I can get the best 14km/l in city run

I experienced it many times. Asked SC but they can't solve it.

Hi Mirage/Attrage users. Does anyone of you experience voltage fluctuation when driving the car especially the lights are turned on? I can feel the front lights (low beam) will dim quite seriously when the air-conditioning's compressor kicks in, and other lights even the information display (showing km running, fuel consumption, etc.) at the dashboard meter fluctuating.

As I have brought this to the service centre they checked my car's battery and concluded that the car is okay provided the battery is in good condition.

I have not done any modification to the car so I can say there must be something that is caused by the car itself. Does anyone know the appropriate way to detect maybe there might be leakage or component malfunction that causes the voltage fluctuations? Feedback would be very much appreciated.