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