[OnePlus 3 Guides, News, & Discussion] charackthe:
AFTER DOING EVERY SINGLE ONE OF THESE INCLUDING THE OPTIONAL ONES YOU'LL GET 12 HOURS OF SCREEN ON TIME

IF YOU EXCLUDE OPTIONAL ONES YOU'LL GET MINIMUM 7+ HOURS OF SCREEN ON TIME

First,obviously, you need a rooted OP3 or OP3T

If your device isn't rooted check this out.

There is several steps to achieve this results,

TAKE A BACKUP

This is the most important. In case you mess something up or this guide doesn't quite work for you, you can always go back to your old setup.

Pick a ROM that works best for you in terms of battery life (I am using this ROM)

Pick a kernel that works well with your requirements and the ROM (I am using Boeffla Kernel)

So the key to achieve the best amount of screen on time is underclocking your CPU and GPU. An average user shouldn't have any problems underclocking but some users that play games or care about benchmarks shouldn't underclock. There is optimal values that I found while messing around in the config app.

After flashing ROMs and kernels don't boot yet! We have more to flash!

Download AKT profiles and flash!

After you've done these you can boot up your OP3/OP3T

Now we have several things to do on the device:

First thing's first, let's tweak the frequencies! The optimal values are around 844 MHz for the little and 1324 MHz for the big CPU and minimum frequencies should be at lowest possible. For the GPU the best value seems to be 401 MHz and again minimum frequency is lowest possible.

Next! Let's set our ROM's battery profile to performance! Wait hold up there wasn't I talking about longer screen on time how is performance profile is going to help? Here is the answer: Since we have already underclocked our CPU and GPU setting this profile to performance will allow things to be used up a little more. And it will make you feel good :D

Now remember when we flashed AKT Profiles? Now we'll set that to performance.. Oh wait what? Performance again? YES! But we'll pick the most smooth one. Set it to Fusion Performance!

(Optional) Now we have things to do with apps. Download Greenify to hibernate things when you aren't using them. Select apps according to your preferences and hibernate.

(Optional) Now we'll play with some wakelocks. Download a wakelock detector app and monitor the wakelocks. After some amount of time you'll see some useless apps waking your device every now and then. Go to your ROMs wakelock disabled and disable those wakelocks! Show them who is the owner of the phone! (For example you can block The File Manager HD's wakelock since it has no job when screen is off and MX Player's wakelock because again what can a video player app do when you aren't watching anything.)

After this point if you see more lag increase the CPU and GPU frequencies or reduce them if you see no lag and if you want more battery!

This method might not turn out the same for everyone! You might disable some wakelocks that are important for the device and mess up things so be CAREFUL and when hibernating note that all the hibernated apps will stop pushing notifications! So don't hibernate WhatsApp, Messenger etc.

There is a screenshot below as a proof.

I know I've done nothing but writing this but still if you want to donate here it is LOL

*UPDATE*: Blocking the useless wakelocks allow OP3 to survive 24 hours (if you aren't using it at all) only 3G on (no wifi, no GPS ONLY 3G)