I have a strange problem with my battery and alternator. the battery sometimes goes dead and I need to ask my friends to connect another battery for starting the engine.
I measured the dc voltage of the battery. when the car is off it is 12.5V and when the engine starts it is still 12.5V . I turned on the AC and Radio but while the engine was on and the voltage on the battery did not change. but when I turned on the lights, the battery voltage went to 14.2V which means the alternator started to work. I also disconnected the battery cable while the engine was working and it continued working that means the voltage was ok from the alternator.
but my main question is that why the alternator does not work when the engine starts and even when the AC or Radio is on. it just starts when I turn on the lights.

 

the alternator is designed to charge in low output mode(12.60 volts) when there is low load, which includes with the ac on but the compressor is not working

you need to test for a parasitic drain

https://www.youtube.com/results?sear...itc+drain+test

 

I checked and there is no drain from the battery.
I am confused that why the alternator is not charging the battery unless the lights are on. when the engine starts, the battery voltage is 12.5V same as when the car was off, but when I turn on the lights, the alternator works and the voltage of the battery is raised to 14.2V
I hope someone could help me on this issue.

 

Have the alternator and battery checked. It's pointless to guess unless you've checked both first.