Glos Guy,
The battery on a PHEV doesn’t get fully depleted, it just looks that way.
When the battery level gets to a point determined by the battery management system, the car switches to hybrid mode. The car then operates like a non plug in hybrid such as a Prius. It goes about its business putting charge in the battery whenever the car decelerates or brakes and taking charge from the battery to aid acceleration. The full combined power of the engine and electric motor is always available.