Garmin HRM-Dual not working

Yeah, the reverse polarity thing is bogus. There is zero chance the engineers didn't include a diode. It's more likely just the reset from a full drain (battery out for 30 seconds) that reset it. 

Good to know, i will try before throw the thing out of the window

Too late for me. Replaced the battery, left battery out long enough for capacitor to drain. Shorted the battery terminals (battery upside down for ten seconds), washed the strap in the washing machine. All to no avail. I saw a replacement strap for $30 online, but since they couldn't dane to make the HRM-Dual work with their connect app, I feel too singed to give Garmin more money. One to try a cheapo strap and see what happens. Good luck to you.

This worked actually

HRM Dual is very sensitive to battery voltage.  And don't assume a new, even brand-name battery, is not already below voltage.  I just tested 4 new batteries with a multi-meter and 3 were already at 2.5V - should be at least 3.0V!  Only one worked in the HRM Dual, at 2.8V.

Yep! This worked. Old, beat module that I thought for sure was NFG, but it lives on!

Thanks, this was my problem. I replaced the battery and the monitor worked for 30 min. Measured my "new" battery and it was 1,9 V. I bought a new one and everything is working fine now. 

This worked for me!!! YESSS!!!!! I bought new batteries because I thought the ones I had had gone bad over a year but turned out it was the reverse polarity trick that did it