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The alternator recharges the battery to restore power that is depleted during starting and keeps the electrical accessories on your car powered while it is running. The alternator is driven by the engine belt and may be controlled by the engine computer or voltage regulator to prevent overcharging. Over time, the internal windings and brushes of the alternator can wear out or become contaminated with grease or oil, and this can keep the alternator from producing adequate voltage for recharging or operation. When the alternator loses capacity or fails, the electrical systems of your vehicle will only stay on until the battery's reserve charge has been depleted. Your car might die, but you might be able to restart it with a jump start or by recharging the battery to park somewhere safe. If you notice that your headlights dim when using your power windows, or if your battery just won't stay fully charged, it may be time to replace your alternator or other charging system parts. If you're not sure whether your alternator is the source of the problem, check the tension of your belts to make sure the alternator is spinning while the engine runs, and stop by your local O'Reilly Auto Parts where we can test alternators on or off of your vehicle. If you need a new alternator or other charging system parts, you can shop our selection of replacement alternators, car batteries, and starters to find the parts you need to make your repair.