Electric Vehicles

What is Regenerative Braking & How Does it Work to Charge an Electric Vehicle Battery?

This cutaway diagram of General Motors’ 2005 Sequel hydrogen fuel cell concept car showcases its regenerative brake system. (Image/GM)

When battery electric vehicles (BEVs) and hybrid electric cars started really showing up in the early part of the 21st century, they brought with them a term that many gearheads weren’t familiar with: regenerative braking.

And while the concept of regenerative braking was new to a lot of automotive enthusiasts, the technology and its application have been around for a long time. In fact, the core principle behind regenerative braking goes hand-in-hand with the first electric motor ever made.

But first things first…

What is Regenerative Braking?

In the context of a hybrid or electric vehicle, regenerative braking describes a vehicle’s ability to transform kinetic energy (AKA motion) into electrical energy. That energy is then used to recharge an electric vehicle battery while it’s being driven.

In other words, when you take your foot off the accelerator pedal in an electric vehicle, the regenerative braking system kicks in to automatically charge the EV’s battery.

That means a regenerative braking system essentially extends the usable range of an electric or hybrid vehicle, having a significant impact on the viability of an electric drivetrain.

How Does a Regenerative Braking System Work?

While we could dedicate a few thousand words to the basics of electrical engineering, we’ll distill it down to this: an electric motor and an electric generator are basically the same thing.

So, for brevity’s sake, if you apply electrical power to an EV’s electric motor, the output shaft will begin to spin and propel the vehicle. But when you stop supplying power, the vehicle coasts and the wheels manually spin the same shaft on what’s now become an electric generator.

Or said another way, on deceleration, that manual rotation from the vehicle’s wheels are now turning the motor, in essence changing it into a generator, which creates electricity to charge the battery.

Again, this is a very, very simplified explanation. If you really want to get into the weeds on the electromagnetic principles behind how all this works, you may want to start with a guy named Michael Faraday.

Say you want to slow down from 45 mph. You take your foot off the accelerator pedal to coast and electricity stops flowing to the vehicle’s electric motor(s). But obviously the car won’t come to an immediate, screeching halt. Since the car still has momentum, it will continue to coast until that kinetic energy is gradually bled away (from friction, air drag, etc.)

That’s where the regenerative braking system kicks in. It converts the vehicle’s electric motor into an electrical generator, driven by the vehicle’s wheels, which then begins sending a charge back into the electric vehicle’s battery.

Regenerative brake systems continue to evolve. Nissan’s e-Pedal technology is a good example. It essentially lets you use the accelerator pedal to speed up AND slow down. Click here to see how it works. (Image/Nissan)

Even better, this system will typically slow the vehicle down faster than normal deceleration. That means less wear on the vehicle’s traditional brake components, like its pads and rotors.

…Which brings us to another good point.

Does a Regenerative Braking System Replace a Traditional Friction Brake System?

No. While a regenerative braking system can positively impact an electric vehicle’s ability to slow down, you’ll eventually have to press the brake pedal to get the vehicle to come to a complete (or sudden) stop.

That’s where a regular hydraulic friction brake system jumps into action.

An electric vehicle with regenerative braking still has a traditional friction brake system—meaning it has brake rotors (maybe drums in the rear), brake calipers, pads or shoes, a master cylinder, and hydraulic DOT-spec brake fluid.

In other words, an electric vehicle can essentially have two braking systems. So, perhaps in the sole context of slowing down and stopping, it’s easiest to think of a regenerative braking system as a supplement to the hydraulic friction brake system we all know and love.

Advantages of a Regenerative Brake System

We mentioned the two main benefits above. A regenerative brake system can:

  1. Recharge an electric vehicle battery while driving
  2. Slow a car faster to reduce wear on the traditional brake system components

While not all regenerative braking systems operate in the same way (they’re part of each respective manufacturers’ EV “secret sauce”), they typically offer those same benefits.

But there’s one other important facet to note here. Nowadays, a regenerative braking system works automatically. In most cases, a driver will barely notice it’s operation; the system seamlessly switches its regenerative mode on and off with no effort or input from the vehicle operator. You may simply notice that the vehicle just decelerates a bit faster.

Cadillac introduced “Regen-On-Demand” in its hybrid-electric ELR. Through paddle shifting, it lets the driver temporarily control the regenerative braking system. Read more about it here. (Image/Cadillac)

With specific regard to battery charging, regenerative braking is a big deal. Since each EV maker is on a perpetual quest to increase the viability and practicality of electric vehicles, battery range is at the forefront of those efforts.

Given regenerative braking’s ability to recharge an electric vehicle battery through normal driving (which results in significantly increased electric vehicle range), it has become a fundamental system within seemingly every EV manufactured today.

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  4. Terry Malone says:

    Does the Regenerative system recharge the battery which is normally charged by plugging it in>? …or does it charge the Hyybrd battery….ie…the battery that is charged from the turning wheels of the car?

    • Hey Terry, If I understand your question correctly, here’s my shot at an answer: While all HEVs are different, it’s fair to say that, generally speaking, regen charges the main battery pack used for powering the electric motor (drive wheels).

      HEVs do have an additional 12V battery like the one you’ll find in a traditional ICE vehicle, used for starting the engine and powering basic 12V vehicle accessories. This battery is typically charged by the HEVs engine.

  5. Robert Spence says:

    this may seem like a dumb question because I have no expertise whatsoever. I can’t purchase a traditional electric vehicle care which requires a cord and charging system as I have only on-street parking. Does regen braking work so that charging from an external source is done away-with and all the charging comes from the regen braking alone?


    • Hey Robert, the short answer is no. Think of brake regen as a way to extend a charged battery’s life–not a way to completely recharge the battery.

      In other words, regen braking can resupply the battery with some charge while driving, but not enough to maintain charge independent of other external sources (EG, plugging it in).

  6. larry thomas says:

    Another downside of regenerative braking: Impeding or slowing the car to below posted speed limit when traveling downhill. While driving downhill on a prolonged grade, depending on the degree of slope, the regenerative braking would impede the vehicle’s downhill momentum, necessitating use of the accelerator where otherwise it would not be needed. This is another instance of a negative when using regenerative braking.

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