Garmin Fenix 1 Replacement Battery 361-00061-00 3.7V 300mAh

Garmin Fenix 1 / Fenix 2 — 3.7V Li-ion Replacement Battery (361-00061-00)

This is a 3.7V, 300mAh Li-ion cell replacing OEM part 361-00061-00. It fits the Garmin Fenix 1 and Fenix 2 multisport GPS watches, as well as the PD3555w. Dimensions are 34.10 × 34.00 × 4.10mm — measure your original cell before fitting if you are unsure.

• Fenix 1, Fenix 2, and PD3555w compatibility: All three models share the same cell footprint, connector pinout, and 3.7V nominal voltage rail. The BMS on each platform reads the same charge termination signal, so no firmware change or adapter is needed when swapping cells across these devices.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on Fenix hardware. The BMS accepted the cell without fault codes, held charge termination at 4.2V, and cut off cleanly at the low-voltage threshold without triggering a protection trip on startup.
• Post-swap charge requirement on Fenix platforms: After fitting the new cell, place the watch on its magnetic charger immediately and bring it to 100% before first use. The fuel gauge IC on the Fenix platform loses its reference point when the cell is disconnected. Without a full charge cycle from near-zero, the percentage readout will be inaccurate for days.

Why Fenix GPS and continuous heart rate monitoring drain a new cell faster in the first week

A new Li-ion cell has slightly higher internal impedance until it completes three to five full charge cycles. During that break-in period, the Fenix GPS receiver and optical HR sensor draw against a cell that cannot yet deliver peak current as efficiently as a fully conditioned one. This causes the fuel gauge IC to report faster drain than you will see long-term. By the end of the first week of normal use, current delivery stabilises and the percentage drop per hour will level out noticeably.

Fenix not pairing with phone after battery replacement

Removing the cell clears the active Bluetooth session and can cause the watch to drop its paired device record. The phone retains the old pairing entry, but the watch no longer recognises it on reconnection. Go into the Garmin Connect app, remove the Fenix from paired devices, then re-initiate pairing from the watch's Bluetooth settings menu. Complete the pairing handshake with both devices within 30 seconds of the watch broadcasting.