batteries augusta ga

Spent almost 5k on stereo and now i need battery in trunk?

So i went to Anything audio in augusta, ga (BEWARE). I bought a jl 1000/1, 300/4 all jl mids and hi’s and a jl 13.5 w7. They wired 4gauge power wire to trunk for amps and compacitor, and now they struggle for voltage. I really would like to strangle them but i moved and it’s probaly to late now. But i went to a shop here in al, and they want almost 500 to run new power wire to trunk for more power, but what i was thinking is just to parallel another batter in trunk off of 4 gauge and it should be fine right? I will ground into trunk as well. Optima yellow top is my choice, but i heard it could be bad to do that. Now i am going to upgrade my alternator probably in future(after new battery kills it possibly). But is this a good idea? I need more power! Oh ya, my amps cut off when i turn my stereo up and stereo shop showed me they were struggling for voltage and cut off when it got real low. Thank you.

well a battery does not generate power it only stores it…an alternator generates power and the generator is what charges the battery…and if you add another battery then your alternator will struggle to charge the extra battery while it powers all of your system…what I’m trying to say is that adding another battery will only make the problem worse…the battery only powers the system when the engine is off…a better alternator is your best solution