The Polar F11 Fitness heart monitor has a strap with electrodes that you fasten to your chest. It detects your pulse rate and transmits the information wirelessly to a wrist unit so you can keep track of your vitals while you do cardiovascular exercise. Polar estimates that the average battery lifespan of the transmitter is two years, when used one hour per day, seven days a week. If the transmitter stops working, it might be time to change the battery.
Insert a coin into the slot on the chest strap transmitter connector's battery cover, and turn the cover counter-clockwise from the "Close" position to the "Open" position.
Remove the battery cover from the connector.
Lift out the battery.
Insert the new battery, with the negative side of the battery facing down against the bottom of the container, and the positive side facing up against the cover.
Remove the sealing ring of the battery cover, and replace it with a new sealing ring.
Place the battery cover back on, with the arrow pointing to the "Open" position.
Press the cover until its exterior surface is at the same level as the connector.
Insert a coin into the slot on the chest strap transmitter connector's battery cover, and turn the cover clockwise from the "Open" position to the "Close" position.
You can obtain a new sealing ring for the battery cover at an authorised Polar Service Center (see Resources).