There's a couple different ways: When running 2+ alternators, you can use one at 14.4v(although most will sit around 15v)) for your stock electrical, starting battery, head unit. The second and/or rest of the alts can be set to 16v for a 14v battery(s) or 18v for a 16v batter(s). These batteries would be completely isolated from your stock electrical, they could be a large battery bank in the rear of the car for just the amplifiers. Some amps aren't stable on 16v or 18v, so you have to check with the manufacture, but the upside is increased power. Or you could run 2+ alternators all set at 14.4v for stock electrical, battery bank, and amps. This is easier since nothing is separated. But when running multiple alts you have the choice to use some for stock electrical and some for amps with a higher voltage. This is handy because then you don't need to use step-downs if you have a single alt but want 16v or 18v. The case of an alternator is the ground, when it's mounted everything else in the vehicle (chassis, battery negative terminal, etc) is grounded to it (alt case). Hope that helps.