The Polar T31 Heart Rate Monitor is a tool for athletes to gauge how hard their heart is working while exercising. Using this information, they can also calculate how many calories they've burnt. These devices have 2,500 hours of battery life, an might require a new battery at some point during its life. Polar welds the case shut to protect it, so you can't change the battery yourself. You must have Polar fix it for you.
Make sure the battery is actually dead. Occasionally another problem, such as water getting into the transmitter, keeps it from working. Place the monitor in a cool, dry place and check it later to make sure it's the battery.
Locate a Polar dealer in your area. Polar maintains a list on its website of authorised dealers all over the world.
Bring or send your T31 to a Polar dealer. Explain that you think your battery is dead. The company will test the device and let you know whether or not you need a new battery.
Purchase your new transmitter. The Polar distributor can set the transmitter in the chest strap for you. Polar sells these for £29, but depending on your warranty you might get it cheaper.
Clean your transmitter and store it in a cool place to increase the battery life.