Tech’splaining: What Are Regenerative Brakes, and How Do They Work?

  • Christian Wardlaw has 25 years of experience serving in automotive editorial leadership roles with Autobytel, Edmunds, J.D. Power, and Tribune Publishing. A married father of four, Chris is based in the Los Angeles suburbs and believes fuel cell electric vehicles will power the future.

can be reached at christianwardlaw@gmail.com
  • Christian Wardlaw has 25 years of experience serving in automotive editorial leadership roles with Autobytel, Edmunds, J.D. Power, and Tribune Publishing. A married father of four, Chris is based in the Los Angeles suburbs and believes fuel cell electric vehicles will power the future.

can be reached at christianwardlaw@gmail.com
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In a traditional car, when the driver applies the brakes, the kinetic energy that the gas engine creates to propel the vehicle converts into heat through the friction of the brake pads against a disc or a drum. The brakes dissipate that heat into the air, wasting the otherwise useful energy.

Furthermore, modern cars are loaded with electronics, from infotainment systems to advanced driver assistance systems. They need electricity, and lots of it. With a traditional powertrain, a belt-driven alternator supplies that electricity, and the gas engine provides the energy to turn the belt that powers the alternator. As a result, the belt-driven alternator reduces the engine’s efficiency.

A regenerative braking system is designed to capture the kinetic energy traditionally lost during coasting and braking, eliminating the alternator and making a vehicle more efficient.

How Regenerative Brakes Work

Regenerative Braking Diagram
Most regenerative braking systems capture the kinetic energy that is otherwise lost during coasting and braking, converting it into electricity and storing the juice in the vehicle’s battery. (Image: Bosch)

Regenerative braking systems are common in hybrid and electric vehicles (EVs), though they are not exclusive to them. Mazda recently offered one with its gasoline-fueled 2.5-liter 4-cylinder engine to power onboard electronics and improve fuel economy, reducing the load on the engine and making the car feel more powerful while improving gas mileage.

Most regenerative braking systems rely on an electric motor that becomes a generator when the vehicle is coasting or stopping. This is why they’re most common in hybrid and EVs. In generator mode, the electric motor captures the otherwise lost kinetic energy, converting it into electricity and storing it in the powertrain’s battery.

Mazda’s i-ELOOP system was an exception to this rule. Since the cars weren’t hybrids, the technology used a freewheeling alternator to harness the kinetic energy. The alternator transferred the energy to a capacitor, and the capacitor immediately used it to power the vehicle’s electrical components. That reduced workload on the gas engine, making the car more efficient.

Getting back to hybrids, when the electric motor operates as a generator to capture the energy and transform it into electricity, it keeps the battery charged up and ready to assist with acceleration and to power accessories when the automatic stop/start system turns off the gasoline engine. At lower speeds, the hybrid can operate for longer periods of time in EV Mode, further reducing use of the gasoline engine.

In an EV, regenerative braking helps to preserve battery life, extending the driving range or reducing the amount of necessary recharging. And in both types of vehicles, it reduces the amount of active braking a driver performs, extending brake life and reducing brake dust.

Regenerative Brakes Reduce Your Carbon Footprint

2019 Nissan Leaf
The latest Nissan Leaf’s e-Pedal system allows a careful driver to operate the car while rarely using the conventional braking system. (Photo: Nissan)

Early regenerative braking systems felt funny to a driver. The added friction in the drivetrain produced more slowing than normal when lifting off the accelerator pedal, and the brake pedal could feel like an on/off switch during brake application, making them hard to modulate smoothly.

Newer systems feel more natural to a driver. In hybrids, blended regenerative braking systems improve pedal feel from behind the wheel, making the cars more enjoyable to drive. In EVs, drivers may be able to calibrate how aggressively the regenerative braking works. Some EVs, like the Chevrolet Bolt EV and the Nissan Leaf, provide one-pedal driving because the highest setting can bring the car to a full stop. A careful driver who plans in advance doesn’t need to use the brake pedal at all.

Regardless of the type of vehicle in which they’re used, the point of regenerative braking systems is to increase efficiency and reduce carbon dioxide emissions. With a hybrid, regenerative brakes improve gas mileage and reduce the amount of gasoline the vehicle burns. With an EV, battery preservation reduces electricity consumption, and if your local utility relies on fossil fuels to create electricity, using less of it reduces carbon dioxide emissions.

Clearly, aside from odd pedal feel when the driver presses on the brakes, there is no downside to a regenerative braking system. And in the latest hybrids and electrics, you get used to that right quick.


About the Author

  • Christian Wardlaw has 25 years of experience serving in automotive editorial leadership roles with Autobytel, Edmunds, J.D. Power, and Tribune Publishing. A married father of four, Chris is based in the Los Angeles suburbs and believes fuel cell electric vehicles will power the future.

can be reached at christianwardlaw@gmail.com
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