When the red battery light illuminates, that means there is a problem with the charging system. The alternator is responsible for keeping the battery charged when the engine is running. SO its safe to say that there is somehting wrong with the alternator.

I had the light go on in my 04 x5 after only 65k miles. After the red battery light went on, I took out the alternator and got it tested. It failed the alternator test. Turned out to be the voltage regulator on the alternator. The shop I took the alternator to said its common for the regulator to go out on these cars.

I think what your mechanic wants to determine is if the alternator is completely shot or if its just the voltage regulator. IMO, with your alternator being 8 years old, I would just pull it out and exchange it for another one. I'd feel more comfortable knowing I had something as new as can be and not something that has a possibility of failing on me sometime down the road.