You use an alt with an external regulator. The regulator allows you to adjust the voltage to whatever you desire.
Personally, I would run a dual alt and dual battery system if you want to run voltage over 16V on a regular basis. Most systems spike up to about 16-17V but are not designed to stay at this voltage (12V system).....
If you run a dual charge/battery system, you can keep your 12V system for your engine, ECU, gauges, etc.... and run the 2nd 18V system for your sub amp(s) only so you don't burn out any components for your engine, etc. //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif