Without power, every car will fall out. The alternator supplies the power that is required and the battery stores spare power.
With that supply of power, the starter motor can be activated when the engine is not running. There is always a positive and a negative cable from the battery to components under the hood. Those are thick cables. Usually there is also a somewhat thinner cable connected to the plus of the battery. Its color may vary, but it is an important cable. This comes from the dynamo and supplies the power to the battery. If the battery is removed from the car and then put back in, there is a good chance in some cars that the cable will be forgotten to connect. The battery is full and at start-up there is nothing to indicate that a cable is not connected. But when I drove to Rotterdam in the dark with a just-bought Alfetta in 1981, the headlights went dimmer. When I arrived at a telephone booth, I called the salesperson to discuss the problem. He knew what to do. Just connect that red wire to the battery and see what happens, was the advice. The problem was indeed resolved. You just have to come to it. Because usually the alternator itself is to blame. It must be broken and must be overhauled.
Photo and text: Jacques van den Bergh