Dell Latitude E6420 Black screen of death but external monitor working fine.

My Dell Latitude E6420 has black screen of death. Let me explain what happen. My friend bought a battery for his E6440 laptop and his laptop not starting (no display) he asked me to check on my laptop for that battery.

My laptop display gone after inserting his battery and changed to my battery still didn't worked. I connected external monitor display coming there (external monitor is showing primary). but laptop display not coming to make it secondary.

I did steps.

1. drained power 30-1-2. still didn't work

2. changed RAM slots with another still didn't work

3. tried D+ power, did not see anything.

this morning I observed If close display 70 to 80% I can see barely light on the screen window login in the angle. 

Is that means back light gone? What are solutions have now? Do I need to change display? give me suggestion.