Mild hybrid, full hybrid or plug-in, and why the badge does not tell you

The word hybrid on a boot lid covers three unrelated pieces of engineering. They cost different amounts to buy, save different amounts of fuel, and carry different risks on a used car.

Most buyers do not know which one they are looking at, and the badge will not tell them. Here is how to work it out, and what each is actually worth.

The one question that separates them

Can the car move on electricity alone, with the engine off?

If no, it is a mild hybrid. If yes at low speed for short distances, it is a full hybrid. If yes for forty kilometres or more and it plugs into a wall, it is a plug-in hybrid.

That is the whole test, and everything else follows from it.

Mild hybrid

A mild hybrid keeps a conventional petrol engine and adds a small electric motor, usually running on a 48 volt system, along with a battery roughly the size of a briefcase.

The motor replaces the alternator. In most systems it sits on the accessory belt, which is why you will see it called a belt-driven starter generator. It does four things:

• Restarts the engine for stop-start, faster and more smoothly than a conventional starter
• Adds a small amount of torque when pulling away, typically ten to fifteen kilowatts
• Recovers energy under braking instead of wasting it as heat
• Lets the engine switch off while coasting

What it cannot do is drive the car. Not for a metre, not at any speed. The motor assists the engine and is never an alternative to it.

The advantage is that it is cheap, light and takes up almost no space, so manufacturers can fit it across a whole range without redesigning anything. The Kia Stonic 1.0 MHEV and a large number of recent Continental models work this way.

Full hybrid

A full hybrid carries a much larger battery at a few hundred volts and a motor big enough to move the car by itself.

At low speed, pulling out of a carpark, crawling in traffic, the engine can be entirely off and the car runs on the motor. The engine starts when you need more than the motor can give, or when the battery needs charging. You never plug it in. All the electricity comes from the engine and from braking.

There are several ways to build one, and the differences matter more than the label. Toyota splits engine output through a planetary gearset so the engine can drive the wheels and charge at the same time. Nissan's e-Power never connects the engine to the wheels at all and uses it purely as a generator. We covered how those layouts differ, and what it does to the way each drives, in How a hybrid actually works, and what torque really means.

Plug-in hybrid

A plug-in is a full hybrid with a much bigger battery and a charging port. Forty to eighty kilometres of electric range is typical, enough that a short commute never uses petrol at all.

The catch is that it only works if you actually charge it. A plug-in that is never plugged in is a heavier, thirstier version of a full hybrid, carrying several hundred kilograms of battery it is not using. If you do not have reliable charging where you park, it is the wrong car.

What each is actually worth

This is the part that decides it.

A mild hybrid saves you somewhere around two hundred dollars a year. A full hybrid saves four times that.

The gap is not a small difference in degree. A mild hybrid recovers a bit of braking energy and smooths out stop-start. A full hybrid genuinely runs on electricity for a meaningful share of urban driving. They are not two points on the same scale.

Figures are illustrative, worked at $2.75 a litre on comparable cars. Your own mileage and driving pattern will move them, and the mild hybrid's advantage shrinks further on a highway commute where there is little braking to recover and few stops to restart from.

What this means in Singapore specifically

Road tax. LTA calculates hybrid road tax twice, once on engine capacity and once on motor power rating, and charges the higher. A mild hybrid's motor is far too small for the motor side ever to win, so in practice you pay exactly the same road tax as the equivalent petrol car. The badge saves you nothing here.

COE category. Category A requires engine capacity up to 1,600cc and maximum power output up to 97kW, both conditions. If a mild hybrid's rated output includes the motor's contribution and that pushes it over 97kW, the car lands in Category B. Check the log card rather than assuming, because on a small-engine car this is the difference that matters most.

VES. A mild hybrid usually improves CO2 enough to matter at the margin of a band, occasionally enough to shift one. That affects the rebate or surcharge the first owner paid, not you, but it is baked into what the car cost new.

What to check on a used one

A mild hybrid has far less to go wrong than a full hybrid, which is the honest upside of doing less.

The 48 volt battery is a wear item, but it is small and it costs a fraction of what a full hybrid pack costs to replace. The starter generator and its belt are the other components to ask about, and a belt-driven unit is a serviceable part rather than a major assembly.

On a full hybrid the high voltage battery is the significant item, and on any used example it should be assessed before you commit rather than after. Ask for it in writing.

On either, a service history with real intervals tells you more than the technology does.

The part nobody says out loud

Some cars are marketed hard on the word hybrid while delivering the mild version of it. If a listing leans on the badge and the fuel figure is only slightly better than the petrol equivalent, you are looking at a mild hybrid whether or not it says so.

That does not make it a bad car. A mild hybrid is a sensible piece of engineering that makes a petrol car slightly better at almost no cost. It just is not worth a full hybrid premium, and it will not deliver full hybrid running costs.

Ask the one question. If it cannot move on electricity alone, price it as a petrol car with a useful improvement, not as a hybrid.

Before you decide

1. Ask whether the car can drive on electricity alone. If the answer is vague, it is a mild hybrid.
2. Check the log card for engine capacity and maximum power output, and work out which COE category it actually sits in.
3. Compare the real fuel consumption against the petrol version of the same car, not against the brochure.
4. On a plug-in, be honest about whether you will charge it. If not, buy the full hybrid instead.

For whether a hybrid suits Singapore driving at all, see Why a hybrid suits Singapore better than almost anywhere else.