Chosen Solution
Recently after replacing my screen on my 6s the battery has been having issues, for the past couple of days it has been charging for hours and hour (at least 16 hours the longest) and it still wont turn on, when it does turn on, it dies immediately on the charger, I heard something about heating up the back of the phone while plugged in, does this work?
Do you still have the original screen? If you do, then I would try to install it to see if the phone behaves any differently. Alternatively, disconnect the screen (always disconnect the battery first!) and connect it to an iTunes enabled computer to see if it boots properly. If it does boot (and assuming it isn’t locked with a passcode) then you could also download and install 3uTools which will allow you to see the battery condition. While you have the phone opened up, do a thorough visual inspection, preferably with magnification, to see if there is any physical damage or debris that could be causing issues.
Don’t heat up the back of the phone it makes it worse as it overheats the phone and drains the phone and most probably damages the battery. You can get a free battery replacement by Apple themselves if the phone is still Valid. Or you can get a third party battery for Ebay then install it yourself. Watching a simple tutorial on YouTube can help a lot and cut back cost. But my suggestion if you can’t buy another battery is to scrap the phone and sell of as battery damaged
