I have had a Nexus 6 Smartphone since 2014 and over the last few months it has started to shut down seemingly at random. This, understandably, made it a bit unreliable to use, so I pretty much just stopped using it.
Since I seldom really "use" it I couldn't justify a replacement or a new phone. I did some research and supposedly the issue is caused by the battery- given it's age, this didn't seem wholly unreasonable.
I bought a replacement for about 12 dollars off eBay and when it arrived was somehow able to take apart my Nexus 6, replace the battery, and put it back together again.
Aside from completely resolving the issue with random shutdowns, the battery life seems to have been greatly extended, which is probably just me not noticing how much it had degraded.
I thought I'd share this for anybody else who might be considering a phone replacement to address their old phone "acting weird"- replacing the battery may be a simpler solution.