Is a new batter for sound system enough?

The answer posted by alex is completely correct. No matter how many batteries you add to a system and no matter how big of a capacitor you get it is impossible to get more power out of your alternator than it puts out. To be honest you can pick up a decent high output alternator for 240 bucks from power bastards and thats a whole lot less than any CEA compliant 1800 watt amplifier or even the subwoofers your running. All a bunch of batteries will do is allow your system to have more storage space for that power, but once the stored power gets used up by the amp your back to the same issues you had before. The best way to run a big system is a high powered alternator and an AGM battery. Theres a reason you won't see people running caps on the bigger builds and its because capacitors are designed to help stiffen the charging system and are used when you are going just beyond what your alternator can handle so your battery has more time to charge between bass hits due to the super fast charging and discharging of the cap. It does not matter if your current alternator is good or not, it simply doesn't have the power to keep up with the amp.