An alternator is a simple device that converts mechanical motion into electrical energy through the use of a motor and a magnet. When the engine is running, the alternator uses some of its energy to create electricity that your vehicle's systems need to run properly. The alternator also recharges the battery, which is why you are able to restart your engine after jump-starting it and running it for a period of time. If your vehicle’s electrical systems begin to malfunction when your engine is running, the likely culprit is a faulty alternator.