culzero wrote:if you have the space its always better to put a second battery
Ok, since I can speak out of a long winded journey of discovery and experience, you probably DO NOT need a second battery.
The only reason you may need a second battery is if you would like to listen to music without the car started, or you use power accessories such as winches or hydraulics. And if you go through this route, you will need a battery isolator.
An extra battery only manages to put an extra strain on the alternator because it requires charging. Capacitors are just as useless, and can run up as expensive as an extra battery.
The problem is, most amplifiers have unregulated power supplies and work best with a voltage of 14.4V. Your alternator provides that voltage. Only when your alternator fails to keep up with your system, will any battery kick in its reserve. So that means, if your battery is charged at 13.0V, it will only provide you power if your system drops below 13V.
If you got a good 200-300 to spend on power upgrades, please upgrade the big 3, power wire and alternator before you even look at a capacitor or extra battery.