1. Which cell each one takes
Most chest straps take a CR2032. The Polar H10 and H9 use a CR2025, and bike computers and speed sensors often take a CR2032 or a CR2450. The code is printed inside the strap.
2. A strap with no signal
If the receiver finds no heart rate, the strap is the first thing to check. A flat cell and a battery fitted the wrong way round look exactly the same from the watch.
3. Moisture causes the real failures
Sweat gets into the strap and corrodes the contacts. If a fresh battery doesn’t work, wipe the contacts with a dry cloth and check that the cover is sealed properly.
4. The sensor needs a little moisture
A chest strap reads better with a film of sweat, but not with a flooded contact. Wipe the inside of the strap clean and let it dry before you fit a new cell.
5. Keep other fingers off the electrodes
A bare finger across the contacts is enough to stop the reading. Make sure the strap sits against your chest and that nothing else is touching the electrodes.