Likely the battery or alternator, but probably the alternator
The alternator may not be recharging the battery .
dead or low cranking amp on battery or if that checks out, most likely the alternator.have you mechanic run a resistance load and that will tell the story
The battery light indicates that the battery is not receiving a charging current. You may have a bad alternator or a broken drive belt (if you have a broken belt you'll have overheating and power steering problems too, though).
Defective alternator.
The battery light stays on when the alternator is over or under charging.
so you can see the battery after dark!
That light doesn't mean that your battery needs to be replaced. It means that your battery is not being charged while the car is running. Have your alternator tested.
Your battery is not charging due to a problem with the battery cables or a bad alternator.
It's not charging the battery. Check the alternator.
Your belt is correctly tensioned?
The battery light can stay on for a couple of different reasons. Assuming your new battery is good (which may not be true and should be checked if all else fails) then most likely problem would be the alternator (generator in old Beetles). The light will stay on if the charging system is malfunctioning or the batteryhas completely failed, a bad battery itself may not light the light.If you don't have any test equipment then a professionals help will be required.
why das my interior light stay on and run the battery down.