Welcome to the board. My first guess would be the belt. It may be slipping at the higher RPMs causing the alternator not to charge, and therefore turning on the Battery light. Plus the more accesories you have on the harder the alternator works and that would cause it to come on at a lower RPM. What you should do is get the car running, turn you high beams on, your A/c the Stereo, and everything else electrical, and have someone rev the engine to the RPM where the light usually comes on while you listen for any squeeling noises, and look at the belt to see if there is any excessive movement. Also put a volt meter on the bater and check to see if the voltage stays abouve 13v through out the RPM range. It should be around 13.5-14.5v while it is idleing, and should go up a little when you rev. If it goes down any your belt is either slipping, or the alt is bad. I know the tester we used at the parts store I worked at onty told us to rev the engine to like 3000 RPM or so, and it your light comes on above that the test is no good. If the car has an automatic tensioner, the spring in it could be bad causing the belt to loosen at higher RPMs as well. Make sure you also check the connectors and cables as well before you do anything just to be safe.