[Galaxy Note II General] aukhan:
The KERNEL can do some important things to help with battery saving as it is the controller of all things working in your phone:

1.

Underclocking

- if you feel your phone is fast enough, go ahead and lower the maximum frequency of your CPU, it will save power as the faster the CPU goes, the more energy it uses.

2.

Undervolting

- it's more complicated; every CPU requires certain amount of supplied voltage to run and the amount increases with the speed of CPU (clock frequency). For example 200Mhz requires only 0.9V while 1600Mhz requires 1.25V by default. The thing is, the higher the voltage, the higher the heat and of course power consumption. So the best way to lower it is to lower voltage - Samsung had to set voltage at the high enough level that every CPU they produce would work correctly but every CPU is different and some of them allow for lowering voltage and still remaining fully stable thus using less power to do the same work. Typically you can save about 0.05V but some CPU will allow as much as 0.1V to be saved. The same really goes for our GPU part, it can be undervolted as well. There are other parts in our phone that can be undervolted, like memory or controllers of various part but I have found (well in my phone) that saving were very small and caused instability so I would not recommend playing with them. We could think about undervolting our display as it is the biggest consumer of energy in our phone but actually we are doing it all the time The voltage supplied to the screen decides its brightness so if we were to lower the voltage it would just get dimmer

3. There are small savings to be had in various other parts controlled by the kernel:

- first and second thing are tied with SDcard - using it carries high power requirements - the less we use it, the better. Now we can't reduce to completely as all our data, apps and whole system is on it but we can reduce it's use by setting various caches.

a) read cache for internal and external SD combined with scheduler that minimizes reads and writes - so far the best scheduler created specifically for mobile SD use is FIOPS, so using that with a large buffer (maximum of 4096) is actually the best from energy standpoint.

b) system swap space - some kernels allow for creating a very specific kind of swap space, Android will use it once the free memory falls below certain point. Normally this swap space would be placed on SDcard but in this case it's inside a specific region of RAM. Why it is created like this? Because it can be easily compressed to keep more data, so basically we are using Android mechanisms and compressing memory so we can run more apps and keep them in physical RAM That means they are accessible faster than if we were to read them from SDcard and they use less power. Compressing and decompressing data as they go in and out of swap space is still far less energy consuming process then reading them from SDcard.

- third is governor configuration - governor is a system service that decides at what frequency should the CPU be working at every moment and how much cores should be enabled - this of course has great impact on energy consumption and on the smoothness of our experience with our phone. There are two schools of setting up governor and they base their decisions on two premises:

a) sharply increase CPU speed to get the work done fast and sharply decrease speed once it's not needed.

b) slowly increase speed and only so much to do what must be don then slowly decrease speed once you are done because you may have to do something again in a moment

There are pros and cons of both ways - way A means jumping to high frequency for a short time but high frequency uses comparatively large amount of energy, way B means slow increase but also means remaining in intermediate states for longer actually using energy for longer. I don't have any way to actually measure the resulting energy consumption but way A has a distinct advantage of creating much smoother experience so I use that myself.

- fourth is hotplug configuration - our CPU can dynamically enable and disable additional cores - the process is called hotplugging. Some governors are created specifically for controlling this process, the best, as far as I have tested, in this is Lulzactiveq. Hotplugging has to be wise as to the IF and WHEN to enable and disable additional cores, it measures how many "packets" of data are in queue to be processed and based on short history anticipates increase and decrease of workload.

All those interesting options are configured in scripts created for main contemporary kernels: Nadia, Devil and Agni and available HERE.

Latest OC / UV Scripts for Devil / Agni and Nadia Kernels for Note 2 are HERE

Guide to EXT4 to F2FS migration for Note 2 is HERE

Courtesy:@Mat9V