91 audi 100 - strange alternator issue

The Audi 100 won't charge the battery. Looking around online, it seems like a problem with the wiring from the alternator to the battery.

Instead of fixing the old wire, I ran a new wire, rated for 105 amps, from the alternator directly to the battery. Current alternator is 90 amps.

I was told the battery and alternator were replaced a month or so before I bought the car. The alternator looks like a referb unit.

When it is hooked up, the engine drops 50-100 RPM's or so, so the alternator seems to be working, but I only get 12.5v on the battery, which is the same as when it is off.

I did a few tests to find out what the problem might be.

Voltage of the battery with the car off - 12.7v
Voltage of the battery with the car running, but alternator not hooked up - 12.4 - 12.5
Voltage of the battery with the car running, alternator hooked up, engine @ 2500RPM - 12.5
(The voltage actually goes down the higher I rev it, with the alternator hooked up, by .05v from 2000 to 4000 RPM)

Voltage at the alternator, not hooked up to the battery, while the car is running @ idle - 20.9v
Voltage at the alternator, not hooked up to the battery, while the car is running @ 2500RPM - 15.5v
Voltage at the wire I ran next to the battery, from the alternator, but not hooked up to the battery @ idle - 20.8v (only a .1v drop over the wire, fine with me)
Voltage at the wire I ran next to the battery, from the alternator, but not hooked up to the battery @ 2500RPM - 15.5v

I don't understand, a direct connection should bring the battery to around 14.5 volts @ 2500RPM or less. Is this a bad alternator? Bad belt? My wiring solution crap?