New Voltage

Alternators do in fact produce rectified dc voltage but Dave is correct about it not being a clean signal. If you look at it on an ocilliscope you will see this. I know, I know, who uses an ocilliscope on a car? Whether you do or not the sine wave (voltage) is not clean coming right off the alternator. The battery does in fact clean up the voltage signal like a capacitor.

I do agree that it is best not to run alot of current through old wiring and old bulkhead connections so that is the only reason I see to wire high current devices to the alternator directly. If your going to do this it would probably be best to get a large capacitor (like the kind used to accent stereos) to add to the circuit (in parallel with the alternator output) to smooth out the ripple.