I was reading about load testing and full fielding alternators in a few different books I've got, and one of the points they all made was that to test an alternator's maximum output (or close to it) the rpm's need to be raised to around 2000 to 2500 since alternators can't provide full current at idle speeds.
This seems a bit counter intuitive to me as I would think that by design an alternator logically needs to be able to provide sufficient current at idle to power all accessories plus keep the battery charged.
As a practical example, I took some measurements on my 99 Nissan Almera 1.6L (which I assume is in good working order.) I turned on the bright lights, A/C and radio.
At idle (about 850 rpm due to idle up) I measured a DC current of 59 amperes at the B+ alternator cable, and 11 amperes coming into the battery positive cable.
I then raised the rpm to 2500 and measured again, getting a DC current of 69.2 amperes at the alternator and a DC current of 14.5 amperes at the battery. According to what I've read, normally only about 5 amperes is needed to keep the battery charged, but I'd been running some loads without the engine on for a few minutes before I did these tests so the battery probably needed a bit more charging than usual.
So clearly even with the idle up the alternator is not capable of providing all the current the system actually needs, yet at the same time it is providing sufficient current to run the system without drawing from the battery.
So what are the underlying reasons that charging systems are set up this way?