[G4 Q&A, Help & Troubleshooting] Lennyuk:
Like a lot of people I found myself with battery woes on the G4, at first my battery was comparable to my G3, if not a little bit better but over time, and certainly since MM was released, my battery life was quite frankly awful.

Long Story

I was losing power rapidly, even when I was barely touching the phone - so I finally decided to look into it. After carefully monitoring apps, wakelocks etc nothing looked out of place, everything was working normally until I installed my old faithful app - SetCPU, this is a nice basic app that just shows what your CPU has clocked too and how long it spent there in a given timescale and one thing alarmed me greatly. I was getting less than one minute of deep sleep in every 60 minutes the phone was running - now if I had been using the phone for 58 minutes of that hour I would expect this, only I hadn't!

I did more testing, and more testing and more testing but the results were the same, the phone barely entered deep sleep, and when it did it didn't stay there very long.

Now as I said, using all the other apps, and android stats, nothing looked particularly out of place and after a few days of trying everything under the sun to fix the problem I decided to go nuclear. I wiped the phone, installed no third party apps but setcpu and began testing. I left the phone for a period of 10 minutes to see what the deep sleep value was (looking for it to be somewhere between 80 and 100% of time, which it was!) and it seemed to be back to normal, working fine. So then I started to install my core apps, one by one running the same 10 minute test between each one, idea being eventually I would hit whatever app it was that caused me problems.

So I installed upwards of 30 apps, and everything was still fine, in 10 minutes of not using my phone I was getting around 9 minutes of deep sleep, which is what you would expect.

And then came facebook, now I know there are lots of reports of facebook being a bad app blah blah blah, but I have never seen facebook react like this before, and I don't know if it was marshmallow or a mixture of marshmallow and LG - but when facebook is installed my phone stopped sleeping again, at first it wasn't a huge change, but was noticeable it dropped to deep sleeping at about the 40% mark so I had 4 minutes deep sleep in my 10 minute test - force closing facebook made no difference, I tested it again and again, rebooted and again - same results.

I then installed facebook messenger, and my deep sleep dropped again, to about 20%.

I then uninstalled messenger - no change.

I then rebooted - back to 40%

I then removed facebook - no change

I then rebooted - back to 80+ %.

TLDR: The "Solution"

Now for some of you this won't be a solution as you will claim you need the apps, but there are third party alternatives out there that work pretty well without causing this issue.

Facebook (and facebook messenger) on stock MM builds (at least on the G4) causes the phone to hardly ever enter "deep sleep", this means it is always running a CPU clock, albeit the lowest setting, but it consumes far more power than it needs too especially when the phone is not in use.

After removing them my phone is now sleeping fine and my battery life is back to being very good - lost 4% in the last 8 hours (albeit idle, I was asleep), which is much better than losing 4% an hour that I was before facebook was removed.

Go give it a try, install setcpu, reset the timer close go back to your home screen then leave your phone for 10 minutes then check your deep sleep values, then try it after removing facebook (and rebooting).

Let me know how you get on.