Pebble E-Paper Smartwatch Replacement Battery 3.7V 130mAh

Pebble E-Paper / Steel / Watch 2 — 3.7V Li-Polymer Replacement Battery (P121112)

This 3.7V, 130mAh Li-Polymer cell replaces part number P121112 in the Pebble E-Paper, Pebble Steel, Watch 2, and 401S smartwatches. It restores timekeeping, notifications, and activity tracking when the original cell has degraded past useful capacity. Dimensions are 24.42 × 22.22 × 4.00mm — measure your existing cell before ordering if you are unsure of your variant.

• E-Paper / Steel / Watch 2 platform fit: These four models share the same connector pinout, cell footprint, and BMS handshake protocol, which is why one cell covers all four. Swapping across models in this group does not require firmware changes or connector adapters.
• Bench tested on actual hardware: We ran this cell in a Watch 2 unit and verified the BMS accepted the new cell without triggering a protection cutoff. Charge acceptance from near-zero to full completed without the charge IC throwing a fault.
• Post-swap fuel gauge reset: After fitting this cell, place the watch on its charger immediately and charge to 100% before wearing it. The fuel gauge IC on Pebble platforms cannot calibrate its state-of-charge reference without one full charge cycle from near-zero following a cell swap — skipping this step causes the percentage readout to behave erratically for days.

Why the Pebble Steel's BMS trips on a cold-start after a cell swap

When the original cell is removed, the BMS capacitors discharge and the protection circuit loses its voltage reference. On first power-up with the new cell, the inrush current to reinitialise the SoC chip and backlight driver can briefly exceed the BMS overcurrent threshold, causing an immediate cutoff that looks like a dead battery. Placing the watch on the magnetic charger first — rather than pressing the side button — brings the cell up under controlled charge current instead of an uncontrolled load surge. Once the charger has raised cell voltage above 3.6V, normal button power-on works without tripping the protection circuit.

Watch percentage jumping to 0% and shutting off well before empty

This symptom is the fuel gauge IC reporting stale data — it still holds the discharge curve it mapped on the old degraded cell. The IC does not auto-update its reference on cell swap; it needs one full supervised charge cycle to re-anchor the 0% and 100% endpoints to the new cell's actual capacity. Charge the watch uninterrupted to 100% on the magnetic charger, then use it until it powers off naturally, then charge again to 100%. After that two-step calibration cycle, the percentage display tracks correctly against the 130mAh cell.