Garmin Fenix 2 STD-810G Replacement Battery 3.87V 340mAh

Garmin Fenix 2 / Instinct Solar — 3.87V Li-Polymer Replacement Battery (361-00164-10)

This 3.87V, 340mAh lithium-polymer cell replaces the original battery in the Garmin Fenix 2 smartwatch (STD-810G) and Instinct Solar. It matches the OEM footprint at 36.80 × 24.70 × 3.00mm, so it seats correctly against the watch's connector without modification. Voltage and capacity are drawn from the product data, not estimated.

• Fenix 2 and Instinct Solar compatibility: Both watches share OEM part numbers 361-00164-10, 361-00164-00, and 361-00136-10. The connector pinout and cell dimensions are identical across this group, so one cell covers both platforms without adapter work.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a Fenix 2 unit. The BMS accepted the new cell without fault codes, and the fuel gauge responded correctly once a full charge cycle completed from near-zero.
• First charge after cell swap: Place the watch on its magnetic charger immediately after fitting this cell and charge it fully to 100% before use. The fuel gauge IC on these Garmin platforms loses its reference when the cell is removed — it cannot recalibrate without a complete charge cycle from near-zero.

GPS and continuous heart rate monitoring load on a fresh 340mAh cell

The Fenix 2 runs GPS, barometric altimeter, and optical heart rate simultaneously during activity tracking. That combined draw is substantial relative to a 340mAh cell. A new cell at full impedance will show a steeper apparent voltage drop during the first few charge-discharge cycles than the worn cell it replaced — this is normal cell settling, not a capacity defect. After three to five full cycles, the voltage curve stabilises and the fuel gauge reads accurately.

Watch not pairing with phone after battery replacement

Removing the cell cuts power to the BLE stack entirely, which drops the active pairing session stored in volatile memory. The phone's Bluetooth settings still show the watch as a saved device, but the handshake no longer completes. Go into your phone's Bluetooth settings, forget the Garmin device, then re-pair through the Garmin Connect app from scratch. The watch needs to be powered on and showing the pairing screen before the app scan will find it.