The Secret to a Great Electric Car? How It Brakes

By · October 19, 2011

What's the secret sauce of electric cars? In a speed-obsessed society, you’d expect it to be acceleration—which most EVs have in spades. But actually the thing that distinguishes one electric car from one another is not how quickly it gets going, but the way it stops. In recent weeks, I’ve reached out to engineers from BMW, Ford, Toyota, Honda and Tesla to learn about each company’s approach to regenerative braking—the system that provides the feel or flavor of their electric cars.

I say it’s secret, because not everybody is talking. Toyota isn’t saying much, which is ironic because this investigation was inspired by my drive, last April, of the “phase zero” version of the new RAV4 EV—a co-production of Toyota and Tesla. That vehicle had the most aggressive regen of any EV I’ve driven—bringing the car to a rapid halt when you let off the accelerator pedal. The Tesla Roadster-esque acceleration and deceleration made it a blast to drive, but I expect it will get dialed way back in the production model, based on the single line that Toyota was willing to offer for this mini-survey of electric car braking. “What exactly is it going to look like when it comes to market? I can’t tell you that,” Jana Hartline, Toyota’s environmental communications manager, told me. “Toyota always strives for the best drivability and smoothest ride for the customer.”

Toyota RAV4 EV

An early prototype of the new Toyota RAV4 EV. (Photo: Brad Berman.)

I thought Tesla could shed light on the subject, both in terms of the RAV4 EV and the upcoming Model S—but after multiple emails over several weeks, I’m still waiting for a response.

Like Toyota, Honda only offered a banal comment about regen braking, and only as it relates to its IMA hybrid system—declining to say anything about plug-in hybrids or EVs. The message from the central office in Japan: “Honda's IMA System is designed to be a very familiar system to the driver, so that drivers who are used to the internal combustion system can use our hybrids without awkwardness or sacrifice.” I can only assume that the same would hold true for the company’s electric cars.

Ford: Seamless Driving and Maximum Regen

On the other hand, Sherif Marakby was happy to talk. The Ford executive is in charge of engineering and business regarding the Ford Focus Electric and the Ford C-Max Energi plug-in hybrid, as well as the company’s hybrids.

Marakby believes that Ford’s series regen brake-by-wire offers the best of all possible worlds: maximum regen under all conditions and a brake feel that is seamless to the driver. On, there’s been a lot of discussion about where to place the regen functionality: on the accelerator (go) pedal or on the brakes? “We blend it,” said Markby. “This is trying to simulate what a gas vehicle does. When you let off, you start the regen process, and as you apply the brake pedal, it continues.”

Ford C-Max Energi Plug-in Hybrid

Ford C-Max Energi Plug-in Hybrid

According to Marakby, this system captures 94 percent of the brake energy from the brake system in a city cycle—allowing almost all of the braking to be regenerative, with almost no friction braking. The system can throw as much as 15 kW to 20 kW into the battery pack.

Ford utilizes the same brake-by-wire system on all its electric-drive vehicles, from hybrids to pure EVs. The company is on the third-generation of that system. “What we learned from the first couple of systems is that if the brake system is any different from what the customer would expect as normal operation, we get complaints,” said Marakby. “And when you give them more regen, we actually don’t see a big benefit. If you get more aggressive, you’re not going to put any more energy into the battery pack.”

This means no special eco, performance, or coasting modes for Ford’s EVs—to increase and dampen acceleration or deceleration. “This is one of the things we debated internally,” explained Marakby. “But braking is purely a pedal map. You can do the same thing [as special modes do] by yourself [by how you apply the pedals]. If you’re cruising on the highway, it doesn’t make any difference. It’s just a matter of how deep you are into the pedal. You’re really not saving anything.” Marakby reiterated his confidence that Ford’s EVs and PHEVs would require no special modes or actions from drivers accustomed to gas-powered vehicles, and still absolutely maximize the efficiency and range of the electric-drive vehicles.

BMW: The Ultimate Braking Machine

BMW is taking a much different approach—trying to create a new category of driving and braking experience that is unique to electric vehicles. Aaron Singer, BMW product planning and strategy manager for electric vehicles, graciously answered my questions by email. It’s so different than the competition and such as departure from the current path of other EVs coming to market that I wanted to share his answers in detail. What's your overall strategy for regenerative braking in your electric cars?

Aaron Singer: Feedback from the MINI E field trial clearly indicated our driver’s preference for aggressive regenerative braking coupled with the ability for “one pedal driving.” This provides immediate feedback that they are driving something special, and is a unique feature of eMobility. The BMW strategy, as demonstrated in the upcoming ActiveE (all electric 1-Series coupe), is to maintain this aggressive regenerative brake feel as a unique eMobility selling point.

BMW ActiveE

The driving feel of the upcoming BMW ActiveE will reveal the company's progress on its EV-specific approach to braking.

How do you strike the balance between maximizing regenerative energy, and providing a braking "feel" that mainstream drivers are accustomed to?

For pure electric drive, emphasis will be placed on the regenerative aspects of the braking system. However, for increased driver acceptance, a “sailing mode” has been implemented, allowing the driver to coast at highway speeds – similar to a gasoline vehicle.

Will you provide the ability for drivers to control how much regen is happening via buttons/modes? Why or why not? Other driver-selected controls?

Yes. The ActiveE will be equipped with an “Eco Pro” mode. Once selected, this mode will slightly reduce HVAC and throttle response, and provide additional regenerative braking response. This mode allows the user to place a higher priority on range, extending the driving distance of the car by slightly reducing the creature comforts.

What's your strategy for balancing the benefits of coasting (periods when no regen occurs) versus strong regen (for example, when traveling downhill)?

Allow customer to choose. Due to the one pedal driving, the user can select whether the car is sailing or generating power.

How do you plan on displaying regen, state-of-charge, and range on the dashboard?

The dashboard will have a small display, allowing the customer to quickly assess their remaining battery state of charge. Additionally, a digital display shows the battery percentage, remaining range, and histogram of past performance. There is an analog gauge displaying the motor performance; communicating whether the motor is using or generating power.

New to EVs? Start here

  1. Seven Things To Know About Buying a Plug-In Car
    A few simple tips before you visit the dealership.
  2. Incentives for Plug-in Hybrids and Electric Cars
    Take advantage of credits and rebates to reduce EV costs.
  3. Buying Your First Home EV Charger
    You'll want a home charger. Here's how to buy the right one.