During your workout the HRM Blue will remember and search for the smart phone to which it is paired.
If you are not using one of your paired sensors during a workout, HRM Blue will use a small amount of battery life as it tries to connect to your smart phone.
If you do not want that we recommend after your workout that you unsnap at least one side of the sensor from the chest strap. The HRM Blue turns off within max. 5 minutes after removing the chest strap. Depending on your work out ratio the battery life of the HRM Blue should last up to more than one year.