From my understanding of car batteries, which is not a lot, you should test them after a certain time (I think it's 24 hrs) because if you've driven the car/van it will obviously be charged. I think (again?) if it's under 12.3 (?) then you need to replace it. I really should listen more.

Basically you shouldn't test it until it's been sitting a while because it'll give a false reading like tyres do.