Q: Battery light coming on faintly after installing new alternator

Hi there. Have you had the battery tested? Use a digital battery tester that provides current voltage, cold cranking amps, and state of health. Any corrosion on the terminals? This could present an opportunity for electrical disturbances and create concerns. However, this does sound like the alternator or belt is at fault. Check for proper belt tension and belt condition to ensure that it is not slipping. If it is slipping, then adjust/replace as necessary. If this checks ok, then the alternator may be at fault. Even new parts can be faulty and a proper load test will uncover whether or not it is indeed faulty. When you rev the engine, the alternator spins accordingly and can provide additional voltage/amperage which is probably why the light starts out dim and gets brighter when you do this. I suggest having a qualified technician perform an inspection to avoid replacing unnecessary parts. Your Mechanic has several available technicians that can assist you with a charging system inspection.