Fossil APP00276 Q Marshal Gen 4 Smartwatch Replacement Battery 3.8V 330mAh

Fossil Q Marshal Gen 4 — 3.8V Li-Polymer Replacement Battery (APP00276)

This 3.8V, 330mAh Li-Polymer cell is a direct swap for the original APP00276 battery in the Fossil Q Marshal Gen 4 smartwatch. It fits the 29.60 x 24.00 x 3.80mm battery bay and matches the original connector pinout. Replace this when the watch can no longer hold a charge through a full day of normal use.

• Q Marshal Gen 4 fitment: The Gen 4 platform uses a BMS handshake tied to this cell's impedance profile. Swapping to a cell of the correct voltage and capacity keeps the fuel gauge IC reading accurately and prevents false low-battery shutdowns.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the Q Marshal Gen 4 board. The BMS accepted the new cell without error flags, and the fuel gauge recalibrated correctly after a full charge from near-zero.
• Post-swap charge requirement: After fitting this cell, place the watch on its magnetic charger immediately and charge to 100% before first use. The fuel gauge IC on the Q Marshal Gen 4 loses its reference point during a cell swap and cannot recalibrate without a full charge cycle starting from near-zero.

Why the Q Marshal Gen 4 shows 0% immediately after a battery swap

The Q Marshal Gen 4 uses a coulomb-counting fuel gauge IC that tracks charge state relative to a known baseline. When you remove the old cell, that baseline is wiped. The watch reports 0% — or refuses to boot — because the IC has no reference point, not because the new cell is flat. Fit the battery, connect the charger, and let it reach 100% without interruption. The IC re-establishes its reference at full charge, and the percentage display returns to normal from that point forward.

Battery draining in one day instead of multi-day on the Q Marshal Gen 4

Always-on display mode combined with continuous heart rate monitoring pulls a sustained current that a 330mAh cell was not sized to sustain for multiple days. Check Wear OS settings and confirm that always-on display and continuous HR tracking are both turned off — tilt-to-wake and on-demand HR extend charge intervals significantly. If drain remains heavy after those changes, check that no background app is holding a persistent BLE connection. Under typical notification and step-tracking use, charge level should remain above 20% at end of day.