#12651, "RE: What powers radio when car is idle...battery or alternator?" In response to In response to 3
If your running an under drive pulley, that might explain your symtoms. An underdrive pulley (depending on manufacturer and type) will make your alternator rotate at a slightly slower RPM at idle than a OEM pulley and result in the alternator not being able to fully charge the electrical system at idle condition. With this information, try the previous poster's suggestion and check the system with a voltage meter at idle and while running.