Mazda's i-ELOOP regenerative braking system.Mazda
It’s counterintuitive to think that braking your car will make it go further, but when it comes to electric and hybrid vehicles, it’s a unique idiosyncrasy that isn’t necessarily well understood.
Regenerative braking, or regen braking for short, is becoming an increasingly important component in extending the range of such cars, which means it’s a technology that drivers may want to get to know better.
“It’s here to stay because it makes sense,” says Olivier Trescases, an engineering professor and director of the University of Toronto’s Electric Vehicle Research Centre. “But it takes some getting used to.”
Most hybrids and virtually all plug-in electric vehicles, including the likes of the Chevy Bolt, Nissan Leaf and Tesla’s entire lineup, have regen braking with variable settings built in. In most cases, drivers can choose to turn the feature off, in which case they can coast as normal when they take their foot off the gas pedal.
If regen is turned on, however, easing off the accelerator kicks in a commensurate amount of braking. Most cars with the feature allow low and high regen settings, where removing your foot results in either a little braking or a lot.
Whichever setting is chosen, the braking – as its name implies – helps to regenerate the vehicle’s battery and thereby extend its range.
The way it works, Trescases explains, is that the car’s motor acts as both an engine and a generator.
Ford Escape's regen braking mechanism.
When the accelerator pedal is being pushed, the motor cranks out energy from the battery to propel the vehicle forward. When the driver eases off the pedal and regen is active, the motor acts as a generator, the current reverses direction and therefore charges the battery.
“You have kinetic energy from the vehicle that’s transferred into electrical energy,” he says. “You are physically causing negative torque so you’re applying force to the wheel that slows it down.”
By using the motor itself to handle some deceleration, regen also saves wear and tear on a vehicle’s actual brakes. Regulations require all vehicles, including EVs and hybrids, to have physical brakes, but they end up being used less frequently as a result of the motor offset. They’re still there for drivers to hit when the regen function isn’t slowing the car fast enough.
Trescases estimates that physical brakes in cars typically last up to five times longer as a result.
As for the actual range benefits, estimates vary and depend on road conditions. Someone who drives downhill a lot with regen active, for example, will see bigger benefits – they’re likely to have a greater stored range when they finish at the bottom of the hill than they did when they started at the top.
Guesses from experts and manufacturers thus range from single digits to as high as 25-per-cent extra. Chris Reed, senior vice-president of research and development at Nissan Technical Center North America, is among those who believes gains are in the single percentages – but he says it is only one of several factors that affect range.
“We have lots of little incremental things where you’re trying to get every last drop [out of the battery], so that few per cent is huge at the end of the day,” he says.
The range extension isn’t necessarily the result of physical, mechanical factors, but rather how the car’s computer synchronizes the motor and the “gas” pedal. Manufacturers are still learning how to optimize the software that handles this, which means that further tweaks could lead to more regen gains down the road.
David Larsen, general manager of product management for Jaguar Land Rover, says this potential is one of the main reasons why his company is participating in Formula E electric racing.
“The findings that we have on the track are making their way right into the [Jaguar] I-Pace,” he says.
Regen braking also ultimately allows for single-pedal driving, with the secondary brake pedal needed mainly for fast or emergency stops. Many electric vehicles offer this capability, which has won fans among those who have tried it.
“That feeling of not being able to coast is a little bit annoying if you’re so used to it, but eventually you start to think of that pedal as a speed control rather than a torque control,” says Trescases, who uses the function in his Nissan Leaf. “You’re now setting your desired speed based on the position of your foot.”
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