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Polar S720i Heart Rate Monitors
. I’ve had one of these for quite a while and the battery if finally dead. Got two years out of though. Fortunately you don’t have to send back to Polar, but you do get an additional year warranty if you do:

Battery life is estimated at 1.5 years on the watch and 5 years or 2,500 hours on the chest transmitter · Watch batteries are standard 3 volt lithium and may be changed by a jeweler

It is $13 to replace it with Polar and they guarantee water resistance. So not a bad deal for a watch tjhat is designed to brave the elements Here is the repair form which is kind of hard to find.