Originally Posted by Braineack
if you're only reading 12v while running your alternator isn't working.
The battery should read around 12v when the engine is not running.
A basic principle of electronics is that in order for current to flow from the alternator into the battery, the voltage produced by the alternator much be higher than the battery voltage.
A comparable scenario here would be if you had two balloons connected to one another via a straw. In order to force air from one balloon into the other, you squeeze the balloon, causing the pressure (voltage) inside it to rise. Air then flows from one to the other until the pressure equalizes between the two.
Same deal here. In order for the battery to actually charge, the alternator is run at a higher voltage than the battery's normal resting voltage; the exact voltage differs somewhat from car to car, but 13.8v is often cited as the ideal, with observed readings ranging from around 13.5 to 14.5v.
If you read less than this at any point in the system while the engine is running, you have a problem. Maybe a bad alternator, maybe a wiring problem in the car, maybe your meter is just out of calibration or malfunctioning. These things happen.