Alternator Question

Last summer I installed underdrives and a 100 amp alt. on my 85 GT. This winter I installed an electric fan that pulls about 13amps. I've noticed at idle the volts go down to 10.5 to 11. The other night after a 3 hour drive with about an hour stuck in a construction zone I notice a white corrosive build up around the positive terminal on the battery. Am I frying my battery? Please tell me I don't need to now shell out $300 for a 3g.
Also I have two amps driving the stereo with subs.
PS ... I would have thought a 100 amp alternator would have been enough.

And what's so different about a 3 G from any other 130 amp Alt.

I didn't have this charging problem until I installed the electric fan. I'm thinking of changing the alternator pulley, if that doesn't work, having the alt. rebuilt to around 160 amp. Just found out about a guy in my area that does that for the local PD, FD and EMT's for their vehicles. Hopefully cheaper then buying a new alt.