About this replacement
FitBit Inspire 3 — 3.87V Li-Polymer Replacement Battery (FB424)
This 3.87V, 75mAh Li-Polymer cell replaces the original FB424 battery inside the FitBit Inspire 3 smartwatch. It fits the full Inspire 3 lineup — the watch that tracks daily activity, heart rate, sleep stages, and workout metrics. Swap this cell when the original no longer holds a charge or the watch dies before the end of the day.
- Inspire 3 compatibility: The Inspire 3 uses a specific 19.30 x 11.80 x 3.40mm cell footprint with a fixed connector orientation. This cell matches that footprint exactly — voltage rail, physical dimensions, and connector pitch all align with the FB424 spec.
- Bench tested on actual hardware: We ran this cell on an Inspire 3 unit. The BMS accepted the new cell without fault flags, heart rate and SpO2 sensors initialised normally, and the fuel gauge IC began calibrating correctly after a full charge cycle from near-zero.
- Fuel gauge reset after swap: After fitting this cell, place the watch on its magnetic charger immediately and charge to 100% before use. The fuel gauge IC on the Inspire 3 cannot calibrate its reference point without one full charge cycle from near-zero following a cell swap.
Why the Inspire 3 shows 0% immediately after a battery swap
The Inspire 3 uses a coulomb-counting fuel gauge IC to report battery percentage. When the original cell is removed, that IC loses its stored reference data entirely. The new cell has no charge history, so the gauge defaults to 0% or an undefined state. Plug the watch into its charger straight after reassembly and allow it to charge to 100% — one full cycle resets the IC and restores accurate percentage readings.
Charge contacts not making on the magnetic charger after reassembly
If the watch sits on the charger but shows no charging animation, the rear charge contacts have likely shifted during back-panel reassembly. Even a 0.5mm misalignment breaks the magnetic contact. Remove the watch from the charger, inspect the rear contacts for debris or adhesive residue, re-seat the back panel flush, then confirm the charger snaps into place magnetically with even contact across all four pads.