Your description is a bit confusing. "No battery light the 2 or 3 day the light came back on". Was the battery light on before and that's why you went to the shop, or was there some other problem?
Worn brushes inside the alternator are a very common problem and that will cause intermittent operation. Any testing has to be done while the problem is occurring, meaning the warning light is on. The brushes can be replaced separately but since it takes extra time, most people just replace the entire alternator.
A break in one of the wires can also cause intermittent operation and a few quick voltage tests will verify that before the alternator is replaced.
Friday, May 10th, 2013 AT 8:30 AM