The alternator is the automobile part that helps maintain the charge in the electrical system. While the vehicle is running, it also keeps the battery fully charged. In a perfectly functioning electrical system, the alternator only has to run about half of the time after ignition. So, even though it is imperative to the operation of the vehicle, the car can run for a short way without it if the battery is fully charged.
Watch your dashboard. One of the easiest ways to know that an alternator might be faulty is that a warning light is displayed. In many vehicles the light comes on when the ignition key is turned, then goes out when ignition has been accomplished. If it stays on, this may be an indicator that the alternator is not charging properly, or if it comes on while the vehicle is running, it would be beneficial to check into the reason.
Poor Electrical System Performance
Observe the performance of the vehicle's electrical system. This is best accomplished in the dark while lights are pointed at a dark object. Turn on the headlights and the car accessory that typically requires the most power, the rear window defogger. While in a parked position depress the accelerator. If the headlights dim during acceleration the alternator may be failing. Remove the negative battery cable from the battery if you have the proper tools for another check. (The negative cable is the one that goes to the engine, the positive one is grounded.) If the car stops running, then the alternator is not working and the car is running solely on the charge of the battery.
Check the alternator's belt to see if it is functioning properly. The alternator is belt-driven. When all systems are fully charged it clicks off for less strain on the engine and to prolong its life. (A new alternator should last at least four years.) If the voltage shows symptoms of being low, the alternator itself may not be failing, but it could be not charging because the belt may be loose or broken.
Have a mechanic check the flow of electricity through the vehicle. (Have a certified automotive technician perform diagnosis of the alternator. Safety is important, as electrocution is a risk.) When the alternator is failing the voltage runs low in the vehicle. If you are having a problem with the electrical system of the vehicle, the reason could be something as simple and inexpensive as a faulty battery cable. You may not have to go to the expense of replacing the alternator.