Garmin Forerunner 205 Replacement Battery 3.7V 700mAh

Garmin Forerunner 205/305 Series — 3.7V Li-ion Replacement Battery (361-00026-00)

This is a 3.7V, 700mAh Li-ion cell for the Garmin Forerunner 205, Forerunner 305, and Forerunner 305i GPS running watches. It replaces OEM part number 361-00026-00. Fit this battery when the original cell no longer holds a charge or causes unexpected shutoffs during a run.

• Forerunner 205, 305, and 305i compatibility: All three watches share the same battery bay dimensions, connector pinout, and BMS voltage thresholds — 38.90 × 34.25 × 5.50mm with a matching 3.7V charging curve. One cell covers the full 200/300-series lineup.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on the Forerunner platform. The BMS accepted the cell without fault flags, charge termination triggered correctly at the top of the 3.7V nominal curve, and low-voltage cutoff engaged cleanly before cell damage threshold.
• Cold-start satellite acquisition after swap: After fitting this cell, power the watch on outdoors and let it acquire a full satellite fix before your first run. A full power interruption clears the GPS almanac cache, so the receiver runs a cold start — allow 5–10 minutes for that first fix. Subsequent warm starts take under a minute.

Why the Forerunner 205/305 shuts off without warning at low battery

The Forerunner's battery indicator is calibrated against the original cell's discharge curve. After years of cycle aging, or after fitting a new cell, the indicator and the actual voltage threshold can fall out of sync. The watch may show one or two bars remaining, then cut power when the cell hits the hardware low-voltage cutoff — typically around 3.0V. To recalibrate the indicator, run the watch through two full charge and discharge cycles after fitting the new cell. The displayed level will track actual capacity more accurately from the third cycle onward.

GPS accuracy dropping as the Forerunner battery runs low

The Forerunner 205/305 GPS receiver draws consistently while a satellite fix is active, but the watch firmware reduces receiver sensitivity at low supply voltage to protect the cell from hard cutoff. That sensitivity reduction narrows the number of satellites the receiver can track, which degrades positional accuracy — pace and distance data become less consistent. This is not a fault with the replacement cell; it is firmware behaviour triggered below roughly 3.2V. Keeping the cell charged above that threshold, especially on long runs, prevents the sensitivity reduction from activating.