in my last post I meant
"Actually I COULD have pushed the DC jack's legs further into the corresponding motherboard's holes but it was very late when I noticed this because I had already soldered it to the mother board."
As for your comment, whether I use just the plug without the battery or with the battery, the effect is the same, it turns on for just two seconds and after that it turns off. I do not even get to see anything at the laptop's screen.
Any other idea?