What is a mild hybrid car? Everything you need to know
Confused about what a mild hybrid is and how it differs from a regular one? Then here's your complete guide to this low-cost, fuel-saving technology.
- What does mild hybrid mean?
- How many batteries do mild hybrids need?
- How to drive a mild hybrid car well
Mild hybrid is a generic term for using electric power to assist a petrol- or diesel-engined vehicle. Unlike full hybrids, these don't qualify as a specific category of vehicle in their own right and few of them are even badged as such. Yet this blend of technologies is a welcome approach for affordable lower-emission cars in Britain's diverse and challenging driving conditions during this phase of electrification.
Unlike plug-in hybrid (PHEV) or 'full' hybrid (HEV) cars, the mild hybrid (MHEV) label covers a broad variety of fuel-saving solutions — the term is clearly aimed at generating credibility if not total clarity around a collective, complementary group of innovations that together work to reduce emissions and save fuel.
Automotive innovations tend to be synonymous with the brand that successfully introduced them, the association remaining strong even when rival manufacturers have launched their own versions of the technology.
There is no 'go-to' mild hybrid car to set expectations, however. High performance combined with all-wheel drive used to conjure notions of Audi’s quattro while more recently car buyers associated 'hybrid' only with the Toyota Prius. Both set the benchmarks for other brands to match, or beat.
Mild hybrid technologies are different because competing systems were developed by component suppliers and sold to sold to manufacturers when the demand — or legislation — made adopting them worthwhile. As a result, there's no single car model — or make — that's instantly associated with MHEV tech.
Toyota's first Prius was sold in the UK from 2000, though it wasn't the first hybrid offered in the UK. That honour belongs to the Honda Insight, a lightweight, two-seater economy coupé that technically, could lay claim to being the first mild hybrid. Japanese brands were first to market, but 1982's Lucas-Reliant Hybrid demonstrated many principles of modern solutions, in prototype form.
Mild hybrid technology is much younger and benefits from evolution in stages rather than the whole system having to be reinvented when individual elements improve.
From a buyer's perspective mild hybrid is more of a marketing term than it is a description of one particular technology. Chances are, if you've bought a new petrol or diesel car recently, it may use mild hybrid tech to save fuel even if you weren't sold those benefits. For cleaner, lower-cost motoring, mild hybrid technology is usefully effective in real-world conditions and it’s simple and easy to maintain.
How do mild hybrid systems work?
Mild hybrid systems in the majority of cars are a refinement of stop-start technology. By taking a holistic approach to the car's energy needs, mild hybrids cut fuel comsumption and emissions primarily but in many cases can also provide a short increase in power for better acceleration and overtaking.
This is done by recovering kinetic energy when braking or coasting similar to the KERS system in a Formula 1 racing car — though it’s unlikely to slingshot you to a podium finish unless you’re competing for the lowest CO2 output.
At the heart of mild hybrid is a second battery. Not a large EV-style multi-cell pack that takes up floor space or boot capacity, but something compact that fits wherever there's a convenient space. Typically it's is a 48V Lithium-Ion one, comparable to a laptop battery.
This allows a higher power density in the available space and the shift to a 48V system allows thinner wiring — or greater current in conventional wiring — for high-demand devices, such as motor capable of moving the car for sustained periods.
The next innovation is making as many engine-driven features as possible run on this additional electric power. From air conditioning to water pumps, the reasons your old fossil-fuel engine had to keep turning to keep the car alive have been removed, plus the technology got much cheaper in the process.
So the main benefit of a mild hybrid car isn’t in keeping the car moving but in keeping it safe and comfortable when it isn’t. Stuck in traffic on a cold day? Your mild hybrid will keep the lights on and the heater running for much longer without needing the engine’s power.
You may think this is just a nice bonus feature every car should have. Many modern cars already feature one or two systems along these lines, and often have much larger batteries and more powerful alternators to support them. What manufacturers have decided makes a car mild hybrid is the ability to use an electric motor to help move the car when it’s coasting or pulling away from standstill — and use that same motor for regenerative braking, charging the battery rather than the engine doing all the work.
Manufacturers that make good use of mild hybrid technology include Ford, Honda, Mercedes — particularly in recent AMG models — and Suzuki.
How did mild hybrid systems come about?
