No backlight and video with two different logic board

3 years ago, my MacBook Pro Retina Mid-2012 display lost the video signal and the backlight while running. A little bit of smoke came out from the ventilation on top of the keyboard in the left corner and it made a strange sound in the left speaker. Since then I had to use it with an external display. No video signal and no backlight goes to the internal screen.

I took it to Apple and after checking it, they told me they had to change the logic board. It was a 900€ repair at the time and I couldn’t pay for it.

A year ago I went to a shop in Estonia where they perform micro soldering, hoping that they would accept to change the GPU. They managed to fix the backlight of the screen but not the video signal. They told me that the GPU replacement is not something they take on. They said that if I get a new logic board it would fix it.

This year I found a trusty seller on eBay selling a logic board for about 350€ including shipping and import fee. I decided to get it.

I changed the mother board, turned on the laptop on battery (not plugged in with power cord) and literally 3 seconds after starting, white smoke coming out from the top left corner of the vent! I stopped it as soon as possible and took a deep breath. No backlight again and no video signal. The computer works with an external display like with the old logic board, but same problem again.

I finally had save enough money to buy a new mother board and it seems that the problem came from somewhere else.

Since the change of logic board, the mbp says that the battery needs servicing. Could it be that the battery is the cause of all this? Do you think there is something I can do?

I would really appreciate help.

Niels