Good info above and as stated, a typical reading would be in the 12.6-volt range when the battery is fully charged. If the reading is 12 volts or less, then the battery needs charging or needs to be replaced with a new one. Does the vehicle sit often? Short trips or the vehicle sitting will shorten the life of a battery and it might not be a bad idea to invest in a battery tender.
1974 Porsche 911 Base Coupe/Targa - Miscellaneous - Page 10