FitBit Luxe LSS271120P Replacement Battery 3.87V 50mAh

FitBit Luxe — 3.87V Li-Polymer Replacement Battery (LSS271120P)

This is a 3.87V, 50mAh lithium-polymer cell for the FitBit Luxe smartwatch. It replaces part number LSS271120P and restores power to the display, heart rate sensor, and activity tracking hardware. Fit the new cell when the original no longer holds a charge or the watch shuts down before reaching 0%.

• Luxe platform fitment: The Luxe uses a compact 22.00 × 10.00 × 2.50mm cell with a specific flex connector and pressure-sensitive adhesive mount. The BMS on this platform communicates directly with the fuel gauge IC — a cell outside these physical tolerances will not seat correctly and may trigger a protection fault.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the Luxe platform, confirmed the BMS handshake completed without fault codes, and verified the fuel gauge reported accurate state-of-charge after a full conditioning cycle.
• Post-swap charge protocol for the Luxe fuel gauge: After fitting this cell, place the Luxe on its magnetic charger immediately and run it to 100% before first use. The fuel gauge IC on the Luxe loses its reference point when the cell is removed — skipping this step causes the watch to report incorrect percentages or shut down early.

Why the Luxe shuts down before hitting 0% after a cell swap

The Luxe uses a coulomb-counting fuel gauge IC that tracks charge by accumulating current flow over time. When you remove the old cell, that counter resets and the IC has no reference for the new cell's actual capacity. Until a full charge cycle recalibrates it, the IC can miscalculate the cutoff threshold and trigger a low-battery shutdown at 15–20% displayed charge. One full charge to 4.35V from near-zero after the swap corrects this. After that cycle, the reported percentage and actual cutoff voltage align.

Luxe not pairing to phone after battery replacement

Removing the cell cuts power to the BLE stack entirely, which wipes the active pairing session stored in volatile memory. The Luxe does not automatically re-establish this link on power-up — it waits for a fresh pairing handshake. On your phone, go to Bluetooth settings, forget the Luxe, then open the Fitbit app and add the device as new. The watch must be charged to at least 10% before the BLE radio will initialise and show up in the device scan.