About this replacement
Fossil Sport / Gen 4 / Gen 5 Smartwatch — 3.8V Li-Polymer Replacement Battery (APP00221)
This 3.8V, 260mAh lithium-polymer cell replaces part number APP00221 in the Fossil Sport, Gen 4, and Gen 5 smartwatches. The original cell degrades over charge cycles, causing shorter battery life and eventual failure to hold charge. Swapping this cell restores full watch operation across fitness tracking, notifications, and timekeeping.
- Sport, Gen 4, and Gen 5 compatibility: All three models use the same 29.10 x 23.40 x 4.00mm cell footprint, the same 3.8V nominal rail, and share the APP00221 part number — the connector and BMS handshake are identical across the lineup.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on the Gen 5 platform. The BMS accepted the cell without fault codes, and the fuel gauge IC registered voltage correctly after a full charge cycle from near-zero.
- Post-swap charge requirement: After fitting this cell, place the watch on its magnetic charger immediately and charge to 100% before use. The fuel gauge IC in these watches loses its reference point during a cell swap — it cannot recalibrate unless it sees a full charge cycle starting from near-zero.
Watch shows 0% immediately after battery swap
The fuel gauge IC on Fossil smartwatches tracks state-of-charge by logging cumulative charge and discharge from a known reference point. When the cell is removed, that reference is lost entirely. The IC has no way to estimate where the new cell sits in its charge curve until it runs a full cycle. Place the watch on charge straight away and let it reach 100% — the gauge resets its reference at that point and will read accurately from then on.
Battery draining in one day after a new cell is fitted
Always-on display mode combined with continuous heart rate monitoring can pull significantly more current than the standard polling interval. On the Gen 5 platform, continuous HR monitoring alone keeps the optical sensor active at a constant draw rather than sampling in short bursts. Check the Fossil Wear OS app and confirm HR mode is set to automatic rather than continuous. If drain persists after that change, check whether always-on display was re-enabled during the pairing process after the swap.