Cliff Topp expressed precisely :
All modern cars will have an amount of quiescent current draw to power things
like the alarm, the clock, the radio presets and so on when the car is parked
up and switched off. I've seen it written somewhere that around 50mA can be
considered 'normal'.

My question is - if the quiescent current draw is 50mA (0.05A), how do I
calculate voltage drop per hour?

For instance, if I park the car up at 10pm and the battery is showing 12.5V,
with a 50mA draw overnight what will the voltage be at, say, 9am?

OK, thanks everybody.