capacitor keeps losing volts ? why?

Hello, I'm trying to figure out why my cap stays @ 12.5 volts when I start my car. Is it suppose to be @ that number? also I have a 800watt kenwood 2c connected to 2 12' rocfords he2 800watts. I have my dvd player and 1 tv running form the cap. I changed my battery to a optimum 800 red cap. has 800 cold cranking amps something like that. I turn my radio on amp,cap,crossover all power on,system booking 15-20 minutes later cap reads 10.5-11 amp keeps turning on and off.... Why is this happing I have 4 ga running---can you please help anybody???? thanks

Sounds like your charging system is being way to over worked and you need a bigger altenator. A cap only helps if everything else is up to speed. It sounds like your alt cant keep up with your power requirements and hence the drop in voltage and the amp turning off.double-secret reverse-osmosis speaker-cone-induced high-level interference distortion, Its a killer

you may have drained your cap. have you ran your system without your car running. if so this will drain your cap especially with all those accesories hooked up to it.94 Pontiac Sunbird 3.1 V6

alpine deck

audiobahn 3.5 fronts w/ tweeters

audiobahn 6.5 rears w/ tweeters

* sold rest of system to save up towards new car. $3000 spent on it.

You know what I had the same problem probably running the same amp as you are but I was pushing a single MA Audio sub 10 inch, anyways all I can say  is that if you are driving a toyota compact you are bound to have this problem so your best bet is to change your alternator for anything above 120 amps and you should be fine,  OF COURSE if you stick with the set-up you currently have....