It does however give you some indication as to when to charge the battery before it loses too much charge!
Stevie, when it's reading 12 volts it's already over half empty.
I took a battery off the boat yesterday, it was too flat to turn the small bilge pump, it's so knackered it won't take a charge, but it still gives a reading of 9.6 volts on the meter. That's a multi test meter not the Aldi one.