I had a 6 month old battery on my RX-8 when suddenly over a period of a few days it appeared to be dying. The car would crank more and more times before starting. Finally, I was driving from Vegas to Phoenix (a 350 mile trip) and upon entering Phoenix, the engine just cut out and died, while driving. I towed the car home and took the battery to auto zone, where they told me the battery was dead. My initial thought was that the alternator was dead and had killed the battery and the car, so I drove the car with the new battery to auto zone, but they ran tests and told me that the alternator was good. I have heard of a bad battery killing the car before, so figured that was the case. Well, 3 weeks later, I was driving for about 4 hours (out of town), when the same thing happened - the engine just died in the middle of driving. Pulled over, could not start. Got a passerby to jump start the battery, the car started, and we charged the battery from the other car for 10 minutes. Then drove it to Autozone, who checked the battery and said it was absolutely good! Took the car to dealer, who told me they couldn't find ANYTHING wrong with it! I am afraid of the 3rd battery dying in another 3 weeks. One other thing that I think might contribute, is I have a subwoofer I put in - nothing big, about 500w. My grounding wire came off a bit so I was getting a "humming" noise when accelerating for the past week. Could the poor grounding have discharged my battery like that?

This is quite a puzzle for me, and not sure where to go from here - so if anyone could recommend anything or have any ideas, please let me know.

 

I dont really trust Autozone's Alternator test.

Anyway, you should check the dark current of the car. That simply means, how much power its drawing when the car is OFF.

If its within spec, you start looking at other areas.

To charge the battery, your Alternator should output at LEAST 13.5, and the usual range is about 14.2-14.7V at all times.

and make sure your trunk light is off.

 

I'd try disconnecting your sub and see if it still happens. 500 watts = around 40 amps on a 12 volt system. Not sure what the stock alternator is rated at, but your battery may be getting voltage, but not enough amps to charge the battery.

bottom line is youre having a charging problem somewhere. Youve replaced the battery multiple times. take that out of the equation. Either your alt. is taking a dump, a sporadic wiring problem, or sub is the problem.