Unlike outdated electrical systems that used generators and could function without a battery, modern automotive electrical systems need a battery in order to function properly. A dead battery means a car that won’t start, and an alternator that will have to work too hard—potentially to the point of failure—which is why it’s so important to understand how to correctly, and safely, charge and maintain a car battery.
There are two ways to charge a car battery: with the alternator, or with an external charger. Under normal circumstances, the alternator charges the battery whenever the engine is running. When a battery dies, then an external charger is the best way to charge it back up.
This is why most normal battery usage, like running the radio or dome lights while the engine if off, is replenished naturally the next time you start your car. As the RPM of the engine increase, the alternator’s ability to generate electricity also increases, and any power that isn’t being used by accessories like your headlights is available to charge the battery.