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What's a mild hybrid? And how's it different to a full hybrid?

Engine and electric motor combos come in different flavours. Here's how to taste, sorry, *tell* the difference

Published: 23 May 2023

Mild hybrids are the most basic type of electrified cars you can buy. Although many drivers are considering the switch to something fully electric, mild hybrids minimise the change: no faffing around with a plug or cable, you see.

They allow you to enjoy some of the benefits of electric tech - mainly better fuel economy - without needing to spend hours and hours hunting around for charging points.

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Meanwhile full hybrids, like Goldilocks, straddle the middle ground between internal combustion and electric motors, although they're slightly more sophisticated than mild hybrids.

And then you've got plug-in hybrids, which are more complicated again and require some cable input to get the best out of them.

Basically, the main difference between all three is the extent to which the car can drive on pure electric power. Allow us to enlighten you in more detail...

Let’s have it then, what’s a mild hybrid?

Most mild hybrids (also called MHEVs) run a conventional engine, but employ a small generator (instead of a starter motor) and a lithium-ion battery (separate to the 12V one you might jump start occasionally).

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Their job is to assist the engine, helping it to accelerate efficiently and make the start-stop system work more smoothly. That's it, in a nutshell.

Mild hybrids are mostly for tackling exhaust emissions. In order to meet varying government targets and take advantage of tax incentives calculated by the amount of CO2 emitted per kilometre, car makers have shoehorned electric gizmos into their models. Wahey for tax breaks (more about them in a bit).

This also means there are fewer manual gearboxes out there (boo). However, near-silent start-ups and an increase in fuel economy ensure drivers are kept happy.

Right. And a full hybrid?

Manufacturers all use subtly different systems, but generally speaking a full hybrid can use its engine to act as a petrol generator for a battery and electric motor. Said battery is bigger than that of a mild hybrid, but smaller than what you'll get in a plug-in hybrid.

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Unlike the mild hybrid, a full hybrid will allow you to travel short distances - usually no more than a mile or two - under electric power alone, letting the engine shut off briefly and save you fuel.

'Full hybrids' are usually just called 'hybrids', but in the same way that all thumbs are fingers but not all fingers are thumbs, the term gets applied to all types of hybrid. So be wary.

Oh, and a select bunch of manufacturers call their full hybrids ‘self-charging' hybrids.

What the Merry Christmas is a self-charging hybrid?

It’s nothing more than a clever marketing term for a full hybrid, which is ultimately still powered entirely by the fuel tank.

By virtue of using regenerative braking, these hybrids can go further on the fuel put into the tank. Impressively, cars like the Toyota Corolla can push 60mpg in real-world driving. However, you can forget the visions of perpetual motion that 'self-charging' seems to imply.

By contrast, mild hybrids can't give you a fuel economy boost that's anything close to as big.

That just leaves the plug-in hybrid...

Plug-in hybrid vehicles (or PHEVs for short) are the most sophisticated of the lot, using a battery that's so big it would be unfeasible to charge it using just the engine and regen braking. As the name implies, you have to plug them in.

Depending on the car, this will give you a certain amount of all-electric range to deploy without once waking the engine. The Mercedes C-Class PHEV is one of the longest-range plug-in hybrids you can buy with its 68 miles, although 30-40 is more common.

As long as you charge the battery frequently, you'll be able to do all of your short-distance driving - the commute, the school run, the big weekly shop etc - entirely on electric power, preserving those fossil fuels for longer trips.

Right, I’m up to speed. Pros and cons for each please.

With mild hybrids, you get better fuel economy, fewer CO2 emissions and the transition is easy-as-pie to make, as there’s no need to modify your driving. At all. The downside is there’s no EV range.

With a full (or self-charging) hybrid, you get the same perks of improved fuel economy and lower CO2 emissions, just more so. And those short bursts of electric-only running could help you cruise silently through sleepy villages. The catch is that full hybrids are usually more expensive than mild hybrids because the tech is more advanced.

Plug-in hybrids are significantly dearer still, but can potentially make the difference back by plugging in regularly and doing all of your short journeys - the commute, the school run, the big shop etc - on electric power, which is generally cheaper per mile than fuel.

Don't be fooled by the brochure which claims mpg in the triple digits: actually bothering to do the charging is the only way you'll get close to those numbers in the real world. Because if you do, you’re literally just driving around a regular car with a ton of extra weight. Thirsty work. That’s where hybrids and mild hybrids are better off.

One other thing: more often than not the batteries for PHEVs get shoved under the boot and beneath the rear seats, which can eat into boot-space and legroom compared to non-PHEV models. Something to bear in mind if practicality is a big concern.

Can we go back to the part about tax breaks?

Sure. Until 2024, electric cars and some hybrids qualify for lower road tax (VED). There’s also more attractive benefit-in-kind (BIK) rates offered for EVs and PHEVs than with petrol and diesel cars.

So if you're sacrificing some of your salary to run a company car, remember you could sacrifice far less of it by going electric.

Which type of hybrid is right for me?

That's the million dollar question, and it depends entirely on how you use your car. Mild and full hybrids make almost every kind of driving slightly cleaner and cheaper in the long-run, so don't be put off. In fact, they’re often the entry-level powertrain on many models these days, so you might not have a choice. Don't fret, there's nothing to fear here.

Unless you own a caravan or a trailer, perhaps: hybrids aren't always rated for towing as much weight as non-electrified cars for complicated, motor-related reasons. Something to look into before you part with your cash.

As mentioned, a PHEV might give you the best of both worlds, but they're only worth it if you pinkie promise to recharge the battery often and always. That's a tough pledge without your own driveway (or deep pockets).

If you frequently drive long distances, you won't be stopping half way to charge the battery for the sake of a few green miles. Too much faff. But start with a full battery and the fuel saving will still add up over time.

Hang on, couldn’t I just go fully electric?

You could! More expensive again than PHEVs of course, but if you're doing 90 per cent of your driving on electric power anyway, why not go the whole hog?

Skip the stepping stone that is hybridisation and you'll maximise your saving on running costs (and possibly make the extra spend back quicker), plus you'll cut your tailpipe emissions to nil. No tailpipe, duh.

The big fear for most is charging on longer journeys, and given the horror stories you hear about folks getting stranded at broken chargers, it's easy to see why.

Yes the infrastructure in the UK is still growing, and yes reliability is an issue for some providers, but there are plenty of people who make it work, especially those with access to Tesla's Supercharger network. A little planning goes a long way.

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