[Note 7 Themes, Apps, and Mods] donce1991:
As the title suggest, its possible to put Note 8 battery in our beloved Note 7 :) the capacity is a bit smaller (200 mah less, but still 100 mah more than what gimped Note FE version sports) . Its inspired by similar thread by Mr.Ultimate, he did some testing and modification with s7 battery.

I did some research for compatible batteries:

Manufacturers codes for Samsung S7 battery is GH43-04574C, for Note 8 its GH82-15090A, its just a shame they used much wider batteries in S8, S9 and plus models, but they might use narrower battery in upcoming Note 9, so upgrading and getting bigger battery might not be out of question ;)

As far as I could tell the Note 8 battery is the most promising for its size and device line (still Samsung Note series, just newer) so maybe better compatibility too and so I went with it :)

For everyone who would like to repeat this process, of course first you need to open your phone. You can look at ifixit.com note7 teardown or youtube for how to open it up and general guidance before you try anything, especially if its your first time opening a glued phone.

You need to remove at least the motherboard and the old battery for this modification (better yet, to remove all the components - easier to clean after), then you have to cut that frame part colored in red between battery and motherboard, cos original S7 or Note 8 battery is a tiny bit wider and removing that part gives fit just enough space to either one. You can use the dremel or any other similar tool to grind it off (I don't recommend cutting it, cos it might stress the frame too much and break the screen, but its just my preference). Then I used some black paint to mask exposed parts like here and here (for a closer look), optional, but I think its safer that way.

Now then new battery can fit comfy inside, had to bend the flex cable for reference, cos there enough space, also the new battery already comes with its own adhesive strips, if you get a diff battery, you can buy some 3M adhesive or similar double sided tape. What's left now is to remove the controller from the new battery and solder on the controller from the old battery.

Because batteries don't like heat the best bet would be to use spot welder, like they do in factories, but that's a bit on the expensive side (unless you do other electronics projects and happen to have one or made DIY one and even then the size of the tabs would be challenge too). So more realistic option would be to use soldering iron and some solder with flux, but because battery tabs are aluminum just any soldering flux won't work, because solder just wont stick properly and wont make permanent connection, which can add some resistance or even make a loose connection over time. So you must use specific soldering flux which eats into battery tabs and makes perfect connection. I tried few generic flux types I had on hand and basically ruined the endings of the tabs cos the solder just wouldn't stick to it, so I got some cheap zinc flux from local shop and it worked perfectly even using the same solder. After that you can use isopropyl alcohol to remove the remaining flux off the tabs, just don't short the battery. ;)

Some pointers:

Try to get original battery, fake or low quality crap from ebay wont cut it, not only because of lower capacity (there are no chinesium batteries who can beat Samsung in battery capacity in same size, period) but because of quick charging too (you shouldn't risk fixing Samsung fire hazard with your own DIY hazard).

MUST use battery controller from original battery, using other controller can and will produce some problems, the controller itself doesn't measure capacity, only voltage, so diff capacity doesn't matter, but diff controller wont "talk" with the phone properly, only using Note 7 controller the phone can charge from 0 to 100% and give full incremental readings and battery charging application actually see the right capacity (about 3300 mahs in Note 8 battery case).

Solder the old Note 7 battery controller directly to new battery tabs, using wires, extensions, etc could add some resistance to connection and you won't get correct battery readings or end up with crappy or slower charging or worse.

I did tried using Note 8 battery without Note 7 controller, even had to make some DIY connectors. The Note 8 battery connector is the same as the one on Note 7 motherboard so even not taking into account different flex cable length and form, you cant just psychically plug Note 8 battery to Note 7 motherboard. So I took some connectors from dead Samsung S6 motherboards and frankensteined them to flex cable and with some wires I got this and this, thus I could use new the battery without any disassembly or soldering directly (to the battery) just by plugin it with DIY inter connector. It looked all good and tidy inside, that outlined area is there the old flex cable was routed (for reference then closing the phone), but it didn't worked properly, I got crappy indication when charging/discharging, the phone couldn't properly detect exact percentage when charging, like, it could sit on 5% for half an hour, when jumps straight to about 80%, or go gradually to about 30-35 and jump to about 70-80 and just sit there and if rebooted it would go to 99% and so on. Also the official wireless battery cover didn't worked properly too.

As for software side I did jumped for a few weeks through diff roms to find the best one for me (Marshmallow based ones works great, but for some you need diff kernel because of charging cap, while Nougat roms are newer, but can be less stable, lack iris support and official covers, that you can get really cheaply, doesn't work too). So I ended up using original Note 7 Marshmallow rom (the one with 30% charging cap) and hydra kernel version 1.3.5, had to extract this version from hydra rom , cos you can separately download only 1.3 version (from here), recovery twrp 3.1, rooted with SuperSU 2.82 and with Europe Multi-CSC selection from CSC Selection. Of course I still get that nagging notification every time I charge the phone, but its a small nuisance. Also removed some bloatware with No Bloat Free app to make it snappier.

So far it works great for me as everyday driver. I charge the the phone on average every two days, it takes less than two hours on average to charge (I try not to use fast charging much) and its holding about same or even better than my old rooted Note 4 (n910c) with new battery. SOT is great (like more than half a day on wifi playing youtube, ect), (SOT time, usage, by apps. After using the phone for a few months I get average charging/discharging and usage time like this). Of course the system or other apps won't detect the exact capacity because they take the capacity from the model information not from measurements, so because the model is n930f, the system reports original hard coded Note 7 capacity of 3500 mah, but because I used BuildProp Editor app to change the model to n935f other apps shows 3200 mah (like it should for Note FE model), while only battery measuring apps like Accu​Battery report actual 3300 mah (estimated capacity is measured, designed capacity can be changed manually, by default it would show 3200 mah) capacity.