Battery yes, it is just me or is the guy whispering or talking very softly?
https://www.youtube.com/watch?v=EvV07kAttIc
Wow, I hit that link yesterday and clicked the maximize icon and my desktop immediately/spontaneously restarted!!! Serious system error, said Windows, when finished rebooting.

I'm way overdue to rebuild that system, it's running XP, the mobo won't even run Windows 7!
Edit: Watched from one of my Win7 laptops. Voice OK. He said getting the back off is "tricky." Hard to know how good you can get at that, I can get the back off my 520 in 5 seconds or less. The battery for the 635 is a lot bigger than the 520's, I can see that in the video. Likely, battery life is also a lot greater.
I installed an app the other day and it said I had to turn off running in background for some apps before it would work, and I was offered all the apps that were running in the background and I turned off a bunch. I had no idea so many apps were running in the background, including a bunch that I've NEVER used! Later I went into settings and couldn't find where I could see what apps are running in the background. Surely there must be a place in settings to see that.
I'm wondering a few things. A few times my 520 has woken me up with noise. Does airplane mode turn that off? I know you can set quiet hours, but I'd prefer to just turn off all sounds because I don't want to specify sleep time, that can vary.