[Excaliber]

You know the big reason alternators are superior to generators is that they put out more amperage, and in turn a higher voltage, at low rpm. A generator can make just as much power, but it needs to be big, heavy and turn at a higher rpm to do so.

That said, even alternators need at least a minimum rpm to begin producing decent power and it's a rare occasion when even an alternator will be putting out 100% capacity even at high rpm.

At idle rpm lower than a 1,000 with a "heavy load" like light and fan(s) it's normal for an alternator to not put out enough amps/volts to meet load demand.