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Your car has a 12 volt system and uses a battery to maintain that 12volts. However cars us an alternator to produce power to charge your battery (at a slightly higher voltage, usually 13.8 volts). However if the engine is running slower, the alternator stops charging the battery and your system goes back to it's 12 volt battery power and therfore 1.8 volts less. All the lights dim slightly, the heater will slow down, everything that uses electricity will be effected. A weak alternator will make this worse as does a worn, slipping alternator drive belt. Should probably be checked.

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16y ago
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14y ago

The headlights in a car usually dim slightly when the starter motor is used. This is because the starter generally pulls substantial current from the battery, and that causes the battery voltage to decrease.

However, they should not go too dim...

Excessiving dimming can be caused by a bad or undercharged battery, and bad starter, or bad wiring connections.

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10y ago

As you add more load, the load current increases. This causes an increase in the internal voltage drop taking place inside the battery, due to its internal resistance. The terminal voltage will then reduce by an amount equal to that increase in internal voltage drop.

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15y ago

Because when a car is started the starter motor draws power from the battery at the same time as the lights, leaving less power for the lights so they dim.

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Q: Why did your headlights dim while you start your car?
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