What does MHEV mean? Your guide to mild hybrids

Jesse Crosse

22 Sep 2023

​Understanding that a hybrid car has an engine helped by an electric motor to reduce fuel consumption is easy enough. What can be confusing though, is that there are three different types: full hybrid (also known as a self-charging hybrid), plug-in hybrid electric vehicle (PHEV - a full hybrid with a larger battery you can also plug-in and charge) and mild hybrid electric vehicle (MHEV). 

What’s different about an MHEV? 

MHEVs are so-called because the electric motor isn’t that powerful compared to the other two. Unlike those “full” hybrids, it cannot be driven for short distances under electric power only. 

What it does do is to boost engine power when you accelerate, giving the engine less work to do and saving you fuel. Like EVs and the other two hybrids, an MHEV has regenerative braking which means the electric motor becomes a generator as you slow down, charging the hybrid battery.  

The electric motor is usually attached to the outside of the engine and alternatively assists the engine or drives it via a drive belt. An MHEV electric motor is officially called a belt-driven starter generator (BISG). As the name suggests, it also adds another feature to the car, which is stop-start. 

Is stop-start easy to use? It seems a bit scary

When you pull up at the traffic lights or in a queue, in fact anywhere for more than a few seconds, the engine will shut down to save fuel. With a manual gearbox car, this is usually when you put the car in neutral.

When you want to move off again, the engine automatically starts again in a heartbeat. 

Depending on whether the car is a manual or automatic, that may happen when you de-clutch and select a gear, or simply press the accelerator pedal. Either way, once you’ve acclimatised to your car, it happens so quickly you won’t notice it and there's certainly nothing to worry about.

If the engine is cold or you are using a lot of power (for example, while defrosting the rear window) then the stop-start may automatically decide to keep the engine running. It will all happen automatically though, so you don't need to do anything.

The MHEV Ford Puma has two batteries. Seems a bit greedy

Are MHEVs expensive to buy?

Because MHEVs are not designed to drive under electric-only power, the hybrid part of the car works at much lower voltage (48 volts instead of 400 volts like the full hybrids). The advantage is, the car costs less to make and is cheaper to buy as a result. The good news is, you still get the benefit of a hybrid, but without the ability to drive for short distances only on electric power.

Like full hybrids, MHEVs have two electrical systems so that means the car has two batteries. There’s a small 48-volt hybrid battery to help power the car and store energy from regenerative braking. All the other electrics, such as lights, wipers, infotainment and so on, are powered by a 12-volt battery just like any other car. MHEVs like Ford’s Puma also have a conventional 12-volt starter and if the 12-volt battery goes flat, you can still use jump leads to get started.

How does an MHEV reduce fuel consumption?

One thing all hybrids have in common is being more effective at improving economy in urban areas where the car is constantly speeding up and slowing down. It’s during the slowing down that a hybrid scores most, clawing back energy used as the car accelerated. On long motorway journeys where there’s far less starting and stopping involved, they get less chance to work their magic.

Will an MHEV save me money?

Because MHEVs are a relative low cost way for manufacturers to improve their official emissions and fuel consumption figures, petrol MHEVs are relatively widespread. In 2022 13.1% of all cars sold in the UK were petrol mild hybrids and the figure is higher than that as of September 2023.

Buying a mild hybrid doesn’t cost significantly more and although savings are modest, MHEVs do less use less fuel than a conventional car, especially in mixed road or urban conditions. You don’t have to plug them in or do anything differently to running a conventional car. 

MHEVs are not much more expensive and can save on tax and fuel

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