Chosen Solution
I have had issues with this iPhone 6 that is about 3 or 4 years old. It was not charging so I took it to an apple certified technician and he replaced the charging port. It worked for about a year and a half before it stopped working again. It started having battery issues (battery draining fast, heating up, suddenly powering off) leading up to it eventually not charging and staying stuck on the red battery screen (however it was the one saying it was charging). So I replaced the battery with one bought from this site. I ran the battery down to dead and then fully charged it up once at which point I used it for the day and then plugged it in that night. It was still working in the morning, however I had to power it down to keep it from ringing for about an hour at which point I tried to boot it back up but it would not turn on. I tried force restarting it, to no avail. I allowed it to charge all night again and tried force restarting it while plugged in. There is no response from the phone whatsoever (vibrations, dings or anything). I have taken it apart again and inspected the cables and connectors for any damage and have not found any. This is leading me to believe it is something in the logic board. I would love any input as to the cause of the problem.
It may very well be a bad replacement battery. If you have access to a multimeter, try measuring the voltage directly at the battery connector (with the battery disconnected) You should measure at least 3V but typically 3.2V will allow the phone to boot. There has been huge demand for batteries following the throttling issue earlier this year and I suspect some poor quality batteries are weeding their way through the supply chain. Contact iFixit Support (help.ifixit.com) to see what they can do for you as well.
