i just recently bought my 1984 firebird and it ran great on the way home but now wont so much as even click when i turn the key. i took the obvious first step and had the battery checked and they claim it was good. but to get my car to start it has to be left on a battery charger for hours and it will only stay on for about 6 min then im back to where i started. i really hope not but could this be a bad altenator?