Re: Need help from a pro
Just as an FYI... forget about and ignore peak ratings. They are meaningless marketing jargon. Also, it doesn't matter the speakers you're driving with the amp (with respect to power supply), only the amplifier's power rating is important.
If you're running an 1800 watt amplifier then you need ~100 amp hours of battery. Most small car batteries are ~50 amp hours and a large marine battery is ~100 ah. I'd use the largest battery I could fit under the hood and then add a second battery the same size in the rear, because you can't have too much battery for the audio system. Of course, for a budget... two 50 ah batteries would suffice.
I would also advise you to install a big three upgrade, as your alternator is the main source of power in the vehicle and if it can't provide enough voltage and current to the batteries, then the number of batteries you have is irrelevant.
Lastly, it would help if you shared the model number or specs of the battery you currently have. We can't help you choose a second battery if all we know is that you bought a new front battery. ;-)
Owner, Audio Anarchy, LLC
2915 Youree Drive, Shreveport, LA