2010 mont sport 4x4 voltage drops at idle
 

hi, on my 2010 montero sport 4x4 the voltage drops at idle, you can see it at headlights they dim a bit and the a/c fans speed slow down at idle at the street light or garage, when you do some gas and rpms go little bit hi all runs fine, I bring the truck to autozone and they check battery and charge rate an found all normal, any ideas??? tks in advance

get a new battery.
Turn your idle up.
Check tension on alt belt.