https://tontio.com/auto-guides/car-battery/car-battery-voltage/
* Voltage when the car is starting
When you start your car, the battery provides the necessary power to the starter motor to turn over the engine. During starting the batteries voltage will decrease for a short time, before going up to the running voltage.
During starting a healthy car battery voltage should be 10v or higher.
* Fully charged voltage
A fully charged car battery voltage should be 12.6v or above, this is with the engine off.
* Voltage when the car is running
The voltage, when the engine is running, should be in the range of 13.5-14.7 volts. When your car is running the cars alternator is charging the battery, this is why you should see a higher voltage.