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.
How long do HRM-Tri batteries last? – Chris Worfolk's Blog
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Battery Replacement Instructions for Premium Feature HRM Straps | Garmin Customer Support