Bob;

Good advice...all else being OK, a squealing belt is slipping because the level of load is too much for that particular amount of tension to transfer the mechanical energy.

All;

Regarding output being RPM independent...it most certainly IS NOT...it is DEPENDENT...without going into di/dt and hi physics to prove that the faster the magnetic lines of force of the field slice through the windings of the stator, the more current they induce. Period.

See this graph of output vs. RPM on a HUMVEE: http://www.humvee.net/hid/electric/altout.html

...and it doesn't matter what vehicle, and the maximum output. The priciple is THE SAME, and the HUMVEE graph was the quickest one I could find through an "alternator output graph" search....and don't forget, the alt RPM is not necessarily the engine RPM since it's affected by the pulley ratios.

Cheers