2002 Fiat Punto

Dear Pro's,

Ive had an audio system installed in my car for over 2 years now with no problems. However this winter just gone, my battery went flat, I assumed it was dead so I just replaced it, but the same thing happened to the new one. My next thought was that it could be a drain. So I had someone check it out and they told me there was no dran when the car is off. So then I thought it could be the alternator. Im not an expert but I used a voltmetre to check the battery voltage and this did not increase when I turned the car on and revved it, I also checked the input and output amps of the battery when the car was on and this was virtually the same at around 2 amps so I can only assume the battery is not charging.
However I was told that the alternator is definitely charging at its max voltage.
I am confused and need help as to what it could be.
April 10, 2012.

Most batteries only last about 4 yrs, charge it and have it tested to see ifit's any good. Then have the charging system checked.

Apr 10, 2012.