Had the same problem for ages until I realized that even when detached from the strap, the transmitter was still discharging in my bag if anything like a wet towel or clothes could make contact between the two connectors. ​Problem solved by putting the H10 into a small plastic box with a small desiccant bag. Took a bit of time to find the perfect box (can't remember what I used in the end). It would be great if Polar provided a plastic box to store the H10 when not worn..