The reason the battery light is comin on and off is because the alternator is not running and ur battery is losing power very quickly. A good test to do to the belt is to use 2 fingers and if you can easily spin the belt over 90 degrees, than it too loose. Tighten up the tensioner but i highly doubt the belt is that loose that it wont even spin at all. It probably has something to do wit the pulley. GL luck wit the fix