Hey guys, I created an account just to post the solution I found to this issue.
Background: Changed the iPhone battery to a new one
Issue: iPhone would not ready battery percentage, it would always freeze and after a while when battery did died, it was constantly restarting with a white screen for 3 seconds and stuck in this loop. I was able to restore and back up when I got lucky and the phone stayed on, but after a while the issue came back and the restart loop of death came one.
Solution: I read on the comments here on the tape piece on top of the battery conector
There is a small circular connection just bellow the battery connection (under the frame/once you take out the 2 screws), This piece gets loose, this is where you need to make sure it is properly making a connection to the metal just below (another circular connection), I noticed this was moved and after correcting it, my iPhone came back to life and upon connecting it with a cable it showed the red battery/need charge sign. Now the iPhone is working normal and reading battery percentage..
You can see the connector thing I mentioned here just bellow the knife, its the round one with the sponge on top (http://www.encorenews.ca/wp-content/uploads/2016/07/iphone-6-battery-4.jpg)
Hope this helps and all the best to all of you!