Chosen Solution
followed the step by step directions and replaced by iPhone 6 plus battery… all seemed to go smoothly, but now the iPhone won’t boot (stuck in the apple logo loop on an apple usb charger).. and “waiting on iPhone” in iTunes restore mode connected to my 2016 iMac with current iTunes… GAH!
Most likely the new battery doesnt have enough of a charge to boot the phone. So, either the charging doc needs replaced or cleaned as it is not charging your new battery with enough charge to boot it or you have a board related (u2) ic chip problem and need to replace the (u2) Tristar IC to get it charging properly. If you can use a charged battery to test with, it should boot up and troubleshoot from there :)
