Batteries dying in meh car?

[Slobalt7]

So I have a Chevy Cobalt, only electrical mods are Cobra 29 LTD peaked and tuned to 45W, Rockford 6x9's, PBR300.1 and Skar MA8 subs. A few months ago when i was running different subs on the PBR my stock ACDelco battery took a dump, 6 years old and slightly more load imposed on it recently made it understandable. I had a used Optima red top in my RX7 for a few months and threw it in the Cobalt, only killed it once which was because I had the stereo running and car was off. This morning I got in the car and battery was dead, took it to Advanced Auto, they tested it and apparently the battery is bad. Results were 9.89 volts and 0 amps... How can that be? All of my fuses are good, could there be a problem in my electrical system draining them or the first one was just tired and the second one is defective? The Optima was made in Jan of 09, I wouldn't think it was old enough to die off... I am stumped, Please shed some insight.

[Slobalt7]

Update, Duralast is the third battery this car has killed... I am going to go with alternator... Still looking for any other answer, the alts in these cars is a bitch to change.

[Tarball]

where did you get the duralast from ?

Sounds like you are trying to fix an issue with used potentially bad parts to begin with.

Did you put these batteris on charge before using them? or load test?

Also, don't play with your car off, all you are doing is draining the battery.