A trend towards smaller petrol engines from diesels gained momentum around the introduction of Euro 6 emissions standards.
Manufacturers were keen to ditch diesels — it's very hard to control the nitrogen oxides (NOx) emissions and exhaust particulates. To do so, technology such as Diesel Particulate Filters (DPFs) had to be invented – devices that catch, then burns those particulates when the car's driven at speeds outside of those typical for urban environments.
Great! Except they don’t last long without maintenance. The need to use sophisticated exhaust measurement alongside additional fluid injection systems adds both complexity and cost. Just ask anyone who has had to have their AdBlue tank or NOx sensors replaced.
The downside of small petrol engines is that cars need torque to get moving. When it comes to using petrol for a quick, contained explosion with little waste, modern petrol engines are fairly efficient at high constant speeds — but you need big pistons and long-stroke leverage for low-rev torque compared with diesel’s slow and steady expansion.
Therefore to get more go 'on demand', the idea is to store excess energy and release it when needed. Mild hybrids incorporate an electric motor to provide an instant torque boost for a very short period. It means small, high-revving petrol engines can give that low-down shove that drivers have become accustomed to from turbo diesels.
Most of the technology used by mild hybrids has existed in other forms for a very long time. The starter/alternator and belt-drive used by many systems was introduced a decade earlier in cars to cut emissions when stationary. Plus the high-density batteries have been powering laptops and smartphones for years.
It needed a few small changes to car design to bring it all together but the end result is almost the opposite of attempts to clean up diesel.
Lighter, simpler and lower-cost than bolt-on emission control systems, mild hybrid systems offer most of the real-world benefits of a full hybrid without a big change in driver behaviour, repairability or car design.
How do manufacturers label mild hybrid cars?
You've probably seen more mild hybrid cars around than you realise as most manufacturers are adopting the technology. Where there's potential for ambiguity is that there's not a standardised whay of referring to them.
Browse different car brands' websites or download their brochures and you may see occasional reference to MHEV — shorthand for mild hybrid electric vehicle — or 48V in reference to the electrical system voltage rather than the engine's surfeit of valves. Other manufacturers won't even make reference to it at all.
There's also a recipe for confusion about the differences between self-charging, 'full' hybrids and mild hybrid systems, not helped by some car manufacturers themselves. For instance, you can buy a Fiat 500 that has a Hybrid badge on the back, yet it's actually a mild one rather than a self-charging type. Not a lie, but not helpful.
Suzuki Swift Hybrids used to be labelled similarly, although more recently it's changed — models such as the Suzuki Vitara are referred to more overtly as Mild and Full Hybrids.
Other examples of mild hybrid cars include the the Ford Puma mHEV and the Volkswagen Golf eTSI.
You don't plug-in a mild hybrid car, it simply operates like a normal petrol or diesel so from behind the wheel you shouldn't notice any difference, apart from hopefully better fuel economy and in some models, better performance.
Mild hybrid cars cannot drive on electric power alone, although the engine may turn off during periods of braking.
Why do mild hybrid cars need two batteries?
You could ask the same question of electric cars, which are essentially giant batteries with wheels. The answer is rather dull, sadly. It’s because some aspects of car design are so entrenched, changing every bought-in component gets unreasonably expensive.
Most 48V mild hybrid cars still use 12V components outside of the motor/generator system. The high voltage (HV) battery is used to recharge the lead-acid starter battery via a DC converter rather than an alternator.
There are good reasons for retaining the traditional battery. Robust lead-acid batteries are capable of thousands of cycles of drain and charge, protecting the HV battery from constant fluctuations and prolonging its life.
It also means the bulk of the car’s systems work on well-understood and easily replaced tech. That means the 12V battery is still tied into many control systems, though, so don’t think having two batteries eliminates an ages-old problem for used car owners.
The legacy 12V battery is one of the most common problem areas on electric and hybrid cars — if it isn’t in good condition a lot of mild-hybrids will disable the extra features. Many EVs and HEVs won’t work at all if the 12V battery is below a voltage that might have started an old car with a push.
When the boot's on the other foot here's your fair warning: it's a really bad idea to jump-start a car from a hybrid or EV using old-school jump leads. Switch it off, disconnect the battery and use it as a booster pack, if you have no other options.
Are mild hybrids exempt from ULEZ?
