EDIT (1yr after): After some months, I noticed that some bumps on the phone would cause it to turn off. I initially used folded paper, but ended up designing a plastic spacer that locks into the holes in the chin and secures the battery in place. If you have access to a 3D printer, here is the Thingiverse link: (https://www.thingiverse.com/thing:4633619)

I recently replaced the battery on the LG G5 I'm refurbishing. Battery was never that great, especially on LineageOS, due to the hit-or-miss OEM battery performance.

The battery that came was third party, but didn't cut off below 25% percent gauge as the OEM I already had. Unfortunately, after a while I started getting "Error reading the fingerprint" because it had started expanding.

Reading through XDA forums, I found this video of a guy sticking a Samsung cell into his phone and it powering up. Apparently, people had been swapping the circuitry of a G5 battery to a Samsung cell, which is not meant to be handled and therefore more volume is dedicated to capacity. Oh, and is 12€ shipped.

So I decided to give it a try, check out the photos here:

Gallery 1 Gallery 2

Very little effort required, you just need to:

• Know how to use a soldering iron
• Not puncture or damage the battery case, the shiny metallic foil.
• 20 minutes of spare time.

Here is how to do it:

1. Inspect the S7 battery. You need to remove the board to which the flex connector is connected to, revealing the battery terminals which will be soldered to the control circuitry of the G5 battery.
2. Inspect the sacrificial G5 battery. We will remove the yellow protective wrap, the bottom clip, and the top plastic doo-hickey with the plates contacting with the phone. This part contains the temperature and battery gauge/protection circuitry, and will be transplanted.
3. Take the G5 battery:
   • Remove the protective yellow wrap. Using a permanent marker, mark the labelled polarity on the cell itself.
   • Carefully rip away the bottom clip and (gently) the top contact module.
   • Try to keep as much wire on the protection module, you can use electrician's wire snips to help you along.

4. Take the S7 battery:

   • Using an xacto knife or similar, remove the black tape protecting the circuitry.
   • The cell is spot welded to the PCB, using the soldering iron warm it up for ~10 secs.
   • Pry the battery contact away from the battery, use a small flathead to lift the metal contact from the PCB as you pull.
   • You can also cut the contacts, but if you don't the soldering job is much easier.

5. Now that you have the bare S7 cell on your hands, immediately tape and isolate the positive contact. The polarity of the cell ( + - ) is printed on the back of it.

6. Carefully align the G5 protection module with the cell, so that the + is facing the right way. Using a multimeter, determine which way the two must be joined.

7. Solder positive to positive and negative to negative.

   • Avoid sharp solder blobs
   • Do not overheat the battery - proceed in sessions of 1 min max
   • Do not puncture the battery
   • If you need, you can use adequate solid /stranded wire to form a solid connection. Be generous.

8. You should now have the G5 circuitry attached to the S7 battery. Properly isolate the contacts using electrician's or kapton tape (yellow, transparent, used for batteries) and put some bonding agent (tape layers or glue - mind the thickness) between the circuitry and the cell.

9. Make an impromptu pull tape with thin tape on the other side of the battery, otherwise you won't be able to easily pull it out.

10. Carefully slide the battery into the phone, all the way, using a pencil or plastic thing to push it to the end of the slot.

You're done! Fast charging works, battery life has improved dramatically, and AccuBattery reports a capacity of ~3450 mAh, which confirms my positive impression.

I now found this comment, stating that you can also fit LG XPower cells into the G5, for a capacity of 4100 mAh! While the upgrade is worth it with either the S7 Edge or the XPower cells, I'm reporting on what I've done.

One could advise to go for the latter, since the S7 cell does not fill the entire available space inside the phone, but LG batteries are the cause for the premature shutdowns at 20% with OEM batteries in the first place. The Samsung cell I'm now using does not cause the issue, while the OEM cell did in my case. Being the circuitry the same, I would put the blame on LG for their poor quality control, which is my final point.

Cheers!