Question: what are the causes of a bad alternator?
Answer: Test the Output. When an alternator wears out, it does so in stages. ... 
Check the Battery. Many times, the alternator will fail from being "stressed" by a dead or dying battery. ... 
Listening for Sounds. Inside the alternator is a rotor that spins on a set of ball bearings. ... 
Losing Ground. ... 
Belt Slippage.