Once again Paul Danner simplifies even the most basic diagnosis.
If your vehicle is experiencing charging problems, it is likely the alternator, but first eliminate other possibilities such as a loose or broken alternator belt or a blown fuse. Alternators, just like any other mechanical component on your vehicle, can fail due to use and age.
Peter shows a sometimes forgotten way to check for low voltage issues. By checking the ground side for voltage drop Peter speeds up the process and shortens the diagnostic time.