The alternator may not be recharging the battery .
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.
Likely the battery or alternator, but probably the alternator
It's not charging the battery. Check the alternator.
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.
Your belt is correctly tensioned?
why das my interior light stay on and run the battery down.
The alternator is failing.
You might have a bad battery, a poor connection between the battery and alternator, or a bad ground from the battery.