Omron HR-310 Instruction Manual page 25

The chest belt uses a CR2032 3V battery, which has already been
installed. If the heart rate signals become unsteady or stop your chest
belt battery may need replacing.
To replace the chest belt battery:
1. Using a coin, twist the battery door counter-clockwise until it
clicks out of place.
2. Replace the old battery and close the battery door by twisting it
clockwise until it is fi rmly in place.
BATTERIES
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