Forget it, the 6P isn't repairable. Even if you could find a battery you'll probably destroy the phone if you try and open it up. I had the Nexus 5 before the 6P. It was possible to open the 5 with the right tools but because they didn't intend for you to replace the battery it was impossible to find a decent replacement battery. I bought one off of Amazon and it only lasted half as long as the original, which I ended up putting back in. If you read the Amazon reviews of N5 replacement batteries my experience was typical, they were all crap. It will be worse for the 6P because it's so much harder to open it up so there will be an even smaller market for replacement batteries and thus no reputable seller will offer them.