The alternator should be putting out between 13.5 and 16 volts on a good alternator with the engine running as measured with a DC volt meter at the battery posts. Any less or any more and the alternator is defective of the belt is slipping.
Everyone says a car battery's voltage is "12 volts" but it varies according to how full up it is with electrical charge. A fully-charged car battery in as-new condition can have a voltage of 13.6...
Assuming your voltage meter is accurate, the battery is fully charged, when the key is turned on, without the engine running, and no accessories turned on, the meter indicated voltage should be 12...
A fully charged 12 volt automobile battery will read 12.6 volts with the engine not running and 13.5 to 15.5 volts with the engine running. It will read 12.4 volts with a 75% charge, 12.2 volts with...