Why is my battery light coming on if I tightened up a loose battery cable.

I was driving my car and the battery light teon. It comes on and off while driving. I had it tested at Auto Zone and Wal-mart. Auto Zone showed me it was at 12 and not 14. They tested the Alternator and it was not charging fully. I tightened up one cable on the battery that was loosed could this be the cause. I had the guy at pep boys thighten it up tighter for me but the light is still coming off and on.