Kia says: A mild hybrid car works in a very similar way to a self-charging hybrid car, but it has a smaller battery, e.g. just 48v. Because the battery is smaller, a mild hybrid cannot drive on battery power alone – unlike a self-charging hybrid in which the motor can take over at low speed or when cruising.

Instead, the petrol or diesel engine does the majority of the work and the electric motor is there to provide assistance. This means the engine doesn’t have to work as hard which, in turn, means lower emissions and increased fuel economy. The battery is charged by recovering energy that would otherwise be wasted – when slowing down for example. See Regenerative braking.