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http://www.heartratecheckup.com/polar-heart-rate-monitor-battery.html...
Polar W.I.N.D Sensor Battery Replacement...
Batteries in electronic devices are getting smaller and smaller. Are you ready for a battery that's the size of a grain of sand?...
A ‘sand battery’ in Finland can store renewable energy for months at a time....
Changing battery to Polar H7 heart rate sensor...