It seems the general populous thinks that alternators are supposed to put out full power at idle. I've never thought that to be the case.
I haven't found one yet that will make max amps at idle unless the idle RPM is above 1500 RPM
When Mopar introduced the alternator to all there OEM made car sin 1962 with mechanical voltage regulators that wouldn't charge at night with the lights on if the RPM where below 1000 RPM they decided to fixed that 9 yrs. later in 1971 with the solid state electronic regulators and two wire alternators that still wouldn't make many amps at low RPM