Yes. The emissions standards that define ULEZ apply from 2006 for petrol cars (Euro 4) and September 2015 for diesel cars (Euro 6). The majority of mild hybrid cars were introduced as a response to the next phase of Euro 6 standards (RDE2 or Euro 6d) that became mandatory in 2019 and reduced the amount of leniency around NOx levels.
As a rule hybrid cars, regardless of technology or fuel type, are exempt from ULEZ charges. Personal imports may not have their emissions correctly recorded, however.
What has changed is congestion charging. Steadily reduced from the original less than 100g/km exemption that encouraged so many 'low CO2' diesel sales, the only vehicles exempt since October 2021 are zero-emissions cars. After December 2025, even those will have to pay to enter the capital's centre.
Are there any incentives to buy mild hybrid cars?
Unlike hybrid cars, which were considered alternative fuel vehicles (AFVs) for some schemes, mild hybrid technology is merely an extra fuel-saving feature. It’s no different to fuel injection or stop-start and doesn’t meet the demands of any legislation or emissions standard directly.
As such there are no tax, road-charging or legislative advantages to buying a mild hybrid. They should be considered alongside conventional combustion-engined models when comparing running costs, emissions and economy.
Are mild hybrid cars reliable?
Given the conventional nature of the cars they’re generally very easy to maintain or repair — and as reliable as their non-hybrid counterparts. Suspension, brakes and gearboxes are often the same, with only detail changes.
This means that the majority of breakdowns, as opposed to wear-and-tear or outright neglect, are down to electrical issues. Most cars disconnect the high-voltage system when the bonnet is opened, though the risks of jump-starting incorrectly can have more expensive consequences.
Both batteries get topped up in normal driving and as the cars get older, the typical 12V battery will need replacement after seven years even on well-maintained cars. The auxiliary pack will generally have a similar lifespan but is not standardised.
Replacement costs vary and can be very high — though they’re generally much lower than full hybrid batteries — so reconditioning services are becoming popular. While the battery issue may be simple, a lot of manufacturers code the battery management system (BMS) so simply replacing a faulty cell won’t always solve the issue.
On systems that offer a larger capacity battery and good regenerative braking, the lifespan of braking system consumables will generally be better. That does mean that problems with corroded or sticking callipers could show up before the pads are worn, particularly in areas where high moisture and salt levels such coastal towns are the norm.
Is a mild hybrid car a better used buy than a full hybrid?
Hybrid technology is generally more reliable than Euro 6 diesel emissions controls but it’s expensive to fix when it does go wrong. Mild hybrids are less complex but still need good diagnostic skills to repair correctly. There’s no good reason for the average owner-driver to choose a mild hybrid over a full hybrid, if the thought of 'a hybrid' is worrying you already.
For owners who enjoy DIY maintenance and repairs mild hybrids are a better bet. The largely conventional engine usually only differs in the starter, alternator and charging department, while the high-voltage components are small.
Most cars are designed to keep mild hybrid batteries within the car body rather than slung underneath or as part of the structure and they are often light enough for one person to move. They can even be posted to specialists for refurbishment, as long as your courier accepts that type of battery pack.
The flip-side of that argument is that the microelectronics and compact laptop-style batteries found in mild hybrid systems need more sophisticated skills than the 'get the pack out, take suitable precautions for safety, and identify the faulty cell' techniques used to rebuild a typical older hybrid and electric vehicle batteries. While the theory of operation (and failure) is similar, the scale is much smaller and the level of integration is greater.
How can you get the best out of a mild hybrid system?
Like any technology, how effective it is depends on how it is used. Mild hybrids are great for extra-urban commuting and slow rural short drives in busy areas, where the car will have time to get warmed up before congestion.
Short trips in congested areas depend very much on how the car handles warming up, but if that’s the majority of your driving anyway then a plug-in hybrid or EV would be more beneficial.
On motorways using adaptive cruise control and building speed gently will allow the car to use the electric motor for coasting where supported and many cars combine mild hybrid tech and cylinder deactivation for maximum efficiency.
Mild hybrid cars with manual gearboxes and conventional automatic transmissions are readily available. The same techniques that give you the most economy for a conventional engine are equally beneficial, with greater results.
You’ll see the biggest impact of a mild hybrid system if you spend a lot of time sitting in traffic with a lot of stopping and starting and at low average speeds.
