The systems power priority is starting the engine. Stop start will not enable until sufficient battery voltage is achieved. Cold weather brings down the battery charge and staring the engine will draw several hundred amps so even with a fully charged battery the heated front screen may only defrost half the screen to begin with.
If you are going to use the heated front screen, make sure all other electrical loads are turned off while the screen clears and then make sure you switch off the heated front screen.
B-Max Titanium 1.0 125bhp manual City Pack.