[Galaxy S6 General] Cuzeg Spiked:
Yeah but who said you need QHD 1440p for battery life. I use 720p HD to make sure I get 6 hours SOT, undervolt heavily by -75mv CPU, Greenify for background apps and apps I'm generally not using, Amplify , Cutting certain wakelocks from Greenify , interactive governor , underclock Min freq from 800mhz to 400mhz or so and max frequency from 2.1ghz to 1.5ghz . Little cores underclock max frequency from 1.5ghz to 1.0ghz and min frequency stays the same 400mhz. In hacker kernel , I could get the frequency to 200mhz but I don't think it's worth it. I can even overclock but that's not necessary if your aim is to battery save. Overclock only if a program needs it.

From my experience , I haven't found anything that even remotely needed 2ghz or all the cores. 6 cores seems to function just fine and less power used. 720p seems fine but 1080p if you don't like 720p. I am against 1440p on a 5 inch screen like the S6 though. Lower resolution, far better battery savings.

Then use 2 of the 4 cores in the big core cluster A57. Also GPU underclock to 544mhz or lower max Freq and min freq being 100mhz but that's because I don't have as many GPU demanding games. If you do have GPU demanding games, don't underclock but definitely undervolt.

I even undervolt memory by -25mv at most and -25 to -37.5mv for the GPU. Even -50mv at most on the GPU side. Again undervolt takes experiment to really see instability and a difference in battery.

QHD uses too much power for a 2550mhz battery tbh.

This also depends on your own apps usage too. I also make sure I use less ram possible by Greenify and low ram Xposed module as a good way.

The kernel I use is Hackers Kernel. There are probably better but this one is most customisable for overclocks and underclocks with auto trims and stuff.

Then of course lower brightness. I think CF.Lumen can probably save a small amount of battery . I had daylight mode 50/50/50 all colours so brightness is halved and theoretically I save 50% more brightness . It could give battery gains but it's hard to say.

GPU Rendering is unnecessary in developer option, unless you absolutely need it. I disabled DVFS so that it doesn't stutter at any frequency in gaming also. Keep in Mind , GPU rendering uses a fair amount of battery in exchange for potentially better performance in certain things that require it in GPU demanding stuff.

Don't use fast charging , especially while gaming on phone but fast charge is fine when screen off. Normal charge is preferable to reduce heat as heat does kill battery quicker.

If you care about deep sleep , Power Nap, Amplify, Greenify .

Oh and AppOpsXposed is a good one too in general. I can stop certain start-ups at boot that I wouldn't want ,disable WiFi scanning in settings by disabling locations for settings app (though not preferable if you need new WiFi to search) and other applications. Also disabling notification of certain apps and other crucial things but do so at your own risk.

I did do the advanced idea of cutting off wakelocks via Xposed Module Better Battery Saver but it isn't truly necessary. It does help though but you do so at own risk. Greenify is safer for wakelock cutting though.

I do remember getting much higher battery life but I ruined my battery life with my bad habits of using charger in games and other habits . I remember having 8 hours at first time with arter kernel and hacker kernel but at 1440p and awfully slow clock speed . 720p , I probably would have got more .

Do make sure not to make your phone hot by any means. Heat is the enemy.

Lastly , you can even try black theme and black wallpapers but that's entirely optional.