charging problem

two weeks ago my alternator went out so i put a new one in, but today the battery light came on but only when the engine is in the lower rpms its not idling super low or anything and was running normal until my battery went completly dead. any body got any ideas anything helpful would be nice.

Have the system tested again. I went through three alts from Auto Zone in a couple days because their entire stock was faulty. I finally had them test the units in store and out of 5 only 1 worked properly out of the box. Thats assuming its fault of the new alternator.