Garmin Forerunner 405 Replacement Battery 3.7V 280mAh

Garmin Forerunner 405 / 410 Series — 3.7V Li-ion Replacement Battery (361-00034-00)

This 3.7V, 280mAh Li-ion cell replaces OEM part 361-00034-00 in the Garmin Forerunner 405, 405CX, 410, and 410CX GPS running watches. The original cell degrades after 300–500 charge cycles, causing the watch to shut off mid-run or refuse to hold a charge overnight. Dimensions are 33.30 × 30.20 × 5.00mm — match these before ordering if you have a modified unit.

• Forerunner 405 and 410 platform fit: All four models — 405, 405CX, 410, and 410CX — share the same battery bay dimensions, connector, and BMS handshake protocol. Garmin did not change the cell spec between the CX and non-CX variants, so one part number covers the full run.
• Bench tested on actual hardware: We ran this cell in a Forerunner 410 unit. The BMS accepted the new cell without fault flags, GPS lock initialised normally, and the fuel gauge IC began tracking state-of-charge within the first full charge cycle.
• Post-swap charge protocol for this platform: After fitting the new cell, place the watch on its magnetic charger immediately and charge to 100% before first use. The fuel gauge IC on this platform loses its calibration reference when the cell is swapped and cannot reset accurately without one complete charge from near-zero.

Why the Forerunner 405 reports 0% immediately after a cell swap

The 405 uses a coulomb-counting fuel gauge IC that stores its reference state in non-volatile memory tied to the original cell's charge history. When you remove the old cell, that reference is wiped. The IC wakes up with no baseline, defaults to 0%, and may trigger a low-battery shutdown before the watch fully boots. This is not a fault with the replacement cell. Charge the watch to 100% on the magnetic charger straight after reassembly — the IC re-establishes its reference at the top of the charge curve and the percentage display normalises from that point.

Magnetic charge contacts not engaging after reassembly

If the watch sits on the charger but the charging indicator does not appear, the back plate has shifted slightly during reassembly and the four gold pogo contacts on the charger are no longer centred on the watch's pads. Remove the watch from the charger, check that the rear cover is fully seated and flush with the case, then reposition on the charger. The charger alignment is sensitive to even 1–2mm of lateral shift. Once the indicator light activates, confirm with a voltage reading of 4.2V at full charge before wearing the watch.