[Galaxy Note II Android Development] jshare:
If you're having battery drain problems...

Important: if your mobile provider is an MVNO or your phone is always roaming for whatever other reason, this will consume a lot more battery power regardless of how optimized your rom and apps are. There are still things you can and should do to improve battery life, but you won't be able to have the same results as someone whose phone isn't always roaming.

Otherwise-

1) It's probably not a ROM issue (so stop bugging your ROM's devs)

2) Your phone probably isn't going into Deep Sleep

If you check the attached screenshots, you'll see that thanks to Deep Sleep, my battery was able to last over 2 and a half days with over 4 hours of total screen time, without power saving mode.

The best free app to use for this is the one recommended by a few people here: BetterBatteryStats (XDA Edition). Choose 'CPU States' and scroll to the bottom. If your phone is able to deep sleep, you'll see it there. Otherwise, you'll probably see 200Mhz as the state your phone spends the most time in.

If 200Mhz is the most used state, something is preventing the phone from going into Deep Sleep, as [MENTION=2160126]blowy666[/MENTION] mentioned recently.

[MENTION=2741707]burgudoy[/MENTION] wrote a great guide about saving battery and in it, he explains how to check what's likely to be the problem, such as a kernel wakelock.

But here's something you can check right away with BetterBatteryStats:

In my case, the blocking kernel wakelock is called "charger-softreg", and it only appears *if I connect my phone to my computer via USB* (as of Agni 4.3.7, this is no longer happening- yay!). From then on, my phone can no longer go into Deep Sleep until I reboot it. Which is annoying, because I used to rarely reboot my phone at all, but it's not that big a deal and it's well worth it.

It's worth pointing out that I'm not using DS Battery Saver, because I don't need it. It was being blocked by the charger-softreg wakelock too, and as long as I charge directly from AC and avoid USB, the phone goes into Deep Sleep very quickly when I turn off the screen.

Other things I've done that help battery life:

Greenify as many apps as possible

Use All-in-one-Toolbox's Boot Speedup (or similar, Rom Toolbox can also do this) to disable any apps that I don't need to have running all the time from starting at boot

Frozen annoying Samsung and Google apps that were constantly polling for updates, such as Google+, Hangouts, etc.

Only use Data Networking when I need to

Schedule Wifi/Data/GPS to turn off overnight using Automate It

Screen timeout = 30 seconds

More things I could do to help battery life:

Use Agni Control's Battery Save or Extreme Battery Save main profile (I use Normal-Optimised on Agni 4.3.0)

Use Auto-Brightness - I prefer to keep brightness at about 15%, and switch to auto at night

Try DS Battery Saver and/or Amplify, such as to turn off wifi/data/gps every time the phone deep sleeps

Screen timeout = 15 seconds

From [MENTION=3838732]Taxi341[/MENTION]: always recalibrate your battery after flashing a ROM: charge to 100%, and then go into Phone > Keypad, type: *#0228#. Tap 'Quick Start'. The screen will go off and the calibration only takes a few seconds. After it's done, your battery % will have dropped but that's ok :)

Finally, here are some more general tips about getting great battery life:

[MENTION=4958387]UtkarshGupta[/MENTION]'s [Tips]How to get a sexy battery life.

[MENTION=5867207]vaisakh7[/MENTION]'s [GUIDE]Extreme Battery Life Thread(Greenify+Amplify+Servicely)

[MENTION=4752115]passenger001[/MENTION]'s DN4 OPTIMIZATION AND POWER SAVING GUIDE

[MENTION=591147]Entropy512[/MENTION]'s [REF] Known identified battery drainers