Chosen Solution
I was wondering if someone could assist me in trying to repair/salvage this iPhone 6s Plus that I was given to by a friend. According to him, he had repeatedly abused his device by smashing the phone’s backside onto hard surfaces causing the phone to lose accelerometer & gyroscope functionality, and eventually causing it die. The phone no longer turns on even with a hard reset, is no longer recognised by my computer and will not charge. I don’t know exactly what has happened to the internals as I do not yet have the tools to take a look inside but I suspect that it has got something to do with the battery - i.e. battery dying, flex cables being disconnected from the logic board. I also suspect that some of the internal circuitry has been damaged from repeat smashes resulting in components such as the logic board to fracture or crack. I forget to mention that the phone is also slightly warped and bent backwards, away from the display. I would really appreciate it if someone could share their knowledge so I can at least get this phone back up and running. Ideally, I would also like to repair the accelerometer and gyroscope, but I think these involve operations requiring a high level of soldering which is arguably beyond my current capabilities. I may also look into replacing the back housing just to make the phone looking new again.
As @icefreak says, other than checking the battery and connections, you likely have a logic board issue. This is what many people would probably refer to as a rabbit hole, a job that will take more effort than it’s worth. I would encourage you to seek an experienced micro-solderer, explain the problem and see what they say in terms of probability and cost. An iPhone 6S Plus is certainly worth investing in but my concern would be any latent issues still to crop up. If your friend abused it as you say, lots of issues will show up over time.
If the phone was as abused as you say, then really anything could be wrong. Once you get the tools to take it apart, you will need to go over it with a fine-tooth comb. Definitely test the battery and charging system first, because power is the lifeblood of electronics. The accelerometer and gyroscope are part of the logic board, so they would definitely require detailed soldering. I shake too much for anything that delicate. I would also look at the possibility that a part broke off inside and possibly caused a short circuit somewhere. Basically, you have quite a task ahead of you. The first step is opening it up and seeing if it is even salvageable. Once you do get it open, posting pictures will help with getting an answer. The link below explains how to add pictures. I would also suggest getting a magnifying glass if you don’t have one, because you will probably want to look very closely at the tiny parts. Adding images to an existing answer
