Within the instrument cluster of most cars built in the last decade is a warning light dedicated to signaling an alternator issue. In most cases the light is shaped like a battery, though some show "ALT" or "GEN," meaning alternator or generator, respectively. Many people see this light and instinctively think they have a battery problem, which is symptom that will be covered later, but that's not really why the light goes on.
This light is linked to computer systems within the car monitoring the voltage output of the alternator. If the alternator's output goes below or above a pre-set limit then the dash light comes on. Once the output is within range the light remains unlit. In the early stages of alternator problems the light can seem to flicker -- on for just a second and then off again. Or maybe it lights up only when accessories are activated. For instance, let's say its nighttime and the headlights are on and everything is working just fine. Then it begins to rain. As you flick on the windshield wipers the warning light comes on. Turn off the wipers and it goes away. While that may initially seem like an aggravating problem, the warning light is doing its job exactly.
Most alternators have an output between 13 and 14.5 volts that they try to maintain at a constant level. As more power is demanded by the headlights, the windshield wipers, your radio, the heated seats, the rear window defroster and so on, the alternator needs to work harder to maintain the necessary voltage. If your car's alternator is not working to its full potential, or demands are placed on it that it can no longer meet, the voltage will either go above or below the set level and switch on the warning light.