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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)?

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A lot of times, I use a "water in a pipe" analogy when I think about electricity. In order to explain why the lights dim when you start the car, consider the following: your car, on average, uses a reasonable amount of electricity. This electricity can be considered equivalent to water flowing through a pipe, let's say a 2-inch pipe.

When you turn your headlights on/off, or your radio, or any other accessory, it is as if there is a branch on the 2-inch pipe, let's say with 3/4-inch pipe, that has just been turned on. Like when you turn your faucet on/off. This doesn't really affect the flow in the 2-inch pipe that much.

However, when you start your car, the starter motor requires a lot of electricity. It is as if you had a 4-inch pipe branched off your 2-inch main and turned it on for a few seconds. In other words, if you have your faucet running then flush 3 or 4 toilets all at once. Because you are placing such a high demand on your supply, the supply to all branches diminished briefly.

That being said, I do not believe that there is a very consistent, accurate way to determine the quality of your battery by observing the lights dimming. The measurement is too subjective.

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  • +1 for a brilliant explanation! So a better battery would have a higher current "bigger main pipe" meaning that there would be more current "water" left over for the headlights as the car is starting, causing them to dim less?
    – Max Goodridge
    Jan 29, 2016 at 17:25
  • Yea, that's a fair assumption to make, however the battery isn't really our main pipe, the battery is the pump that is pumping the water through the pipe. The pipe is analogous to your wire - bigger pipe is equivalent to larger gauge wire. I would be hesitant to add a larger battery b/c I wouldn't know if the rest of the circuit could handle the higher current. I.e. if I doubled the pressure in my pipe, would the pipe burst?
    – wesanyer
    Jan 29, 2016 at 17:26
  • By a "better battery" I was referring to replacing a battery which is in "poor condition" with one which is new, producing the current/voltage as recommended by the manufacturer. But I do like the analogy.
    – Max Goodridge
    Jan 29, 2016 at 17:34
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    In general, the wire size is going to be selected based on the max current of each device. The starter should not be capable of using more current than the wire can safely supply, and in a modern vehicle might even have an over current protection device like a fuseable link. Some people actually put two batteries in their car if they have a lot of accessories (more for commercial vehicles).
    – JPhi1618
    Jan 29, 2016 at 17:35
  • Ah hah, yea so if you replace a bad battery with a good one you should see the headlights dim less. But again, it's a pretty hard measurement to make. Could you tell if your faucet ran a bit faster when you put in a low-flush toilet?
    – wesanyer
    Jan 29, 2016 at 17:36
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Wesayner explains how this works, but admits that the measurement is subjective.

It's not a good way to tell the state of your battery, but it will tell you if a dead battery is the reason your engine won't start (or, more specifically, won't turn over.)

The starter is connected to the battery by a relay: the keyswitch operates an electromagnet, that closes a pair of contacts that handle the much larger current of the starter motor. The starter motor relay is known as the "solenoid" (a word which in more general parlance means simply an electromagnet.)

If the headlights do not dim when you try to start the car, it means the solenoid is not switching on the starter current. There is a problem with the wiring or the solenoid.

If the headlights do dim when you try to start the car, it means current is reaching the starter motor, but it is failing to deliver sufficient force to turn the engine. Either the battery is flat or the engine is seized.

The solenoid is mounted on the starter motor, in order to keep the thick heavy duty wire from the battery as short as possible. This arrangement also means that in some cases the solenoid can also be used to engage the starter pinion and flywheel mechanically.

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I agree with Steverrill but it was partly correct. Let me give the clear explanation, why actually the Headlight Dims? In actual the headlight will not dim, it was turned off during the engine cranking. This is because the starter motor is the battery hogger(It will drain out the complete battery in the matter of 15-30 minutes continues run). So the manufacturers designed the starter motor to work in the isolated manner ie the starter motor alone was directly connected to the battery with a solenoid (Bendex or electromagnetic contact). It is also designed in such a way that if the starter motor solenoid was engaged it will disengage all the other electrical / electronic equipments like Headlight, Stereo, AC/Heater Blower,etc. Once the Engine is started then automatically the solenoid will disengage and so all the remaining systems will come back to normal.

Possibly I have cleared out your doubts.

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  • But if the headlights were turned off, surely they would not produce any light at all whilst the engine was being started?
    – Max Goodridge
    Jan 30, 2016 at 16:44
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    @MaxGoodridge Some cars are fitted with a circuit that disconnects other loads when starter is cranked, many are not. So this answer is correct in some cases. The case I described in my answer is not wrong, but it is mutually exclusive to the case described in this answer. Having a circuit like this helps the battery, but it also means you can't use procedure I described in my answer for diagnosing why your car won't start. I wouldn't like to guess what percentage of cars have this feature, but it's an unnecessary refinement. With 400A to the starter, who cares about 40A to the headlights?
    – Level River St
    Jan 30, 2016 at 19:59
  • @MaxGoodridge The headlight bulbs will not stop glowing immediately instead once it is powered off it will dim slowly for few seconds and then it will be stopped glowing. In the olden days vehicle it was not connected to the main controller relay as Steveverrill said, but today all the vehicles are coming out with the auto cut power relay. It is because to prevent the headlamp and sensitive and most required accessories from failure.
    – Shameerariff
    Jan 31, 2016 at 3:57

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