battery light sporadict off and on

What is your question?

my battery light came on July 2009 and went to our mechanic, they tested the batt and debatable whether needed changing but since we live in AZ battery life is very short. Drove about 20 mi and light came on again. Took back to our mechanic, they checked alternator and other things that would bring on batt light. Everything checked out fine. Light would come on almost everyday for maybe 15 seconds sometimes blinking. Finally after about 4 months it magically fixed itself. Well it is back. Any ideas.