My battery was dying at 10-20% randomly. Sometimes I got it down to 5% but sometimes it was 15%. Figured after owning it for a good 15 months (longest I've ever kept a phone so far), it was time to just replace it and see if I can get better SOT.
I also got my parents' the Galaxy S8+. I got 6-7hr SOT easily with what I did on my phone and was test driving it for my use to see if I wanted one as well. But I just couldn't get into the phone. So I wanted to put some life back into my Nexus 6P.
I bought this battery from ebay for $10. The temperature sensor works, first thing I checked before putting things back together. It also says "charging rapidly" when charging as well.
http://www.ebay.com/itm/252482757101?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT
I used these videos for reference:
https://www.youtube.com/watch?v=sWas3cYS_0Q&list=WL&index=28&t=367s
https://www.youtube.com/watch?v=spz9S5aIiJg&list=WL&index=29&t=315s
Just some tips:
1. be careful when you're prying the battery off from the case. It has some super strong black duct tape that I had to use a credit card to push in and wiggle out. I ended up pushing the card in too deep and bent the black cable underneath. You can see the cable I"m talking about here at 10:37
https://youtu.be/spz9S5aIiJg?t=10m37s . Luckily it's reinforced a bit so whew!
2. Just get a replacement back and cover. The plastic piece on the bottom is pretty fragile, I ended up scuffing it with my scalpel. The top glass piece isn't as fragile, but I also ended up scuffing it when I used my scalpel to pry into it to create an opening. The adhesive holding the visor glass and the bottom plastic piece is tape. If you really go in there and pry it out, the tape is going to get all ruffled up. You'll end up having to need new adhesive anyways. Maybe just ruffle up one side and then slowly pry it up. For now until my replacement back comes in, the adhesive on one side is holding both pieces down quite well, nothing is sticking out.
3. Get more than one of those blue triangle picks. They're pretty much one time use, the plastic will deform as yhou're sliding it around.
4. The display has a super fragile plastic border around it. I ended up breaking off pieces of it and didn't realize it until I put everything back together. It probably happened when I separated the back piece from display piece. I'll probably end up breaking other pieces off so it'll feel smoother. This is the one part that isn't easily replaceable, so be careful here.
Pictures after everything was done.
http://imgur.com/a/FClTW