Garmin HRM 600
It lasts up to 2 months at 1 h a day and stores swim heart rate to forward later. It charges through Garmin's own plug.
What the maker states
| Battery | 2 months, Rechargeable, using 1 hour per day |
|---|---|
| Weight | 61 g, With M-XL strap; module only 18 g |
| Water rating | 5 ATM |
| Heart rate sensor | Chest strap, heart rate and HRV |
| External sensors | ANT+, Bluetooth (3 connections) |
| App | Garmin Connect |
| Stored sessions | Stores heart rate for swims and stand-alone use, forwards later |