While the vehicle is off and not running: approx. 12 volts
During Standby: approx. 12 volts
During Cranking: 7-10 volts
After Starting/Running: 13-14.5 volts
( this is in fact the charging voltage from the alternator that is shown here.)
(if the voltage shown is approx. 12volts or equal to the standby voltage, the charging is not taking place and check the alternator asap.)
Batteries degrade over time!
If you see that the starter motor is straining to get the engine running, this is a sign that the battery may be reaching its end of life. ( Faulty glow plugs may also cause strain to the starter motor and during a cold start and will cause more time to start the engine).