Garmin Fenix 3 - Garmin: Replacing the Battery in your Heart Rate Monitor ( HRM) The CR2032 batteries in your HRM-Run, HRM-Swim or HRM-Tri straps will last approximately 18 months, depending on use.
Replacing the Battery on the Garmin Premium Heart Rate Monitor | Garmin Customer Support
Forerunner 920XT - Replacing the HRM-Swim Battery and the HRM-Tri Battery
Replacement for Garmin forerunner 35 Smart Sports Watch Back Cover Battery Case with Optical Heart Rate Smart Watch Repair Parts|Smart Accessories| - AliExpress