About this replacement
Garmin Forerunner 920XT — 3.7V Li-Polymer Replacement Battery (361-00078-00)
This 3.7V, 300mAh Li-Polymer cell is a direct replacement for the battery inside the Garmin Forerunner 920XT GPS multisport watch. It fits the 920XT specifically — not the 920, not the 910XT. The OEM part number is 361-00078-00, and the cell measures 35.90 × 26.45 × 4.00mm.
- Forerunner 920XT fit: The 920XT uses a slim Li-Polymer pouch cell with a three-pin flex connector that carries both power and fuel gauge data. The BMS on this watch reads cell impedance on first charge — fit the wrong cell geometry and the connector won't seat flush, which causes intermittent contact during movement.
- Bench tested on actual hardware: We fitted this cell to a 920XT unit and ran a full charge cycle. The fuel gauge IC registered the new cell correctly after one complete charge-to-discharge pass. BMS cutoff triggered cleanly at the low-voltage threshold without false shutdowns.
- 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 920XT loses its reference point when the cell is disconnected — it cannot recalibrate until it sees a full charge cycle starting from near-zero.
Why the 920XT fuel gauge reads wrong after a cell swap
The Forerunner 920XT uses a coulomb-counting fuel gauge IC that tracks charge by measuring current in and out of the cell over time. When you remove the old battery, that accumulated reference data is gone. The IC restarts with no history, so it estimates state-of-charge from open-circuit voltage alone — which is unreliable on a fresh cell. One complete charge cycle from near-zero to 100% gives the IC enough data to rebuild its model and report percentage accurately.
Watch not pairing with phone after battery replacement
Removing the battery cuts power to the BLE module mid-session, which drops the encrypted pairing bond stored in volatile memory. The phone still shows the watch as paired, but the watch no longer holds a matching key — so connection attempts silently fail. The fix is to forget the device on your phone, then re-pair from scratch through the Garmin Connect app. Do not attempt to re-pair until the watch has completed its first full charge cycle post-swap.