My Garmin strap recently had symptoms similar to yours. I replaced the battery and it worked fine for a couple of rides, then it started acting up again. I found out that the sensor never shut off, even when unplugged from the strap. I noticed it when I was checking the configuration on my Edge and it detected my HR sensor even though the sensor was unplugged. Basically, it was going through batteries every couple of days because it never shut off. I pried the cover off and that saved the battery but was a pain to deal with. I ended up buying a new strap.