A couple of things, bad voltage regulator can cause this, corroded or bad battery cables can cause this, also having a bad connector at the alternator can cause this. A alternator belt that is too loose or warn will also cause this problem. If the car will stay running long enough, you can take the car to the auto parts centers and they can do a test on the charging system to tell you where the problem lies. takes about 10 minutes to run the test once they get everything set up.