[Nexus 6P Q&A, Help & Troubleshooting] Face_Plant:
After quickly destroying the battery in my Galaxy Nexus I decided to research ways to prolong the life of the battery in my smartphone, especially since the phone I replaced it with (Nexus 6) didn't have a removable battery. So far they've worked great. Several tips I have are:

1. Plug in when you get to ~20-25%. If you want to be super extreme you can use an app like Battery Charge Limit to stop charging at 80%. Like I said earlier, discharging/charging too far is bad for the battery.

2. Don't use fast charging. This heats up the battery and wears it out faster. I've used the 1 amp charger from my Galaxy Nexus on my Nexus 6 and 6P and it has definitely helped prolong the life of the battery.

3. Don't use the phone while charging. This causes it to heat up and wears out the battery faster. Light things like texting or browsing XDA with their app won't heat it up too much, but gaming or installing ROMs while plugged in is very bad.

4. Don't charge the phone overnight while it's on. Obviously after the phone reaches full capacity it stops charging, but if your phone is on and running stuff in the background the battery will drain. When it drops to 98% it starts to trickle charge again until it reaches full capacity. This cycle of charging and draining will occur several times throughout the night and adds unnecessary wear cycles to the battery. It's best to charge while the phone is off to avoid this cycle.

5. Don't let the battery get too hot or too cold. Low temperatures can be just as harmful to batteries as high temperatures. If it's 100°F or 0°F outside and your battery is getting very hot or very cold it's best to just leave the phone in your pocket until you're in a place with a more reasonable temperature, such as inside a building or something. Lithium ion batteries prefer a happy medium, which is usually around room temperature (~68°F).

6. Keep the phone cool while doing CPU intensive tasks that cause it to heat up. When I install a ROM and boot it for the first time, wipe cache/dalvik after installing something, run the auto patcher in WakeBlock, or anything else that causes the CPU to run at a high frequency for prolonged periods of time I remove my phone from its case and place it in front of a fan on high speed. This keeps it from getting too hot, but doesn't let it get too cold. Placing your phone in the fridge/freezer will also keep it cool, but lets the battery get too cold, so that method is not recommend.

7. I use an app like EX Kernel Manager to display the battery/CPU temperatures in the notification area. If I notice the temperatures are getting too high or too low I stop using the phone until it cools down or I'm somewhere warmer. This can be annoying on Oreo because you get notifications of apps running in the background, but that's pretty easy to block

Keeping the phone cool can also help avoid the defect in the Snapdragon 810 that causes the BLOD (boot loop of death), which usually occurs when the solder holding the processor to the mother board develops cracks.

Based on posts I've seen in this forum most people begin to experience battery or boot loop issues after about a year or so. I've had my 6P for about a year and have not had any battery, heat, or boot loop issues. In my experience, taking care of your phone and being "extra nice" to it is the key to prolonging its life. After 2 years of using my Nexus 6, battery life on the day I sold it was just as good as day I unboxed it. I don't want to jinx myself, but the same goes for my 6P. After almost a year of use I can still go 16+ hours off the charger with 4+ hours of SOT and 30% or more left in the battery when I plug it in at night.