when the car is off the battery is everything. Whatever the limit of your battery, in terms of sustained current output, that is your limit for sustained listening. Now remember that batteries are designed to generate a lot of amps for cranking the engine, so for short bursts your battery will supply quite a bit of current...400-1200 amps depending on the battery but remember it won't last for too long without being recharged...enter the deep cycle battery which has a much longer sustained output. As for the voltage, sorry, again you're limited to what the battery puts out. I know you're probably thinking, if i had a battery that was 14 volts that would be great...right. Unfortunately, you would have to change your alternator also to charge at a higher voltage, 15.5-16 volts, otherwise the battery would not charge. Your car's components may not like such a high voltage, they may work, but probably won't last for long. In SPL I'm aware of amps that are 17-18 volts, but I don't think these are daily driven vehicles...someone in spl can correct me if I am mistaken.

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2004 300M SPECIAL
Alpine CDA 9833
Alpine Pxa-h701
Alpine Rux-c701
Rockford fosgate p400-4
Rockford fosgate p4004
Hifonics BX 1205D
JBL c608 GTi
JBL p963
Infinity 2.5" center channel
Alpine type R 12"(2)
200 amp alternator
yellow top battery
0 awg power and ground (welding cable) 300 amp anl fuse
all active SQL set up with 2000 watts rms