2002 axiom XS

My alternator caught on fire yesterday after I put a brand new 725cca battery in a day before, and today me and my friend were replacing the alternator. After getting the bad one out, I noticed a loose wire that the ending was bare wire, 2 copper wires inside, twisted up but they were loose like it connected to something?

It's connected to the big fat wire that runs AROUND the top of the motor near the coils. If you look on the passenger side toward the front it has 1 ribbed plastic tubing with wire inside that runs directly down

I don't want to try and start the car after I put the alternator is without knowing what this wire is for.