[G3 General] mxasf:
Hey Markovich -

I'm about to write a post describing my long 15~20 hour troubleshooting ordeal, that ended with my battery being messed up (?!?), which was the last thing I thought would be the problem. So yes I'd absolutely consider the battery. You say "one would think you would be able to power it on while plugged in, though", which seems absolutely logical, but indeed that was not my experience.

re. device manager: does it show up in modems? I thought that is a requirement as well, but I'm not 100%. (I always looked for a listing in modems).

re. LG flash tools: yeah it shouldn't work there. (in my experience, it doesn't work even if you successfully completed the "hard flash" which is a little surprising; but fortunately LGUP seems very robust).

re. LGUP: Did your phone enter download mode? (i.e. plug in thye USB port while holding up arrow; and seeing colorful "download mode" on the screen). Seeing "unknown model" when you start LGUP did seem to happen to me as well. The solution is just to fiddle with your phone (unplug the USB, plug it in, maybe several times; maybe restart LGUP a couple times, making sure to run it from administrator mode). Eventually it will show the main screen with the correct model #, and allow you to download. But yeah, be patient. And do realize a faulty battery can cause lots of problems (bootloops, etc), even if the phone is plugged in. I just got one of those "powerbear 10,000 mAH" batteries on amazon. it really kicks ass, and the enlarged backplate is not a big deal.

-mxasf

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Funny resolution to my problem that I started describing above the other day:

[a] received a brand-new LG G3 (D850 / AT&T variant) and battery (and two year extended warranty); factory installed with MM / android 6.0

tried LG Flash tools (unsuccesfully), this is expected, because you can't use LG Flash Tools on MM ROMs.

[c] based on this thread; tried LGUP (successfully flashed). But it bootlooped. It would even make it to the KitKat start screen, but then crash and reboot

[d] thinking maybe download mode corrupt, or some other fundamental problem: tried the hard reset procedure (shorting motherboard etc) (here); used the standard D850 TOT file (kit kat) (done successfully)

[e] thinking all artifacts of earlier MM installation had to be flashed away (?), tried LG Flash Tools again (unsuccessfully!? same behavior as before)

[f] tried LGUP again (ok, successfully flashed). But upon booting, the phone continued to bootloop just as before (!?!?) (USB plugged in, or not plugged in)

[g] in desperation; about to invoke the warranty... thinking of how to lie about the phone just dying (and it was obviously nothing I did)

[h] chased a wild hunch (brand new 2014-era phone; maybe the battery has an issue?): tried another battery. Bootloop problem fixed, and phone is now 100% functional

did standard modifications: towelroot; installed TWRP (3.02-0); and installed amazing resurrection remix ROM (D850 thread) (nougat / android 7.1.2).

My conclusions:

Very confused about the battery. The phone/battery was "brand new" (albeit may have sat around for awhile before being sold). But this

"bad" battery holds charge well, and was ~90% full through all of this troubleshooting; no evidence of the battery draining quickly like normal bad

batteries. And even weirder: the bootlooping problem occurred even when the phone was plugged into my computer, or to a 2A external

quick-charger, that I normally use to charge the phone !?!?! But when I swapped the battery for another one, all odd behavior disappeared.

As a final "control experiment": I put the bad battery (with 90% charge) into another working D850. That phone would also not boot

successfully, and would simply boot loop. So there is something really wrong with this specific battery.

good luck all! If you have bootlooping problems or really any anomalous behavior at all, check your battery!