No, batts alone aren't enough. Think about it, how is an alternator that puts out 14.4 volts (ideally) going to charge a battery to 16 or 18 volts? It cant. So knowing this, you have to have an alternator capable of charging up to the system voltage you wish to use (higher actually, so it will charge the batt fully... just like 12v batts are charged via a 14.4v alt).
Now to complicate things worse, much of your vehicle's electronics wont like/accept 16-18 volts, and damage could easily occur to the more sensitive components like the computer brain. This means to have a 16 or 18 volt stereo system, you'll either need to run dual charging systems/alts, one being the 16-18v source, and another for a 12v source... or you use the 16-18v alt for everything and install a transformer to step your voltage down to the appropriate 12-14.4v.