Answer to Question #161410 in Electric Circuits for Peter Frank

Question

Why do the light on the car become dimmer when the starter is opened?

Answer 1

Since, the starter motor draws a large current resulting in a large potential drop in the battery, which reduced the thermal voltage of the battery hence, the light of a car are dim when the starter is operated.

When the headlights dim slightly while trying to start a car, it is usually caused by the high current draw from the electric starter motor while trying to crank over the engine. Because the alternator is not making power the vehicle's electrical system is exclusively using battery power which isn't sufficient to fully power the vehicle’s lighting equipment at the same time. It is, therefore, suggested that lighting used for night driving (but never emergency flashers if one is stalled on the roadway) be turned off temporarily until the engine is restarted.