What is a mild hybrid and how does it work?

Mild hybrid technology is becoming increasingly widespread in new cars today but what is a mild hybrid and should you buy one?

Not long ago, a hybrid car meant one thing: a car with two different power sources, usually a petrol or diesel internal-combustion engine working alongside an electric motor. 

However, now, the ‘mild hybrid’ has arrived. The big distinction is that, while the electric motor in a conventional hybrid can drive the car, the motor in a mild hybrid can’t – it just assists the engine.

How does a mild hybrid work?

Mild hybrids come in several configurations, but most commonly feature a small battery pack that works with the regular 12V battery found in all combustion-engined cars.

Often, this is a 48V system with an integrated starter-generator, which acts as both a starter motor and a power bank to assist the engine. The result is that this unit supplies power under certain conditions, like when you’re pulling away, helping take a little load off the engine to help save fuel, boosting economy and lowering your running costs. There’s a small benefit to acceleration as well.

Some mild-hybrid electric vehicles (MHEVs) have a function that allows the engine to turn itself off when coasting. But in any event, MHEV technology will restart the engine automatically when you’re ready to set off or push the accelerator again.

One thing that’s key to all mild hybrids, though, is that there’s no electric-only running. For this you’ll need a full hybrid or plug-in hybrid.

Are mild hybrids exempt from tax?

Unfortunately not. Only zero-emissions vehicles are exempt.

Do mild hybrids feel different to drive?

Not drastically. Most systems will improve the car’s start-stop feature, so you may coast to a stop with no engine power, rather than have the engine cut out at the last minute.

The engine will drive the car, but the battery may help acceleration, and braking may feel a bit different, if the car has regenerative brakes. 

Are there any other potential benefits?

One company is developing a 48V system to cut nitrogen-oxide emissions from diesel cars by up to 60 per cent – by electrically heating the catalytic converter faster than the car’s engine can.

Mild hybrids

Lighter than conventional hybrid

Pollutes more than hybrid

Cheaper to manufacture

No huge boost to economy

Lower emissions

Short, if any, electric-only range

Now take a look at our top 10 best hybrid cars and best small hybrids on sale

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