Hybrids and plug-in hybrids use a combination of an internal combustion engine and electric power, and will switch between the two automatically, use a combination of both or use whichever power source the driver selects depending on the drive mode and state of battery charge. If the drive battery is completely depleted it will simply operate as a conventional petrol car, although most hybrids automatically feed power back into the battery using regenerative braking, and some give the driver the option of using the engine as a generator to recharge the battery.

In short not topping up the battery will make no difference, although it will mean your fuel consumption is higher, and not recharging the battery will mean you are not fully benefitting from the hybrid powertrain.