Not enough voltage

[Mikestrd]

I have a Camry stock battery under hood. Upgraded 250 amp alt. With a large capacitor in back. Along with 6500 watts car audio system. My question is, what battery or batteries do I need to pair with my current setup to stay at 14.8

[Joe X]

If those are 6500W RMS Watts even your alternator is not enough, on the battey those systems need many batteries not just one, say one on the front and one or more in the back.

[Kyblack76]

16volt batteries. Which i wouldnt recommend going to.  Again, this is a 12v hobby. Seeing battery (12vdc) rest is fine at full rip. Also, i dont know your gear from Adam, but ill bet you arent making 6500w on a daily, even if you held 18v.