Some plug-in hybrid electric vehicles (PHEVs) can self-charge. They do this by overworking working the engine to generate the additional power to recharge the battery but this is a very inefficient way to drive a PHEV (as the mpg will plummet as the engine works hard to generate the extra power).

Generally, a PHEV will recuperate lost energy under braking and use this to top up the battery (adding one or two miles.) The plug-in battery (when fully charged) will provide the car with an electric range of 20-30 miles, which will lower the overall fuel economy on a long journey (with the battery assisting the engine) or allowing the driver to power the car from pure electricity (perfect for driving in a town or city).

The hybrid system will recharge itself in a similar manner to a mild-hybrid car (recovering energy under braking); however, this will only add two or three miles to the battery and will not fully recharge the entire plug-in system. PHEVs are not designed to be driven without a fully charged battery. The additional weight of the battery will sap fuel economy to around 30-40mpg.

If you want to know more about life with a PHEV, read our long term test of the Volvo XC40 Plug-in Hybrid here: www.honestjohn.co.uk/our-cars/volvo-xc40