my light came on a few hours ago and it did kinda scare me since last time my car died right after...but this time all was ehhr ok for the most part...idle stuck at 1200...so i tried to check if it was bat or alt...battery was fully charged and did drop as it should when cranked so i checked if the alt was doing its job and it was, keept it at even rate no spike no nothing so i was puzzled and took off the battery, and the car died as if the alt was bad but how could it charge and show normal reading if its bad? i also noticed that with key on engine off the bat light stayed on- it went on with all the other lights but when they shut off batt stayed on...could this be a faulty alt just starting to go bad or a bad ground? advise plz