What is a mild hybrid car and how do they work?

Hybrid cars and powertrains have become an increasingly popular choice for car buyers, because the combination of an engine and an electric motor helps improve fuel economy, boost efficiency and reduce emissions. 

While plug-in hybrids and full-hybrids have been around for a while and offer the greatest fuel savings, many manufacturers also offer mild-hybrid vehicles (MHEV).

Mild-hybrid powertrains have varying degrees of electrical assistance and drive, working in tandem with a regular fossil-fuelled engine. 

But what is a mild-hybrid car, how does it work and should you buy one? This guide reveals all…

What is a mild-hybrid car?

Generally speaking, a mild-hybrid car is predominantly powered by a traditional petrol or diesel engine, but has the electrical assistance from a small lithium-ion battery and a 48-volt electrical system.

The battery in a mild-hybrid car merely acts as a generator to power systems and components that would have been previously powered by the engine. 

The big distinction between a hybrid car and a mild-hybrid car is that, while the electric motor in a full, or plug-in hybrid can drive the car’s wheels, the motor in a mild-hybrid can’t; it just assists the engine, helping it to operate in a more energy-efficient way.

How does a mild-hybrid car work?

As mentioned above, the battery system in a mild-hybrid is limited to providing electrical assistance for the engine; and unlike regular or plug-in hybrids, mild-hybrid cars cannot be driven on electric power alone.

Generally speaking, mild-hybrids come in several configurations, but the most common set-up is 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 – when you’re pulling away, for example – taking a little load off the engine to help save fuel, boost economy and lower your running costs. There’s a small improvement in the car’s acceleration as well. 

Some MHEVs also 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 are ready to set off or push the accelerator again. 

Some mild-hybrid cars are able to harvest electricity through regenerative braking. Energy is converted to electricity, which is then stored in the lithium-ion battery until it's needed to provide extra assistance.

Running costs of a mild-hybrid car

While mild-hybrid cars generally cost more to buy than traditional petrol or diesel cars thanks to the electric motor and 48-volt battery system, they usually cost less than a plug-in hybrid, full-hybrid or an electric car.

Mild-hybrid cars should (in theory) be cheaper to run than their regular combustion-powered counterparts, but the amount of fuel you’re likely to save is minimal and might not offset the increased purchase price. If you really want to reduce your fuel consumption, a full hybrid or plug-in hybrid would be a better choice.

When it comes to VED road tax, mild-hybrid cars don’t benefit from the reduced rate of VED, which is applied to hybrid and electric cars. Mild-hybrid cars also don’t benefit from the lowest levels of BiK company car tax. 

Do mild hybrids feel different to drive?

Mild-hybrid cars don’t feel drastically different to drive in comparison to a petrol or diesel car. Unlike a hybrid car, it’s also possible to buy a mild-hybrid car with a manual gearbox.

Most MHEV 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. 

Advantages and disadvantages of mild-hybrid cars

If you’re thinking about buying a mild-hybrid car, there are a few advantages and disadvantages you should consider before parting with your hard-earned money.

Advantages of a mild-hybrid car

• Mild-hybrid cars are no different to drive than a petrol or diesel powered car and you’re not limited to an automatic gearbox only, because some mild-hybrids come with a manual gearbox.
• Lower fuel consumption and CO2 emissions compared to an identical non-mild hybrid.
• You don’t need to plug in or charge a mild-hybrid car.

Disadvantages of a mild hybrid car

• Mild-hybrid cars can’t run on pure electric power only.
• Fuel savings aren’t as great compared as in a full or plug-in hybrid.
• Initial purchase price is higher than that of a conventional ICE car.
• Limited lifespan of the 48-volt battery.

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.

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