I got an old Polar heart rate monitor watch that was dead. The back was not so difficult to remove. 4 small phillips screws, but the plastic straps overlapped so they had to be removed. By putting in a thin pin (like paperclip) in the holes in the side, you can compress the spring-loaded pin that holds the strap so you can swing out that side of the strap. The battery, a CR 2025, was easy to insert. I saw the screen flash on, but before I got it in place the screen was dead again. I checked the battery placement contacts, looked for other possible breaks. I tried pressing the main button switch, but nothing happened.
I searched the net but didn’t see anything. Tried looking again closely and found what looked like it was maybe a spring that should be in contact with the circuit board. Decided that maybe that was a reset trip switch. So I pressed it down for a short contact with the circuit board and the screen woke up and started showing time.
I’m trying to change battery in the T31 breast strap heart rate transmitter too. In spite of seeing several video on YouTube – it was rather difficult, it seems like there have been some different sealing/closing methods used. I tried cutting through the plastic front from the ”P” in Polar and towards the left.