I have a 2002 ford excursion 5.4 gas i just replaced the battery as it went dead on me tonight 7/25. since then my battery/alternator light is on and wont go off. i put a voltage meter on the battery terminals and have about 13 when i turn the air, lights, and rear defog on it drops to 12.2 or so . does this mean my alternator is bad, if not how do i get the bat. light off?