Steps to Replace the Battery on an HRM- Run, HRM-Swim, or HRM-Tri Strap
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.
Garmin HRM-Tri Heart Rate Monitor
Amazon.com : Garmin HRM-Dual Heart Rate Monitor - Premium & Real-Time Heart Rate Data via ANT & Bluetooth - Bundle with CR2032 Battery : Sports & Outdoors
Garmin HRM-DUAL Heart Rate Strap In-Depth Review | DC Rainmaker
Battery Replacement Instructions for Premium Feature HRM Straps | Garmin Customer Support