yes correct adding another battery will just cause more stress for your alternator there is also no need to ADD another alternator, if anything repleace it
but heres what you should do first
sell me that amp, and buy a kicker zx2500.1, its perfect for 2 15" l7s, even tho its putting 250more rms watts the subs can handle that, this amp will make those subs POUND, the car next to you will be able to feel their car vibrating (i know someone with 1 l7 with a 1000 watt amp, and him next to me i can feel mine vibrate with mine off) it'll pound
second: upgrade your big three, this is probably one of the easiest and cheapest ways to upgrade the electrical current flow for a system, sometimes this may get rid of any problems such as light dimming or massive voltage drops, do this with 1/0g wire to make it so you shouldnt have to go with doing it again if you plan to upgrade your system again.
third (if needed) : get a high output alternator, something over 200amps, if possible work for one 220amps+ it will help in the long run if you ever wish to get bigger
fourth: upgrade to a deep cycle battery, such as an optima yellow top battery, used basically for stereo systems, this will help a lot as well
and the last thing, dont be afraid to ever bump too loud, just bump responsibly.
but really, ill buy that amp off of you

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