What is the difference between a Volvo Mild-Hybrid (B5/B6) and a T8 Plug-in Hybrid?

B5 & B6 models are called “mild hybrids” but that’s kind of stretch.

Volvo simply added a system that makes the stop/start at traffic lights work a bit better.  Less delay when you hit the accerator pedal.

These cars do not have a traditional starter or alternator instead they have a 48-volt battery and a belt driven Integrated Starter Generator (ISG.)   This 48-volt electric motor allows the engine to start up much faster when you hit the accelerator pedal from a stop.  In theory, the ISG actually starts pushing the car a bit as it starts the engine.

This isn’t really a hybrid system, but it might qualify for tax breaks since this system boosts the city gas mileage projections since the engine stops and starts frequently.

It seems to be a pretty reliable system and the only apparent drawback is that customers complain that their A/C doesn’t work when at stops which does make some sense since the engine is not running, and thus the A/C compressor isn't running either. 

The T8 Hybrid models are true hybrid electric/gasoline vehicles with some pure electric range and optional level II plug-in charging.

Volvo went about this a little differently than most manufacturers and basically put two separate systems in the car that really do play well together.

The front wheels are driven by a powerful gasoline engine while the rear wheels are powered an electric motor tied to a 400-volt battery.  Earlier models had a supercharger and a turbocharge on the gasoline engine but later models (mid-2022 onward) only have a turbocharger.  Both have similar horsepower.

If you plug-in and charge a T8 model, they can be driven on pure electric for some (limited) miles.  If you don’t charge them, they operate a lot like other hybrids and just get better gas mileage.

You can charge the T8 models using normal 110-volt house current, but it can take 12+ hours to fully charge from empty. If you have a 220-volt outlet, you can get a faster level II charger and cut the charge time in half.  You cannot use “fast” chargers- Volvo’s charging socket is only level 2 and does not have the extra electrical contacts needed for true fast charging.

Earlier T8 models had a maximum of about 20 miles on pure electric but somewhere in 2022 Volvo added a higher density battery which doubled the pure electric mileage.  35-40 miles of pure electric can cover many people’s work commute.

T8 models do a good job of using electric to extend the overall gas mileage.  They do the typical gas engine stop/start at traffic lights but don’t always start the engine when you start moving again.  T8 models also regenerate electricity using the rear electric motor whenever possible.  You can gain electric range when going down hills.

T8 models have an Integrated Starter Generator (ISG) but it is very different than the B5/B6 models.  Where those connect to the engine using a serpentine belt, the T8 models have a larger 400-volt ISG sandwiched between the engine and transmission torque convertor. 

The air conditioning and heat on T8 models is powered by the 400-volt battery system so they work independently from the gasoline engine but can reduce battery range when heavily used (i.e. really cold or really hot weather.)

T8 models will start the gasoline engine when the 400-volt battery level gets too low and the ISG will power the electrical system and charge the battery.  The 12-volt system gets powered by the 400-volt system. 

T8 models will also automatically run the engine if the gasoline in the tank gets too old, which is possible if you have a short commute and don’t drive a lot.

It is very seamless when the gasoline engine starts and stops.  You don't normally notice it and when you do, it's usually only because of the exhaust note.

The T8 models have user selectable driving modes:

• The default mode is "Hybrid" which favors electric but will kick in the gasoline engine  if you push the pedal down far enough.
• There is a "Power" mode that gives you all the acceleration and power the system can muster.
• There is a "pure electric" mode that will use electric until the battery gets too low.  There is an option to start the vehicle in pure mode by default.
• There is an off-road and also constant AWD for more adverse conditions and challenging roads.
• There are also options for favoring charging or holding the battery level.

The T8 models have proven to be pretty reliable considering how complicated they are.

There have been a lot of issues with the High Voltage Coolant Heater (HVCH) but supposedly the newer replacement units are lasting longer.

So to summarize: the B5/B6 "mild" hybrid models are more like a traditional gasoline vehicles and the T8 Plug-in Hybrids are true gasoline/electric hybrids.

Both systems get better gas mileage than they would without the "hybridization" but the T8 beats the B5/B6 models because of it's ability to drive on full electric and it's more advanced hybrid system.

If you have the ability to plug-in to electricity, the T8 is a very good choice since you can take advantage of it's features if you plug it in at night.  A B5 or B5 models would probably make the most sense if you can't charge at night.

The T8 models are closers to a full electric vehicle but with less pure electric range and less range anxiety since you can run on gasoline when the battery is empty.

Another factor to consider is climate.  T8 models do not like extreme heat or extreme cold and will favor the gasoline engine if temperatures get too far out of the indended operating range.