2004 Mercury Monterey Replaced Alternator Twice Now Doing Same Thing Again

i have replaced my alternator twice and now its doing the same thing again, battery light would come on and go off one day and the next it won't come on at all til it finaly shut down and then i had to get a jump. i thought it was a bad battery but i had it tested and i was told it was good just needed charging and it was my alternator. what could cause the alternator to keep going bad is there a rely or something that could be causing it. this is getting old