Some of the most common signs of a car needing a new alternator include dim lights on the dashboard or the car's exterior, as well as an illuminated alternator light on the dashboard. Other common symptoms and signs of alternator troubles include a dead or weak battery along with noticeable sounds, such as a whining noise, coming from underneath the hood.
The alternator and battery in a car work in tandem to deliver an adequate amount of power to start the car and keep it running. Symptoms of a faulty or bad battery starter or alternator can initially be similar, such as a car not starting, taking longer to start than usual or dimming lights. However, there are key differences that can help to signal an issue with the alternator, such as the power windows moving more slowly than normal or the interior lights shutting off after having come on for a short time after starting the car.
A useful way to check that the issue is with the alternator, rather than the battery, is to try unplugging the car's positive battery connection. If the vehicle stops, the issue is likely with the alternator. Another option is to test the interior and exterior lights, both of which will come on as brightly as usual and then begin to fade.