Garmin Fenix 3 Replacement Battery 3.7V 300mAh 361-00034-02

Garmin Fenix 3 / Fenix 3 HR — 3.7V Li-Polymer Replacement Battery (361-00034-02)

This is a 3.7V, 300mAh lithium-polymer cell replacing OEM part 361-00034-02 in the Garmin Fenix 3 and Fenix 3 HR smartwatches. It matches the original cell's footprint at 32.10 × 24.52 × 4.88mm, so it sits flush under the watch module without modification. Capacity is 300mAh — identical to stock.

• Fenix 3 and Fenix 3 HR compatibility: Both models use the same 3.7V cell on the same power rail with the same connector orientation. The HR variant adds an optical heart rate sensor, which draws additional current, but the cell specification is unchanged between the two.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the Fenix 3 platform. The BMS accepted the cell without a fault flag, and the fuel gauge re-initialised 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 first use. The fuel gauge IC on the Fenix 3 loses its reference point during cell removal — it cannot calibrate the state-of-charge reading without a complete charge cycle from near-zero.

Why the Fenix 3 reports 0% and shuts down after a battery swap

The Fenix 3 uses a fuel gauge IC that tracks battery state by monitoring cumulative charge and discharge over time. When the cell is disconnected, that reference is lost entirely. The IC wakes up with no data and defaults to 0%, which triggers an immediate low-battery shutdown on some units before the user can even navigate the menu. This is not a fault with the replacement cell. Charging to 100% before first use gives the IC a fixed upper reference point to work from.

Charge contacts not seating on the magnetic charger after reassembly

If the watch charges intermittently or not at all after a battery swap, the charge contact pads on the case back have likely shifted during reassembly. The Fenix 3 magnetic charger aligns to four contact points — even a 1mm offset prevents a clean connection. Remove the charger, inspect the case back contacts for debris or misalignment, and re-seat the watch on the charger at a direct 90-degree angle. Confirm charging has started by checking for the charging indicator within 10 seconds of contact.