What is regen braking and is it actually useful?


What is regen braking and is it actually useful?

Electric motors in hybrids and EVs have brought regen braking to mainstream cars. So, what on earth is it?

Joe HoldingPublished: 18 Apr 2023  External link to Top Gear Magazine Subscription – 5 issues for £5Skip 1 photos in the image carousel and continue reading

Regenerative braking. This clumsy phrase has slowly crept into car-person lexicon as electric and hybrid vehicles have become more of A Thing.

So if you’ve been reading TopGear.com’s reviews and group tests for years – and of course, that is the kind of behaviour we absolutely condone – you’ll already be well aware of what regen braking is and how it works.

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But if you’re encountering regenerative braking for the first time, or if the tech has never been properly explained to you, this article is for you.

Actually, even if you think you know how it all works, read it anyway. There are a lot of dodgy explanations out there and there’s every chance you’ve been handed the wrong end of the stick.

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So, without further ado…

What is regenerative braking?

Regenerative braking – or regen braking – is what happens when you begin to slow down in an electric or hybrid car: if it’s got an electric motor, it’ll likely be capable of regen braking.

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You see, when you lift off the accelerator an electric motor will produce an electromagnetic force that acts against the forward motion of the wheels. So instead of coasting you’ll begin to scrub off speed, in a similar way to engine braking on a petrol car.

Great, but… why do I want that?

Efficiency, friend. Unlike normal brake discs, which simply convert kinetic energy into heat that’s lost forever, regen braking allows electricity to be harvested from the motor and then recycled back into the battery to be used again as you drive.

In an electric car or plug-in hybrid vehicle, this means you’ll get a little bit of range back every time you slow down. And the same is true of less capable hybrids, some of which can manage a mile or two on electric power alone at town speeds by re-using the energy you stored by regeneration.

Gotcha. Exactly how does regen braking work?

Right, let’s not overcomplicate this. An electric motor runs a current through a circuit, creating an electromagnetic force that causes it to spin. This spinning motion is what drives your EV’s wheels and propels it forwards (or backwards when the current is reversed).

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However, when you stop shoving electricity from the battery to the motor – i.e. when you ease off the accelerator – the momentum of the car through the wheels becomes the dominant force in the system. This sends that current in the opposite direction and hey presto; your electric motor has become a generator, with electricity flowing back into the battery.

Most cars capable of regen braking will show you this in real time on the speedo, with a dial displaying how much power you’re using on the throttle and how much you’re generating when you slow down.

Do I need to drive any differently?

You don’t have to. But a small change to your driving style can help you take full advantage. Most electric cars and PHEVs will allow you to choose how strong you want the regenerative braking force to be, often by way of flappy paddles behind the steering wheel. This means you can pick a braking strength that suits the kind of driving you’re doing.

For example, if you’re cruising along on the motorway you’ll probably want weak regen so you can drive as smoothly as possible. But in traffic around town, high regen will be better suited to the stop-start nature of your journey.

And for the avoidance of any doubt, you will still have a brake pedal that operates physical brake discs and calipers. Electric and hybrid cars will blend these and the regen automatically, with the best doing so seamlessly and the worst making it difficult for you to judge how far to press the pedal each time. Beware.

Any tips?

With these blended systems, practise using that dial next to the speedo so that you press the ‘brake’ (actually brake plus regen) pedal just far enough that the needle goes close to 100 per cent regeneration, but don’t then press the pedal any harder. That way you won’t be using the discs. Well, y’know, press harder if the alternative is a frontal smash. But what we’re saying is, try to drive with anticipation and slow up gently.

The bottom line is, regen braking doesn’t make the brake pedal redundant. Unless perhaps your EV has a single-pedal driving mode.

Single-pedal driving?

Yes, that’s the blanket term for stuff like Nissan’s e-Pedal. Single-pedal driving is where the regenerative braking force is set to maximum, and it also brings in the brakes at low speed where regen doesn’t work so much. In pretty much every normal scenario aside from an emergency stop, you’ll be able to come to a complete halt without using the brake pedal at all.

This takes some getting used to but can be a very intuitive way of driving when you’ve got the hang of it. It can also take the strain out of city driving with only your right ankle doing any work on the accelerator. And hands on the wheel of course.

Any other benefits of regen braking?

We’ve covered improved efficiency (and, ergo, lower CO2 emissions on hybrids) and boosted range, but there’s another perk of regenerative braking: brake wear will be significantly reduced because you’ll be much less reliant on the discs and calipers.
That in turn means less brake dust in the air; normally a big contributor to harmful particulate matter, especially in cities. And breathe.

Any drawbacks?

Not really, although we should add that regen braking only makes your car more efficient in certain situations. Remember, braking when you don’t need to in a combustion-engined car will waste fuel as you’ll have to accelerate again, and this principle still stands for EVs and hybrids.

This is because the round-trip conversion of kinetic energy back into electricity isn’t itself 100 per cent efficient, so 50 metres of braking won’t give you back the whole 50 metres of range. It doesn’t work like that.

So how much range will regen braking actually give me?

It’s hard to say, because it depends on how efficiently (that word is getting a proper work-out today, huh?) the system can recapture electricity and how much power that system can cope with in one hit. Imagine trying to drink from a hosepipe: a dribble from the nozzle won’t be a problem, but fully open the tap and you’ll be overwhelmed (and very wet) very quickly. Make sense?

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Some EVs have info screens that show energy consumption as a graph against time. If the graph goes negative, that’s regen. In give-and-take driving the negative area might be 10-20 per cent of the positive area, meaning about 10-20 per cent extra range than you’d get if the car just used its discs to slow up.

Does front- or rear-wheel drive make a difference?

Rear-wheel-drive EVs sometimes have less regen available than FWD. In theory a motor can regenerate with as much power as it can accelerate, but if you did that with only the back wheels on a slippery road, you might skid – try stopping a bike with just the back brake. And 4WD EVs usually allow you more regen again, as they’re more stable with all four wheels being slowed.

It’s more complex with hybrids. They’re often regenerating even while the engine is pulling forward, just to let the engine operate in its most efficient rev and fuel range.

Sum it up for me.

Regen braking could give you thousands of extra miles of range per year by recovering energy from the parts of your journey where you’d be slowing down anyway. That’s good. But it doesn’t make up for energy lost to air and tyre resistance – perpetual motion machines they are not.

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