Just use two alternators that are the same. If you need a custom length belt just talk to your local automotive parts store. They likely will have a pulley that will fit your needs or know where you can get one made.
I agree with tcmtech. Maybe a wire to the battery from each considers thinking about.
The 2 alternator regulators could be fooling each other when you parallel them neck by neck. If you want to play it 100% sure then put a blocking diode on heat sink in series with each line and adjust the regulator output with the forward voltage drop value. Otherwise your batteries will not be fully charged. Can anyone fill me in on the 14.8? I prefer to stop at 14.4 V for lead acid car battery.
I have a Chevy van with twin alternators from the factory. There is nothing different about the wiring. just treat each alternator as if it is the only one on the veh. Heavy wire from the output to the battery pack. Use suitable fusible link wire on each alternator lead right next to the point where it joins the battery, or heavier wire. Ford also uses twin alternators on their ambulance packages. Same details there.
As for pulleys, keep them the same diameter. Do you have V belts, or serpentine? What engine are you using this on. Searching a factory parts manual may help you find a setup that will work for you. One company that I am aware of that manufactures custom pulley and drive setups is March. You will have to contact them to see if they can help you. Good luck.
Dual Alternator Project
Re: DUEL ALTERNATOR PROJECT
