On the Fairphone batteries there is a small line with IPC. On the different FP2 versions there are different sizes mentioned, though both have the same size. Don’t trust this numbers totally. As far as I know that means:
Version FP2-BAT01: 1icp6/44/62
Version FP2BAT02: 1ICP6/42/61
Measured; 65,5mm x 45mm x 6mm
1: It is one cell, 3,6V nominal voltage; some batteries for cameras have 2 cells in line, 7,2V
ICP: Integrated Circuit Protection, the integrated electronics protects the cell(s) from overcharging (I guess, does anyone know for sure?)
6/44/62: the size of the 3 dimensions in mm
Even the fairbuds have the line with IPC on the battery of the charging station.
You can try to search for similar batteries with e.g. “battery ipc x/y/z” on different shops.
Maybe FP wrote different sizes on the FP2 batteries since the electronics and package (the sticker with the written details) are added and once they used the size of the naked cell.
For the FP2 I found a nearly similar battery with a different connector:
A battery with different polarity:
This is a followup to the FP meeting in Hamburg: 🇩🇪 18. April | MeetFairbuds 🎧 Treffen in Hamburg für alle Fairphoner*innen - #722 by nudge