What is regenerative braking?

► Regenerative brakes explained
► How they work
► And what they can offer

As electrified vehicles become increasingly common, you can expect to hear a lot more about regenerative braking. But what exactly is this technology, and why is it so important to hybird and electric cars?

Regenerative braking is one of the key advantages an electrified drivetrain has over a purely combustion-powered equivalent as it enables the car to save energy that would otherwise be wasted. It does this by, as the name suggests, regenerating electrical power.

Electric and hybrid cars: further reading

• What are electric cars really like to live with?
• The cheapest hybrid cars you can buy today
• Future electric cars: upcoming EVs
• Longest range electric cars

It’s how mild hybrids, plug-in hybrids and EVs capture power, range and efficiency in a way that petrol and diesel cars don’t.

Braking is a waste of energy

Let’s start with the basic principle behind the way cars have been slowing down for more than 100 years – friction. When the brakes are applied, two or more friction surfaces are forced together, converting the energy of the moving car (kinetic) into heat energy, and reducing its speed. This is an irreversible process in which the heat is dissipated and lost to the atmosphere around the car.

But that heat was once energy you paid for at the petrol station. The fuel was taken from the tank, burnt in the engine, used to turn the wheels and move the car, so when the brakes generate heat there’s nothing more you can do with it. It is, quite literally, waste.

Wouldn’t it be good if there was another way? Well, there is.

Harnessing braking energy

Generating electricity is hard work. That’s why power stations need vast resources of energy to power their turbines. Whether it’s coal, wind or some other form of energy, lots of it is required to spin the generators.

It’s exactly this principle at the heart of regenerative braking. Instead of simply forcing friction surfaces together, regenerative braking uses the inherent resistance of an electric generator to slow the vehicle when it is producing power.

And the best part is that the function needs no significant extra hardware to perform the trick. EVs and hybrids are fitted with conventional friction brakes, but when you press the brake lightly the discs and calipers do nothing.

Instead, the same motor that’s used to turn the wheels is temporarily repurposed as a generator, and instead of using electricity, it produces or ‘regenerates’ power and the effect slows the car. This freshly generated power is sent back to the battery, where it can be used to accelerate the vehicle once again.

Some systems even start the regeneration earlier, without having to press the brake pedal; simply lifting off the accelerator starts regeneration. This is often called ‘single-pedal driving’, since the brake pedal is not required to slow down during almost all normal driving.

It takes a little getting used to but the aggressiveness of the effect can be adjusted.

Regenerative braking bonuses

Using a regenerative system to top up the battery means a hybrid should get more miles from a tank of fuel and an EV’s battery charge will last longer – particularly if the car is used in lots of stop-start driving, when the regen effect is most frequently used.

If a vehicle is being slowed down by a reversed electric motor, wear and tear on the conventional brakes is drastically reduced. Many service centres report that EV and hybrid braking systems often appear almost new, meaning longer brake life and lower maintenance costs.

What’s not to like?

In short, not a lot. Little extra kit is required to enable a vehicle already fitted with a motor and battery to brake regeneratively, so the extra cost is minimal.

Perhaps the biggest drawback is an odd pedal feel before the brake discs and calipers come into play. Manufacturers are getting very good at hiding the spongy or bouncy brake feel but some drivers still find it a little disconnected and lacking feel.

Nuclear or unclear?

Regardless of the vehicle’s drivetrain, the heat you feel from hot brakes does not originate from chemical energy, kinetic, potential or electrical. To understand the importance and effectiveness of energy transfer a little better let’s follow the heat produced by the brakes back further than the servo.

Petrol and diesel is refined crude oil that was formed by many trillions of sea organisms. These tiny creatures fed on algae, which in turn drew its energy ultimately from the light of the sun, originating with nuclear fusion.

While EVs use a very different energy source, electricity can be traced back to the same source. Solar power cuts out a few steps in the process, but even power from coal or gas-fired generation follows the same path – fossil fuel, things feeding on the sun, nuclear energy.