I replaced my alternator and noticed the same problem - if all accessories/lights on and car at idle, like at a stop light, then the voltage drops below that of the battery. (Alternator not putting out enough electricity to charge the battery.)

If I place my foot on the accelerator and increase the engine RPM's, then the alternator will put out enough electricity to charge the battery. (Or if I am not stopped and thus the engine has higher RPM's - alternator turning faster).

Or if I turn some stuff off, then the alternator will be able to keep up. (I installed a voltmeter so I can keep an eye on this.)

I think this is the way the charging system was designed. Maybe they figured not all accessories would ever be on at the same time? The alternator does seem to be under capacity to me...

My stock alternator is 80 amps. Some people have installed 100 amp alternators. For those people... Does the alternator put out enough electricity at idle to power all accessories and charge the battery (voltage at idle with accessories on above 12.8 volts?)