How Does Regenerative Braking Work?

EVs use the braking system to recycle energy

Every vehicle on the road needs two basic things: something to make it go, and something to make it stop. For over a century, the internal combustion engine (ICE) used by gasoline-powered cars has filled the first need, while wasteful friction brakes have met the latter. One way that electric vehicles (EVs) really stand out is that they can hit both targets with one shot: The same electric motor that makes an EV go can also pull double duty as a generator, helping to slow the vehicle while also banking extra electricity in a process that’s known as regenerative braking.

What Is Regenerative Braking?

Regenerative braking is a descriptive term that means exactly what it says. When regenerative brakes are activated, the vehicle slows down while also regenerating some of the electricity that was originally used to accelerate it. That electricity is fed back into the batteries, where it’s available to accelerate the vehicle again in the future. 

This is different from traditional brakes that generate nothing but heat and noise when slowing a vehicle. Unlike ICE vehicles that only use traditional brakes, EVs make use of both traditional and regenerative braking.

How Does This Kind of Braking Work?

When you use the regenerative brakes in an EV, the electric motor that’s normally responsible for accelerating the vehicle switches over to work as a generator. Instead of taking electricity from the batteries and using it to rotate the wheels and accelerate the vehicle, the generator uses the forward momentum of the vehicle and continued rotation of the wheels to generate electricity that can then be stored in the batteries. In addition to charging the batteries, this process also slows the vehicle.

Illustration of how slowing an EV actually converts energy to feed back into the battery.

Bailey Mariner

Unlike traditional brakes, which only activate when you push on the brake pedal, regenerative brakes often kick in the moment you lift your foot off the accelerator pedal. This process is more pronounced in some vehicles than others, so lifting your foot off an EV accelerator pedal can even result in aggressive braking in some vehicles.

Every EV is a little bit different in how it uses its braking systems. Some EVs are designed to be driven without ever touching the brake pedal, with the regenerative brakes kicking in whenever you ease off the accelerator pedal, and more aggressive braking occurring if you take your foot off the accelerator pedal altogether. Other vehicles use a milder form of regenerative braking where aggressive braking only occurs when you push on the brake pedal, and some allow you to switch between different modes.

Why EVs Use Regenerative Braking

The main purpose of regenerative braking is to increase efficiency and range. Without regenerative braking, an EV is limited by the amount of charge that’s stored in the car’s battery the moment you unplug it and start driving. With regenerative braking, the vehicle can keep producing additional power beyond what was stored in the battery.

That’s because some of the battery power that’s expended to accelerate the vehicle is recaptured and stored whenever the vehicle slows or stops, and that power can be used later to accelerate the vehicle again. All of these actions work to give an EV extended range, even if it’s just a little bit here or there.

In addition to making an EV more efficient and extending its range, regenerative braking also has another beneficial byproduct: reduced pollution. While EVs do come equipped with traditional brakes, they’re used far less than they would be in a comparable ICE vehicle. That means they don’t have to be serviced as often, and they generate far less brake dust. Since brake dust contributes to air pollution and can lead to respiratory complications when inhaled, according to Science Daily, it seems the less of it there is, the better.

Why EVs Still Use Traditional Brakes

While EVs are designed to use regenerative brakes in a variety of situations, even stop-and-go traffic, every EV comes equipped with a traditional braking system as well. This second braking system brings additional stopping power in emergencies, and it also takes over in certain situations, like when an EV is stationary and needs to remain stationary.

Another example involves full stops. Some EVs use the regenerative brakes to bring the vehicle to a stop, and then automatically engage the traditional brakes to keep the vehicle in place until it’s time to move again. There is almost no wear on the traditional brake system when it’s used in that manner, and no brake dust is generated. 

The stopping power of the traditional brakes can also be added to the stopping power of the regenerative brakes when rapid decelerating is required. This type of use does cause some wear, but not as much as you’d see in an ICE vehicle driven in similar circumstances.

Is Regenerative Braking Really That Useful?

Regenerative braking is always useful, in that it performs a necessary task, but it’s more efficient in some situations than others. While regenerative braking can increase the range of a vehicle, and does increase range in many situations, the overall increase in efficiency always depends on factors like driving conditions, how aggressive the driver is when accelerating and decelerating, and even the size and weight of the vehicle.

Regenerative braking is typically said to be around 60 to 70 percent efficient when slowing a vehicle down, generating electricity, and storing it in the batteries. That level of efficiency doesn’t translate to a 60 to 70 percent increase in range though, because regenerative brakes only charge the batteries when the system is actually in use. That’s why driving conditions play such a large part in the effectiveness of regenerative brakes.

Stop-and-Go Traffic

In general, regenerative brakes are much more useful in stop-and-go traffic than they are in long distance freeway driving. That’s because the brakes are used a lot more in stop-and-go traffic, so the regenerative brakes naturally charge the batteries a lot more than they would if the vehicle were driving for long periods of time without slowing or stopping on an uncongested highway or freeway.

Terrain

Terrain can also play a part in the effectiveness of regenerative brakes. Driving downhill naturally provides more of an opportunity for regenerative brakes to charge the batteries than uphill driving. Consistently driving up one hill and down another will also result in more recaptured energy than driving on flat ground, as some of the energy used to move the vehicle up each hill can be recaptured on the way back down.

Vehicle Weight

The size and weight of an EV also impacts the effectiveness of regenerative brakes. Heavier vehicles take more energy to accelerate in the first place, so there’s more energy available for the regenerative brakes to recapture when slowing down. While regenerative brakes are used in vehicles as small and light as electric scooters, they actually have more of an impact the heavier a vehicle is.

Different Modes

Some vehicles allow you to switch between different regenerative braking modes, and that can also impact effectiveness. Switching between these modes will cause a system to regenerate more or less energy, and will also change the way the vehicle handles. For example, setting a less aggressive regenerative braking mode will allow a vehicle to coast longer when you take your foot off the pedal but it can’t recapture as much energy in that lower mode.

Recapture Rates

In real world conditions, EV drivers typically report recapture rates anywhere between 5 to 30 percent. Lower rates are associated with lighter vehicles and lots of freeway driving, while higher rates are associated with heavier vehicles and city driving. So while regenerative brakes are more useful in some situations than others, they still play an important part in increasing the overall efficiency of EVs.