[Oldsguy]

I think you are right. There is an old saying, "fool me once, shame on you; fool me twice, shame on me!". He isn't a good mechanic if he didn't check the alternator and sold you a battery outright, and then at that price! I could buy two or maybe three batteries to fit in your car. It could also be the starter solenoid so don't go buy an alternator or telling a mechanic you need an alternator. Tell him what you told us and let him do his job.

edit; nayner, was it the starter in your car?