Not recognizing battery, very strange symptoms
For starters, I'm aware that this is a very old mac, I'm just looking for advice. It's more of a project than anything and I'm not looking for any responses like "Just get a new one". Anything helps, I appreciate it.
I bought this mid 2012 Macbook Pro Retina off ebay for cheap, though it might be fun to try to fix it and resell it. There's no indication of water damage (as far as I'm aware, I'm not super knowledgeable with this stuff) and as far as I know, the previous owner couldn't even get it started up.
The only way I can get it started up is by holding the power button and then plugging in the magsafe charger. It runs, but fans are at full blast and the battery is not detected through the system or any third party software. The light on the charger is green and then turns orange a few seconds after plugging it in, but when I start it up this way, there is no light.
I'm assuming it's a hardware issue, but I have no idea where to look. I opened it up and took the entire logic board out and didn't notice any damage. The only thing out of the ordinary was that I couldn't find the cord for the keyboard backlight. Not sure if it got damaged or if the keyboard was replaced incorrectly or something like that, but it's obvious someone else has done work on the inside. There's a bunch of missing/misplaced screws.
The only other notable thing that I've noticed is that the fans kinda rev up and then turn off over and over again, but only when the battery and magsafe charger are both connected. If I disconnect the battery and connect the magsafe, it doesn't do this.
I've been scouring forum posts from the past decade and many different things but haven't found a solution. I know it's an old device but I figured it wouldn't hurt to see if I could get any fresh advice. I've tried updates, SMC resets, PRAM resets, factory reset, system diagnostics, messing with the logic board, and probably other things that I'm forgetting.
Like I said, this is just a project. I'm not trying to invest a ton of money into it or anything.
Thanks for reading
UPDATE to include diagnostics (Sorry for the picture quality, the computer runs slow as it is, I didn't really want to deal with uploading from it)