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It will surprise nobody that sales of battery electric vehicles (BEVs) are up, rising 125.1% year-on-year to the end of October.

But BEVs aren’t the fastest-expanding sector of the new car market. That title belongs to mild-hybrid diesels, up 796.4% year-to-date. Mild-hybrid petrols are also out-accelerating battery EVs, up 162.2% compared with 2018.

Both powertrain types are starting from a low base, but combine this year’s petrol and diesel mild hybrid sales and the total is 43,498, compared with 28,259 BEV registrations.

So just what is a mild hybrid? How can they benefit fleets, and are they a stop-gap or a significant new trend?



The definition of mild hybrid tends to vary slightly, but commonly a mild hybrid uses a 48V battery and an integrated starter/generator that allows the engine to switch off when coasting or braking; it must , therefore, be able to cope with repeated restarts. The system can also supplement the petrol or diesel engine under acceleration, while regenerative braking keeps the battery topped up.

What 48V mild hybrids can’t do is propel the car on electric power alone.

The Rise of 48V Hybrids - B5 Volvo XC90 Mild Hybrid

Updated B5 Volvo XC90 Mild Hybrid

From being a rarity just a year or so ago, many mild hybrids are now in the marketplace, from small city cars such as the Suzuki Ignis all the way to big 4x4s such as the Volvo XC90.

What benefits does a mild hybrid powertrain offer? Well, Volvo claims the B5 mild hybrid XC90 is up to 15% more economical than the D5 it replaces. It also puts out lower levels of NOx.

Kia, meanwhile, also employs mild hybrid technology in the Sportage 1.6- and 2.0-litre diesels. It claims CO2 reductions of up to 4% on the WLTP cycle, and 7% under the older NEDC tests.

The Rise of 48V Hybrids -Kia Sportage benefits from mild hybrid tech

Kia Sportage benefits from mild hybrid tech

Worthwhile reductions, but it’s clear mild hybrids don’t have the spectacular CO2 and fuel-economy figures of plug-in hybrid electric vehicles (PHEVs). But the mild hybrid is an ‘always on’ technology, and doesn’t rely on user behaviour to deliver the promised benefits. Fail to plug in your plug-in hybrid, and fuel economy will be poor. With a mild hybrid, the benefits are more consistent, no matter whether or not the driver changes their habits.

NOx levels are very low.  If every diesel were a mild hybrid you’d get around 6% economy and CO2 gain

That’s one reason why Nick Molden of Emissions Analytics sees mild hybrids (and other non-plug-in hybrids) as preferable to PHEVs. “Hybridise everything as quickly as you can. It’s a relatively low-risk way to meeting 2025 emissions targets,” he says. “The trouble with plug-in hybrids is you don’t know people are going to actually use them properly.”

Nick Molden - Co-founder of the AIR Alliance and Publisher of the AIR Index

Nick Molden – Emissions Analytics

His own independent assessment of mild hybrids backs up the results of the official WLTP tests. “We’ve tested diesel mild hybrids and NOx levels are very low. If every diesel were a mild hybrid you’d get around a 6% economy and CO2 gain. So the tech is definitely worth having compared with a standard diesel.”

Mild hybrid technology has a PR benefit as well as a technological advantage, because it enables manufacturers to distance themselves from the perceived ills of the previous generation of diesels. “From a marketing point of view it can be sold as a ‘hybrid’,” says Molden.


With the supply of fully electric cars constrained by the availability of natural resources such as cobalt, Molden isn’t the only expert who sees a value in using smaller batteries in more cars, rather than huge batteries in a limited number of BEVs.

Harvey Perkins, co-founder of the HRUX consultancy, says: “In 2018, we dug up 140,000m tonnes of cobalt globally (64% of that in the Congo). So, assuming we didn’t make anything else with it, that’s enough to make around 20 million vehicles a year with an average battery size of 63kWh.

“We’d need around 11% of that 2018 total just to cover all new UK car registrations. Maybe we should be focusing tax incentives on technologies that use smaller batteries until the battery supply has normalised.”


If 48V hybrids are a stage on the way to full electrification of the vehicle fleet, they may represent two steps rather than one.

According to market research and business intelligence company, IDTechEx, we will see soon 48V batteries used in full as well as mild-hybrid vehicles. These full-hybrid 48V cars will be much cheaper than today’s full hybrids, achieving “70% of an engine-dominant high-voltage hybrid’s benefits at 30% of the powertrain cost” according to IDTechEx’s report, titled  ‘48V Mild Hybrid and 48V Full Hybrid Vehicles 2020-2030’.

The report suggests that 48V hybrids could play a significant role, “possibly beyond the life in the marketplace of the traditional HEV and plug-in-hybrid PHEV as they get throttled between 48V full hybrids on price and pure electric cars (including solar 1000km range versions) on performance and convenience.

“Indeed, it is now realised that incremental improvements to 48V full hybrid powertrains may possibly extend to such exotica as solar and supercapacitor bodywork, electricity-producing suspension, autonomy and other features previously reserved for an EV end game.”

For 48V technology, mild hybrids may just be the start.

David Motton