There is nothiing like buying things you do not need. Further testing is indicated here. I would make up a cigarette lighter end feed for my voltmeter and see what that alternator is putting out over the period of a day or twos use. My guess is it may be falling down. Just use the volts function to make sure it is staying up to about 13.5 or greater when driving around. There is already some indication of a charge problem as you say you get no apparent charge indication at idle with your meter. You will also or should go over any and all connections in the charge circuit before absolutly condeming the alternator. Especially the grounds even if they look good. You are aware you can check the grounds by voltage with engine at idle looking for a voltage drop? Or just run a booster cable from the negative terminal to a clean cinnection on the alternator case and remeasure your idle voltage. It should be higher than when engine is off. Make sure you all electical loads are turned off before test even the sterio. Yes of course it could still be the battery but you will have more information to go by before spending money. Just monitor the system under use. Or I should say thats how I would proceed if my battery passed the load test and I had no indication of charge at idle with no electrical load at that time. If the red charge light is on, very dim, or flickering at idle especially forget the battery for awhile.