The alternator is a critical part of your vehicle, and the first hint of a failing alternator comes with the illumination of the battery light on the dashboard. Subsequently, some of your electrical accessories will begin to fail: The lights will become dim, and the radio will start to flicker.
At that point, it’s probably a good idea to pull over to a safe spot off the road. Under these conditions, you probably have only a few minutes left before the car will completely stop running. (For those of you able to measure voltage on the alternator, the end comes when your meter drops below 9.6 volts.)
The actual replacement of the alternator is often difficult and probably best left in the hands of trained auto technicians.
An alternator won’t charge a bad battery, so the two items need to be tested in conjunction.