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.
Battery Replacement Instructions for Premium Feature HRM Straps | Garmin Customer Support
Replacing the Battery on the Garmin Premium Heart Rate Monitor | Garmin Customer Support
REVIEW: Garmin HRM-PRO | Heart rate belt for running | Read the review - Inspiration