I understand that all headlights do dim briefly when you turn the engine on and that this is normal. I also believe that the headlights will dim more if the car battery is in poor condition. I hope that this question is not too physics related for this Q&A site because I think that electrical circuits may form part of the answer in this case.
I do not understand very well how the electrical circuit works in a car to mean that this brief headlight dimming is actually necessary (as all cars that I know of do it by design).
Why do car headlights dim briefly when you turn the engine on?
How can you determine the quality of your car battery by observing this headlight dimming phenomenon (i.e. how do you know what a good car battery would look like for reference)?