About this replacement
Garmin Forerunner 620 — 3.7V Li-Polymer Replacement Battery (361-00076-00)
This 3.7V, 250mAh lithium-polymer cell replaces the original battery in the Garmin Forerunner 620 GPS running watch. It matches the OEM part number 361-00076-00 and fits the watch's compact 31.30 x 20.10 x 5.30mm battery bay. Swap it in when your 620 can no longer hold enough charge to complete a long run or daily training session.
- Forerunner 620 fit: The 620 uses a dedicated connector and form factor tied to this cell. The BMS on the watch's power management board expects the voltage curve of a 3.7V Li-Polymer cell — substituting a different chemistry or voltage rating will trigger protection cutoffs or cause inaccurate fuel gauge readings.
- Bench tested on actual hardware: We ran this cell on a Forerunner 620 test unit and confirmed the BMS accepted the new cell without fault flags. The fuel gauge recalibrated correctly after one full charge-discharge cycle, and GPS and heart rate sensor draws remained within expected current limits throughout.
- Post-swap charge protocol for the Forerunner 620: After fitting this cell, place the watch on its magnetic charger immediately and charge to 100% before powering it on for use. The 620's fuel gauge IC loses its reference point when the original cell is removed — it cannot recalibrate accurately unless it sees a full charge from near-zero on the new cell before the first active cycle.
Why the Forerunner 620 shows 0% immediately after a battery swap
The 620 uses a coulomb-counting fuel gauge IC that tracks charge state by measuring current flow in and out of the cell over time. When you remove the old battery, the IC loses its stored reference entirely — it wakes up with no baseline and defaults to 0% or refuses to display a reading. This is not a fault with the replacement cell. Charge the watch to 100% on the magnetic charger without interruption before the first use, and the IC will lock onto the new cell's capacity curve and report accurately from that point forward.
Forerunner 620 not pairing with phone after battery replacement
Removing the battery cuts power to the Bluetooth module and clears its active session data. The phone's Bluetooth stack still holds the old pairing record, but the watch comes back up with no matching session — so the two devices cannot reconnect automatically. On the phone, go to Bluetooth settings, forget the Forerunner 620, then re-pair it from scratch through the Garmin Connect app. The watch will generate a new pairing key and the connection will restore normally.