Where are you measuring? Using a hand held voltmeter, place the probes on the battery terminal itself -- not the cable clamps. Take a reading with the engine off, then start the engine and at 1500 - 1700 rpm the reading should increase. Static battery voltage is 12.6 volts. With a functioning alternator voltage should rise to 13.5 - 14.5 volts. Dirty or corroded battery cables can mask charging problems. Clean the terminals and connections at both ends of each cable. If none of this corrects the issue, take the alternator to any auto store and have it tested -- normally for free. Any alternator shop in town can likely rebuild the unit much cheaper than a new alternator.