Matt;

100A is the output CAPABILITY of the Alt...it certainly doesn't put that out all the time, (paraphrased) "dumping it into the Battery"...current only flows into the Battery if its' voltage (determined by chemistry), is less than the Charging System Voltage (determined by VReg)...so Alt only supplies that when a huge load (current draw) up to the output rating of the Alt is put on the system...and gawd knows what could do that...maybe turning on Quad 100W Oscar rally lights, and everything else electrical in the car, and with an almost dead Battery...

Beyond that, my point is that when the Alt is heavily electrically loaded, it will happily transfer that load mechanically to the Pulley, which transfers it to the Belt, which transfers it to the Crankshaft, but ONLY successfully if the Belt is tightened up to such a degree to allow this...and the formed sheetmetal Water Pump Pulley must put up with this level of cyclic load...I believe it cannot and will fail!

Cheers