Alongside the battery, the alternator is one of the most important parts of a vehicle's electrical system. When your car is turned off or when it needs to store any excess electricity, this is where the battery comes in. However, at some point, a car's battery would very quickly deplete if it's kept running all the time. That's why a vehicle should also feature an alternator. The alternator is essentially a generator, which is powered by the engine. It would then use this power to generate electricity, which it could then use to charge up the battery while the engine is running. While you're driving about or are idling, most of your car's electrical and electronic accessories are powered directly by the alternator, with the battery serving as a backup.