Any LI-Battery replacement tips/experiences for Fairphone TWS Earbuds?

I have noticed, that after a bit more than a year of usage the battery capacity of my Fairphone TWS earbuds has rather suddenly dropped significantly. I’m not talking like just 20 or 30 percent less - just earlier today I tried to watch a 25 minute video and one of the earpieces didn’t even make it to the end of it.

Since I don’t really want to throw them away or replace them, I’m hence looking to replace the batteries in the earbuds. I am well aware that this is not intended by Fairphone, but judging by the iFixit teardown it should be doable with some soldering despite the abysimal score they recieved.

The battery that will be needed for this is a 3.7V Lithium-Ion-Battery with a capacity of at least 50mAh. The dimensioms should be somewhere in the range of 9x10x5mm in order to fit in the housing.

I’ve browsed the forum and it seems noone has reported any experiences with earbud-battery-replacement so far. So here’s what I’m currently looking for:

  • If anyone has attempted this before any advice would be greatly appreciated. Any experiences? What battery dimension and capacity did you end up using?
  • So far the smallest 50mAh battery I could find on eBay has around 16x9x4mm, but it is currently out-of-stock, includes a battery management chip and costs around 7€. Since the BMS should be included in the earbud already, I suppose the LI cell alone should do - in that case the linked battery would probably fit once the chip is removed. I found another supplier that sells appropriate batteries for around 0.75€ a piece, but only ships to Egypt. Any ideas where I could find something like that in Germany or Austria (or at least the European Union)?
  • Any other technical advice what to be careful about when installing a new battery.

If that doesn’t work out, maybe 3D printing a custom case and choosing a larger battery will be the way to go.

What I'm not looking for (click to expand)
  • Advice to solve the problem by cleaning my earbuds.
  • Links to other threads complaining about repairability issues or battery problems with the earbuds. - I’m looking to repair them - not complain. This also includes this thread, this thread, this thread, this thread and this thread - I’ve checked those out already.
  • Referrals to the Fairphone Support - I’m looking to repair them, not replace them.
  • Notes that I shouldn’t do that since repairing the earbuds is not officially supported.

If successfull, I’ll be happy to report where this attempt at replacing the batteries lead me in the end.