Yeah, a lot of cars will go into discharge at idle. The only way to fix it is to increase the idle, use a smaller alternator pulley, or maybe even use a higher output alternator (it may not be able to produce enough current at such low speed either). I don't have an amperage guage on my truck, but I do have a voltage guage and if I go much below 500 rpm, the voltage will begin to pulsate from 10-14 volts, it usually runs 14 volts constantly.