I just ran into this the other day myself.. got a "new" alternator because mine was knocking (bearing going out)..
Anywho, this "new" alternator worked until I got about 15 miles from home. Then my battery light came on. You're not only suppost to test the connection when at idle, but also when reved up to about 2k rpm.
You should be able to stick the volt meter pin into the postive and negative conection on the battery, position the volt meter to where you can see it, and manualy move the throttle by hand at the throttle body to rev it up. If volts jump up, then there is your problem. Alternator or volt. regulator.
Easy/simple test. Good luck.