Not charging at idle.

Put a meter on the battery and slowly bring off idle. See what/ when it comes up. Could be a number of things.......poorly rebuilt alternator, dirty slip rings/ poor/ worn brushes, someone may have put a larger pulley on the alternator, on and on. Alternator could have internal problems, bad stator or diodes, or even a partially shorted rotor

Did this just start happening, or what?