Battery voltage should read at least 12.5 volts DC while the vehicle is off, ideally 12.8v or better. Anything less indicates an undercharge condition which could be the fault of the alternator, battery, or both. An unswitched electrical device could also be draining the battery while the vehicle is off. While the vehicle is on and operating normally with a properly functioning alternator, voltage should normally range from 13v to 14.8v depending on load or current draw of the system.