Remove the alternator and take it to one of the automotive retailers like Autozone, they do free alternator testing. I have a vehicle with fuel injection, electric radiator fan and when I ran the ac the volt meter in the vehicle would show the voltage slowly drop over time. I was going to upgrade the alternator but decided to have the one in the vehicle tested. Turns out that alternator was not putting out what it was rated at. I replaced that alternator with a new one, same rating as the original one and now with the ac and headlights and all the other accessories on it keeps up.
