That's why many manufacturers also rate their amps at 12.5 volts. You're right that you won't get the same power at 12.5 as opposed to 14.4 volts, but most spl amps and many good brands under rate their amps anyway, so you probably end up pretty close to the rated power at 12.5 volts. Keep in mind also that many amps that use tightly regulated power supplies won't lose as much power at slightly lower voltages compared to unregulated amps. This by no means makes them better, just different, and may work in some situations better or not as well. I like both types...a good amp is a good amp! There's nothing you can really do(reasonably) with the car off to bolster the battery voltage as far as I know. Your alternator runs at 14-14.5 volts, this higher voltage is what allows it to charge your lower voltage battery(12-12.5 volts). If you were to up the battery voltage, it would not charge at all or it would but very slowly...not good!

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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