My Polar Bluetooth Heart rate monitor (that clips on the chest strap) got real finicky with battery replacement. I thought it was that I was getting bad batteries, but instead it was the metal fingers used inside the device to connect the battery to the circuit board. Had to disassemble and bend the metal fingers a bit so they made better contact with the circuit board and the battery. Now works much better. Here’s how to take apart
First remove the 4 screws. Yeah you need one of those star-bit drives, or work carefully with a smaller flat blade…
Now the front cover will come off. Note the rubber o-ring, remember where it goes and don’t lose it. The remove the two screws which hold the metal fingers that connect the circuit board to the strap snaps. Once those two screws are removed, then the circuit board is lose.
The metal tabs that connect to the battery are on the inside of the back. bend the tabs so they make better contact with the circuit board. reassemble, and you should be good to go.