why is battery light on but battery is in good condition

What is your question?

I have a new battery but the battery light came on and stays - the car is running good until the dash lights came on, the headlights dimmed and the car stalled - I let it set for about 15 minutes then stated the car and it ran good the next 85 miles before the battery light came on again. Been driving it another 600 or so miles and it did basically the same thing; I waited a couple hours and it started up again and was running decent - now starting to act up again. - not sure if it is the alternator or something else.