Do you ever the run the system without your vehicle actually being on? Sorry but if your alternator can push that much there's no need for a battery. I didn't think SAZ used high voltage power supplies to push power either. Maybe I'm wrong?
Either way...I can't stand extra batteries being placed in the trunk without any coherent design. I hope the response isn't..."but it's a battcap!!"
EDIT: Just watched some videos...I really can't stand that type of listening setup but if that was your goal good job. We have two distinctly different tastes. However, I still don't think you need a battery. I'm also a little confused with design goals. You show numbers for a burp, "tuned" box for 32hz, and probably play it as a daily driver. Are you just trying to be loud? It wasn't even your box design, was the box tuned to the t/s parameters of the speakers? I'm just starting to feel like your build is trendy vs. intelligent design. There have been guys who have hit 150db on 500watts. Why do you need two SAZ-3000's? In a perfect world you'll gain 3db but I don't see that happening in your ride.