I bought og "button cell battery" for self-installation in my POLAR transmitter and am super happy with the quality of the product, as it has a very long lifespan. Since I often need the POLAR transmitter for heart rate measurement, this is a very beneficial business! Somal, it is also very important for the proper functioning of the "transmitter". I would Duracell CR2025 each / r athletes / in which operates as much "heart rate training" would highly recommend!