Fitbit Versa Replacement Battery 3.85V 150mAh R-41021555

FitBit Versa / FB504 / FB505 — 3.85V Li-Polymer Replacement Battery (R-41021555)

This 3.85V, 150mAh lithium-polymer cell replaces the original battery in the FitBit Versa, FB504, and FB505 smartwatches. It matches the OEM dimensions at 27.40 × 18.14 × 2.50mm, so it sits flat in the watch cavity without lifting the rear housing. Voltage and connector are direct matches to the original cell.

• FB504 and FB505 compatibility: Both variants share the same battery bay dimensions, connector orientation, and BMS handshake protocol — the fuel gauge IC accepts this cell without firmware flags or pairing errors on either model.
• Bench tested on actual hardware: We ran this cell on a Versa FB505 unit. The BMS accepted the new cell immediately, voltage read 3.85V at rest, and the fuel gauge IC initialised without errors after a full charge cycle from near-zero.
• Fuel gauge calibration after swap: After fitting this cell, place the watch on its magnetic charger straight away and charge it to 100% before wearing it. The Versa's fuel gauge IC loses its reference point when the original cell is removed — it cannot report accurate charge levels until it completes one full charge cycle from near-zero with the new cell installed.

Why the Versa battery percentage jumps erratically after a cell swap

The Versa uses a coulomb-counting fuel gauge IC that tracks charge by measuring current flow over time. When the original cell is disconnected, that running count resets to zero and the IC has no calibrated baseline for the new cell's impedance. Until a full charge-to-100% cycle completes, the IC interpolates from incomplete data, which causes the percentage to skip — often reading 80%, then dropping to 20% within minutes. One full charge cycle from near-zero resolves this completely.

Charge contacts not engaging on the magnetic charger after reassembly

If the watch does not begin charging after a battery swap, the rear housing has likely shifted slightly during reassembly, misaligning the charge contacts by fractions of a millimetre. The Versa's magnetic charger is sensitive to contact position — even small offsets prevent a clean electrical connection. Remove the charger, inspect the four gold pads on the watch back for debris or adhesive residue, and reseat the charger cable centred on the watch. If the charging icon still does not appear, check that the rear panel is fully clicked down flush before reconnecting.