2006 Ford Focus • 103,000 miles

I have a 2006 Ford focus. Last night it behaved as if the alternator went bad
the lights went dim, the radio said "Low Battery" and cut off. We got it home, and when I went to restart the car, the battery was dead.I jumped it, put a voltmeter to the alternator and it came out 8.6. It ran for awhile, but eventually stopped, again battery dead. Today, I disconnected the negative battery terminal as I was thinking of taking the alternator to Auto Zone to get tested, but due to its location, it was too difficult a job. So I jump started it and took it to a local mechanic, he put the voltmeter to it and it tested fine 14.2. Since the battery is over 5 yrs old, I put a new one in. Started it, put the voltmeter to it, 12.5 then after about 30 seconds or so went to 14.23 and stayed there. The only "work" I did was disconnect/reconnect battery terminals. What is going on? Can an alternator work intermittenly before it fails completely? Is there something else that should be tested? Or can this all be from a failing battery?
November 30, 2011.

Have the computer controlled of the alternator's field generation circuit checked/tested

Nov 30, 2011.
Computer controlled circuit for the alternator? Are you meaning the relay for the power or testing it within the ecm? Could you clarify for me Rasmataz.

Jan 25, 2012.