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-Dual™ | Heart Rate Monitor with Chest Strap
Garmin Vivofit 4, First Take: Great battery life, but no heart-rate monitoring | ZDNet
Garmin HRM-Dual battery replacement – Chris Worfolk's Blog