12.6 = fully charged.
12.4 = 75% capacity
12.2 = 50% capacity
less than that..... replace the battery.

The bottom line is that the battery stores energy. They wear out. 38 months is the actual average battery life of all automotive batteries. (you can do better, just take care of it) So... see how old it is.... see how much voltage the battery retains overnight. Anything above 12.5 is good. Any battery below 12.2 is on borrowed time.