send it back to Polar,it is too new for you to ridden it to kill the battery
http://www.polarusa.com/service_repair/serviceprocess_con.asp
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Somewhere it must be happening, as you suggest. I would agree with the cad sensor, only because if it were the Speed Sensor you would have more miles on the watch. The cad sensor transmits 2.5 feet, so when you rode aero, you may have exceeded the 2.5 range, I am not saying you only road aero for 23 miles. Next training ride do an out and back ride, and on one way out, keep it all the same, on the return,hit the split, move the cad sensor away from the magnet and see if any mileage is recorded. If this results in no mileage then sailnfast CSI is solved. You may have to really consider not racing with cad. Since you are leaving the S1 on while riding, do a training ride with the S1 on in your jersey and see if it helps