise, the new all-electric Formula E cars make significant use of regenerative braking and allow drivers fairly detailed control over how much regeneration is used through dials on the steering wheel.

New World

Electric cars are far from any kind of panacea. While they may reduce fossil fuel consumption as well air, water, and surface pollution (and noise pollution at lower speeds), they still require a lot of space to drive and park. Like all motor vehicles, they can be a dangerous weapon in the hands of the driver.

I do believe EVs are a significant improvement over ICE vehicles however, and we’re certain to see increasing numbers of them on the road. Currently, we’re adding about 60 per month to the Twin Cities roadways (primarily Nissan Leaf, Tesla Model S, and BMW i3, with a few Fiat 500e’s, and Smart ED’s) and that’s expected to continue to increase. Altruistic virtues aside, they can also work from an economic standpoint. For many people the savings in fuel and maintenance more than make up for the higher initial cost.

This is a bit outside normal streets.mn discussion but is presented in an effort to keep streets.mn readers the best informed on Planet Earth. Well, at least the best informed in the Twin Cities.


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9 Responses to Why Do Electric Cars Brake So Often?

  1. Mike Hicks April 15, 2015 at 11:40 am #

    Thanks for this, Walker. Here’s a video of brake lights on a BMW i3 that shows this effect.

    Having driven manual transmission cars for most of my life, I often use engine braking to similar effect. But as the video shows, regenerative braking can bring a car’s speed down to practically zero, and do it pretty quickly, something that’s not really possible with engine braking.

    I haven’t driven an EV or a hybrid, so I’m not sure whether I’d like to have heavy regen or let the car coast more. I think it would probably be better to have more aggressive regenerative braking at low speeds and less on the highway. That would be similar to the effect of engine braking now.

    A lot of regeneration is often good, since it can reduce the amount of wasted energy when the vehicle is slowing down. But if you’re cruising on the highway, I suspect it’s better to let the car coast more.

    Some cars allow the level of energy regeneration to be cranked up or down to match the driver’s preferences, but I’m not sure how finely it can be tuned.

    • Walker Angell
      Walker Angell April 18, 2015 at 9:35 am #

      Great video. The i3 I was behind was like someone driving with one foot on the accelerator and one on the brake (NYC cab driver?) with their brake lights coming on constantly for short bits. They may have been a new driver and still getting use to the regen. Really weird to follow though.

      It seems most EV’s have either High or Low regen without much else. Formula E can adjust attack, release, curve, and overall though I think only overall and release are available on the wheel.

      Formula E steering wheel

  2. Ben Franske April 15, 2015 at 2:31 pm #

    I’ve been driving a Chevy Volt for the past three years and it offers the ability to switch between a mode which operates similar to a typical automatic car with coasting or a mode with aggressive regenerative braking. I’ve never followed myself so I don’t know if the brake lights come on in aggressive regenerative mode or not, I have always assumed they do not and I’m pretty aware of cars behind me when I’m driving, giving a brake tap to warn them if they’re following too closely and I’m anticipating a regenerative slowdown.

    The aggressive regeneration takes some getting used to at first and many people may not like it but I have come to love it. I’m of the belief that it encourages you to drive in a much more power efficient manner with longer, smoother slowdowns to stop signs/lights than you would otherwise do and which captures more energy then stepping on the brake as doing that also activates mechanical braking resulting in lost energy.

    • Walker Angell
      Walker Angell April 18, 2015 at 10:01 am #

      How often does the engine run in your Volt? I’d guess you’d have to run it occasionally to keep gas from getting stale?

      • Ben Franske April 19, 2015 at 2:34 pm #

        So I’ve started several answers to your question and scrapped them all. The truth is that it varies tremendously depending on your driving. City vs highway, outdoor temperature, etc. The car handles it all automatically including keeping track of the gas age itself and would use the gas itself if it got to be too old. I’ve never had that happen though. The car does require premium gas with the idea that it will last substantially longer. I believe that with premium gas they expect around a 6 month shelf life of the gas. In Minnesota that shouldn’t be much of a problem as the engine will also come on periodically in the winter to circulate fluids and keep components at optimum temperatures.

        My driving also tends to mix some longer trips in occasionally with a fair number of shorter ones so those longer trips will burn enough gas to keep the age low enough that the engine doesn’t need to run unnecessarily.

        • Walker Angell
          Walker Angell April 21, 2015 at 10:38 am #

          Thanks Ben. Interesting about your experiences with it. One Volt owner I talked to in a gas station recently said that it’s like a gateway drug. He’d had his for about a year, loved it, and was looking at Leafs and i3’s now. I’d guess a full battery electric would be difficult for you if you travel a lot until theres a bit more density in the charger network.

  3. Ben April 16, 2015 at 9:23 am #

    Interesting read. I am a car guy but am on the fence when it comes to EVs. Yesterday for my job I drove 690 miles. In doing so used a fair amount of gas. This is an extreme example and rare for me to travel that much in one day. My point is that until EV range improves to allow for real road trips, and not cost $70k, they will continue to chip away at environmental issues bit by bit unless you start looking at where the raw materials come from:

    http://www.theguardian.com/environment/2012/aug/07/china-rare-earth-village-pollution

    The environmental question is a tough one. But if the conversation is to be had, let’s not ignore where the green tech comes from.

    • Walker Angell
      Walker Angell April 18, 2015 at 10:13 am #

      Hopefully the future is better than the present. EV tech (and hydrogen & petrol) is improving with batteries delivering more power and for longer life. Motor efficiency is improving. Power generation is improving. I agree though that ‘green tech’ may not be nearly as green as we would like. Some folks at the U of M released a lifecycle analysis study this past December that seemed to do a good job of looking at these various issues.

      A neighbor drives around the state in a Nissan Leaf. He said it wasn’t easy at first but has gotten better as he’s found places where he can charge while he eats, sleeps, or meets with clients. He’s looking for a used Tesla for the greater range and so he can use their superchargers.

  4. Colin Lee April 16, 2015 at 9:02 pm #

    There are already several rare earth-free motors available (LG, Ricardo, HEVT, Baldor) and most production EVs have already cut back substantially from early models. Even when you consider environmental costs from building an early stage electric vehicle, they don’t even being to factor against the lifecycle costs from extracting, burning, and refining oil. As numerous experts have pointed out, simply the dirty electricity used to refine one gallon of gasoline can drive an EV over fifteen miles.