Not having any model information makes it harder but a general would be check the alternator first both idle and running. Also check the voltage cold and hot. Another thing i would check would be the fuse in the harness from the alternator to battery if it has one. If all that checks out then see if you can make the battery light come on while testing. If the battery light comes on but the battery and alternator voltages are correct then you have an issue in the wire harness and the ecm is seeing a voltage drop.
This is ONLY to be used to report flooding, spam, advertising and problematic (harassing, abusive or crude) posts.