[LG V20 Questions & Answers] Dishe:
I did my homework first and got the Perfine from Amazon, not a no-name junk battery. When I couldn't fit it without removing the sticker, the seller was really helpful and actually offered to replace it after checking first that it fit in *THEIR* V20 at the factory. Really solid customer support. And according to Accubattery, it really is the capacity it says. Or at least within 99% of it (I think it estimated 4080 or something like that). So I don't think that's the issue.

That's actually really interesting- I did take apart the phone to do the thermal mod thing (wiped off the dried up pink gunk and put a thermal pad there), but it seemed pretty clean otherwise inside when I did. How on earth did you figure out that was the problem?

After speaking with some folks on Telegram, Reddit, and other corners of the internet, I have a different theory. A bunch of people told me to let the rom "settle", that battery life is never good after a fresh format or new rom flash. Honestly, the critical thinker in me says that sounds like nonsense. I've never heard of this before and I've been flashing roms for the better half of a decade. I'd think after it takes the time the load up your apps and settings and configure itself, it would be done. But I Googled it, and it seems to be a hotly debated subject.

Those that swear by it claim that whenever you start fresh or flash a new rom, it takes anywhere from one to a few battery cycles before the kernel calms down. Perhaps it is doing some sort of internal calibration based on your battery life and system demands or something like that. The theory seems to be that the Kernel doesn't allow the CPU to fully doze until it is done with this process, which sometimes takes days. I don't know, but the more I read about it, the more it sort of started sound plausible. When I got the phone, I tried out some things on it myself without a SIM card first, just to make sure everything on it checked out before giving it to the wife. Then, I did a hard reset, popped in her SIM card and let it do its thing. That's when we realized battery life was pretty terrible, as after 10-12 hours she already needed to charge... and this was on the 4100mah perfine!

I tried another hard reset, and when that didn't fix it, I decided it was time to root and see if I could get to the bottom of the bad battery. It seemed the kernel was where a lot of it currently went according to battery stat apps. After downgrading my H918 to Nougat so I could install TWRP, I promptly flashed Alpha Omega Oreo 20H and restored her apps. Turns out the next morning we couldn't sync her smartwatch, somehow we broke bluetooth (going into BT settings would crash the phone everytime). So weird, and while we were at it the battery life still wasn't great. So I did a hard reset that evening and found that even with nothing installed, BT was borked. (looking back at it, I think I should have flashed an Oreo rom like the stock 20H rooted first, and THEN the AO rom. I think going straight from nougat might have led to a driver mismatch or something)

Anyway, after that didn't work I flashed the stock rooted 20H rom, and lo and behold bluetooth and everything else is working again! But the battery has been bad all along and is still doing its generally suckiness thing.

BUT... as someone pointed out, if this kernel settle thing is real, I've been pretty much flashing and resetting this thing every other day. If roms need to "settle" first (still a debatable subject), than I never really gave it a chance yet and that's why I'm constantly seeing battery drain from things like Kernel even on different roms! And [MENTION=5663191]pistacios[/MENTION], is it possible that you were having a kernel do the same thing since it was just reset and it just so happened to be that when it calibrated itself you thought you had fixed it by cleaning out the USB port?

All I know is I'm going to give this a couple of days before deciding on a plan B. Seems too simple to be the reason, but hey- I'm desperate.