Battery light

Got a new hatch its a 89, dx 4spd, When car is at lower rpms the battery light faintly comes on. When I bought it it needed a battery. I replaced it, it started fine. The place said they just changed the altenator. I am going to have it checked tomorrow.

probably the alt, battery light accually measures the output of the alt and not the battery itself... its a shitty system what what can you do.