[T-Mobile Galaxy Note 4 Android Development] aka_GrimJack:
Problem

At first I thought this was a hardware or battery issue, but now I'm not so sure, so I'm posting this here. The 'fix' I'm trying right now seems to work, but it's only been about an hour. (FYI: I'm on 3.2)

Earlier today, I had a notification from Play that I had updates to installed software. When I opened Play to perform the update, I was first given a pop-up asking me to agree to Play's Terms and Services, which I did. I believe this happens when Play does a silent update and/or the T&S changes. I installed my updates without issue and used the phone for (maybe) another hour or two before going to sleep.

I woke up to find the phone off, but I hadn't turned it off. So, I turned it on but it wouldn't get past the boot animation. It would shut down and reboot on its own, like a bootloop. Thinking I had some kind of battery drain issue, I connected the charger and was able to get it boot completely. The battery was at 80%, which isn't far from where I left it before sleeping. This makes me think battery drain isn't part of the problem. But disconnecting it from the charger caused a hard-off within a minute, as if I had removed the battery, not a proper shutdown.

Fearing the worst, I reconnected the charger and backed up all apps with Titanium, then I booted into recovery, which seemed fine, and did a full backup. The battery level went up while in recovery, so charging seems to be working. Rebooted after backup, battery around 95%, but still shut down when the charger was disconnected.

I shut down the phone and pulled the battery. It was warm, but not hot. Visual inspection didn't reveal anything unusual. I used a can of compressed air on the battery contacts both on the phone and on the battery itself. Battery back in, I booted to recovery and wiped Dalvik and cache, then rebooted into system.

The boot animation went on longer than normal, but not so long as a firstboot after flashing. I let the device charge to 100%, then disconnected the charger. Unlike previous attempts, it stayed powered on. I went into Kernel Adiutor and disabled the settings at boot. (fastcharge and swapping mpdecision for Intelliplug). I don't know if that has any material effect, though.

I wanted to see what would happen with prolonged usage or sustained battery drain, so I set the screen timeout to 30m and played an HD video I have on the SD-card. After about 15m of that with no issue, I used my Twitter app and that seemed fine, but I did note that the battery level was draining fast: about 12% down after only 20-25 minutes of use. And now it's still on, not shutting down, and the drain seems to have leveled off some. Even with the screen on the whole time, it's only dropped another 4% in the last 15-20m.

I'll be keeping it off the charger to see how long it stays on, but here's the question I have: The only real change I made was wiping cache and Dalvik, because Kernel Adiutor settings are applied AFTER boot. Could the silent Play update I received needed to be 'set up' by the OS (like all apps are on a firstboot) before working properly? I'd assume that on a factory ROM this isn't an issue. And I've never had to do this sort of thing for a Play update on any ROM in the past. But I did notice that failed boots happened after the device's splash screen, during the Android animation, when the OS itself was loading. I don't know why leaving the charger connected allowed a proper boot if this is the case, but since wiping Dalvik and cache, the phone seems to be working as it should.

Anyone else ever encounter anything like this? Is there (maybe) something in the ROM that would prevent a new Play (or Play services) from properly setting up and cause the booting issue?

Thanks in advance for any help figuring this out and Thanks to ZION959 for all their work!

P.S. Sorry about length, didn't want to leave anything out.

EDIT: I think it's a failing battery. When connected to the AC charger, it works normally. But on a full charge -- ROM and TWRP both report battery is 100% -- the phone dies within seconds of being removed from the charger. On a previous phone, periodic resetting of battery stats was necessary to get an accurate read of battery level. Do Samsung devices have a similar issue?

EDIT 2: Replacement battery corrected everything. Factory battery lasted 14mo. Let's see how long this Anker lasts.