Serpentine Belt and Tensioner Replacement
 

I was on my way to Maine this friday and my battery light popped on. I pulled over and pulled a cable off the battery to see if it was the alternator but the car kept running. I actually changed the alternator last spring so I didn't think it went so soon. After changing it though I noticed every once in a while If I pushed the car over 4000 rpm or so the light would flash on. I thought eventually the it would need a belt and a tensioner. So on the way home this weekend the battery light stayed on the whole time if i did above 50-65 mph but if i let it idle it was fine. The lights began to flicker on the way back becuase it was running so long. I'm actually away in college and since the tensioner is right up top on the 2003 1.8t a4 I figure it wont be bad but since all of my tools are 4 hrs away I'm kind of stuck...I figure I can buy a cheap set of 1/2 inch or 3/8 and do the job.
Do you think with these symptoms its a tensioner problem? And if so besides maybe a 1/2 ratchet or 3/8,sockets and extensions and maybe a large flat blade to help get the belt off when i take the tension off is there anything else im missing? Thanks.

Went to the Auto Parts store today to order the tensioner and belt but after talking with the guy there, we put the meter on the battery. He thinks its the alternator itself and it looked like the it was working and not working and going back and forth and not putting out much power. I think it may be the alternator. Any ideas?