12. Your alternator is what keeps the voltage up above 12 volts. Batteries usually rest around the mid to high 12 volt range. Once your pulling more current than your alternator puts out the voltage drops. But you still need extra batteries, even with HO alternators. Alternators dont put out max current at all times. The more amps you draw, the more they put out until they hit their max. So if you had a HO alternator with just a stock battery, and pulled more than the alternator put out, you would have a huge voltage drop. Also batteries act as a buffer for your amps. Say you hit a hard sudden bass note, your alternator wasn't puttin out much before the note an now all of a sudden it needs to put out more. The extra batteries will keep the voltage from dropping as fast and as far