Hello, this is my first post

About 6 weeks ago, I got a used precision 7520 from Ebay. After some time I accidentally came across a strange issue, where if it's left plugged in after turning it off, the battery led will come on and stay on (without battery) blinking every 15 seconds or so, and the laptop doesnt turn back on. it would do the same after disconecting and connection it again, It also wouldnt turn on with just the battery. I found out, again by accident, how to turn it on; you need to push the power button at the exact moment the led blinks. One day i connected it with the battery on, and it turned on normally, after charging the battery completely, I turned it off, and it was back to normal. the issue didnt come back until a couple of days ago, but this time the battery charging "solution" didnt work (draining residual power works sometimes). idk what's going on, is this some sort of undocumented feature? is it hardware failure? I'm at loss here.

Thanks for your attention