It is actually better if the car is awake, as this will apply a moderately small load to the battery.
This load is not that big but it will give you a far more accurate voltage reading.
Doing what is called an OPEN CIRCUIT reading, this is when no load is applied to the battery, is not a good way to take a reading because the battery can show it is fully charged, but just a small load can drag the voltage down.
So again, the small load you will have when the car is awake is a good time to check the battery voltage.
Can I also suggest that after taking the first reading, turn you ignition on, but do NOT start the motor.
With the ignition on, take another reading at the battery.
The second reading should be no more than 0.2v below the first reading.
if it is much more than that, this would indicate the battery may be fully charged but is low on capacity.
Let us know your findings!
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