I am contemplating alternator replacement. My TEST 09 on the OBC reads 13.1-13.5v at idle after a healthy interstate drive. However, yesterday with some accessory's on, I was dipping to 11.8v and I saw a battery light briefly. Thanks.

The OBC always shows - according to my multiple checking - between 0.3-0.6 V less than actual volts measured at the battery pole in the engine room (on my E32) directly.
Check that again.