About this replacement
FitBit Surge — 3.7V Li-Polymer Replacement Battery (LSSP491524AE)
This is a 3.7V, 100mAh Li-Polymer cell for the FitBit Surge smartwatch. It replaces part number LSSP491524AE and fits directly into the Surge's internal battery bay. If your Surge no longer tracks a full day before dying, this cell is the likely fix.
- Surge platform fitment: The Surge uses a compact 24.00 x 15.50 x 4.20mm pouch cell on a 3.7V rail. This cell matches those dimensions and the connector orientation the Surge mainboard expects. The BMS handshake on the Surge checks cell impedance at startup — a mismatched cell triggers immediate shutdown.
- Bench tested on actual hardware: We ran this cell through full charge and discharge cycles on the Surge platform. The fuel gauge IC accepted the new cell without error flags, and the BMS did not trip during GPS lock or continuous heart rate polling — both are high-draw states for this watch.
- Post-swap charging protocol: After fitting this cell, place the Surge on its magnetic charger immediately and charge to 100% before first use. The fuel gauge IC on this platform loses its reference point when the original cell is removed. It cannot recalibrate without a full charge cycle from near-zero after a cell swap.
Why the Surge shuts down mid-GPS session after a new cell install
GPS on the Surge pulls significantly more current than standard step tracking. A new cell that has not completed its first full charge cycle has higher internal impedance than a settled cell. Under the GPS load spike, voltage sags enough to cross the BMS low-voltage cutoff threshold, triggering a shutdown even when the display shows charge remaining. One complete charge-to-100% cycle before use drops that impedance and stabilises the voltage under load.
Surge showing 0% and refusing to power on after battery swap
This is a fuel gauge IC reset issue, not a dead cell. When power is cut during disassembly, the MAX17048 or equivalent gauge IC on the Surge mainboard loses its state-of-charge reference entirely. It reads the new cell as empty and the watch refuses to boot from what it interprets as a fully depleted state. Place the watch on the charger for at least 15 minutes before attempting to power on. Once the gauge IC detects charging current above its wake threshold — typically around 3.0V cell voltage — it reinitialises and the watch boots normally.