Car Lights Dim When Starting Engine

Weak battery. A weak battery can cause the lights to dim when the engine is starting because it doesn’t have enough power to supply the lights and the starter motor at the same time.
Faulty alternator. The alternator is responsible for charging the battery, so if it’s not working properly, it can lead to a weak battery and, subsequently, dim lights.
Loose or corroded connections. Loose or corroded connections can prevent the battery from getting the power it needs to the lights, causing them to dim.
Electrical problems. Other electrical problems, such as a faulty wiring harness or a short circuit, can also cause the lights to dim.

In addition to the lights dimming when the engine is starting, you may also notice other symptoms, such as:

The engine cranks slowly or struggles to start.
The battery light on the dashboard is illuminated.
The headlights flicker or dim when you use other electrical accessories, such as the radio or air conditioner.

To diagnose the cause of the lights dimming, you can perform the following tests:

Battery test. Check the battery voltage with a voltmeter. A healthy battery should read around 12.6 volts when the engine is off. If the voltage is significantly lower, the battery may be weak and need to be replaced.
Alternator test. Test the alternator by starting the engine and measuring the voltage at the battery terminals. The voltage should increase to around 14 volts when the engine is running. If the voltage doesn’t increase, the alternator may be faulty and need to be replaced.
Connection check. Inspect the battery terminals and connections for any loose or corroded connections. Clean any corrosion and tighten any loose connections.

Once you have diagnosed the cause of the lights dimming, you can repair the problem by:

Replacing the battery. If the battery is weak, it will need to be replaced.
Replacing the alternator. If the alternator is faulty, it will need to be replaced.
Tightening or cleaning connections. If the connections are loose or corroded, tighten or clean them.
Fixing electrical problems. If there are any other electrical problems, such as a faulty wiring harness or a short circuit, they will need to be repaired.

To prevent the lights from dimming when starting the engine, you can:

Keep the battery charged by driving the car regularly or using a battery charger.
Have the alternator checked regularly and replaced if necessary.
Inspect the battery terminals and connections regularly and clean any corrosion.
Avoid overloading the electrical system by using too many accessories at once.