Check my comment on the teardown… Fitbit Flex Teardown I think you can find a suitable 031012 battery if you look around & are willing to try dismantling it (which may destroy it).

The figures printed on the battery the dimensions as far as I can tell. The second line probably indicates capacity/ charging rates?…

031012

D420[can’t read it]

3mm x 10mm x 12mm

Possibly 4v, 20mA? I think it is 3.7v so I wonder why it has the ‘D4’ unless that is markings specifically for Fitbit.

There are similar looking batteries appearing on Ebay etc and a few data sheets around…

http://www.all-battery.com/datasheet/305... (the first page lists the 031012 number, also 4.2v is the charge voltage listed, so maybe that explains the ‘D42’).

https://www.ebay.com/itm/5-pcs-3-7V-30-m...

Other ‘headset’ batteries use a similar naming scheme, e.g. 041430 = 4 x 14 x 30mm

That seems like a viable solution to replace the battery. I’ll post back if I get around to doing it with any success.