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I think that, if you have replaced the rechargeable battery in your heart rate monitor, you will need to charge it up for at least 6 hours, before trying to use it.
Rechargeable batteries that have sat upon a shelf waiting to be bought, have that reluctant quality, in many cases, that require their initial recharge be prolonged, even if the charger says they are charged long before the 6 hours are up.
Let me know how it turns out, please. Thanks!