alternator not charging at idle

hi every have a problem here i own a 2001 kia sportage with a 180 amp ho alternator and it seems when i come to a stop my voltage drops but when i take off it goes back up. now i took the alternator in for testing and everything seem's to be ok from what the place i took it at told me so i know the alternator is good. now here is what get's me i try replacing my aftermarket ho alternator with my stock one and no problem at all this problem only happen's when i use my ho alternator. so i was wondering could it be the pulley i am useing on my ho alternator ? if so would it be ok to swap pulley's out from my stock to my aftermarket one and fix my problem or what do you think i would need to do ?

THe difference is pulley diameter is not a sufficient explanation...anyway the difference must not be that much. Is the HO alternator a direct replacement using same KIA connector? I have replaced my original with a GM alternator and had to figure out the connection..which turned out not to be what would have seemed natural....
ANd...is that HO alternator a new one known as totally nominal or is it coming from anywhere? What you describe looks like an alternator with one shot diode in the bridge but in that case it would never deliver full nominal amperes even at full speed.

well i found out my alternator is fine and it is charging it's my battery. oh i found this out is when i was driving to work early in the morning at night. i had my system up a bit but not much and came to a stop and my dash lights were not dimming. but as soon as i turn it up more i see they started to dim. so i figure it had to be the battery since it's a red top with 1000cca and from what i was told correct me if i am wrong red top's don't hold charge long enough to where the alternator can keep it charge. since my alt only charge at like 105 amp's at idle. so if im right about my problem i need to find me a good battery that can handle at lest 1800 watts. since im getting me a sundown audio sa 12 and running it at 1000 watts and i have a 500 watt 4 channel amp right now for my front and back speakers.

OH.....that's different. With a sound blaster that size, anything is possible. I'm used to cars driven by gazoline engine, not by sound wave ...so maybe my experience is lacking in that matter.....:grinyes: