Why Do Car Headlights Start Bright But Quickly Dim?"

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Car headlights initially appear bright when starting because they receive full power in the "Running" position of the ignition key. As the key is turned to the "Starter" position, the starter motor engages and draws significant current, causing a voltage drop due to the battery's internal resistance. This results in the headlights dimming temporarily. Once the key is released back to the "Running" position, the headlights should regain their brightness. Understanding this process clarifies why headlights dim during engine start.
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headlights on starting car are brighter . they immediately get dimmer . why?
 
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When you turn your key, the key passes "Running" position first, at which point, full power is supplied to the headlights. The key then goes to "Starter" position, at which point starter engages. Starter draws a lot of current, which results in a battery voltage drop (due to battery's internal resistance). Because of that, lights grow dimmer. Once you let go of the key, releasing it back to "Running" position, the lights should go back to being bright again.
 

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