The battery I will not comment here, but tell only the problems that can occur if the battery is not replaced by Polar.
I changed the battery itself (is not really difficult), then the display made problems.
The contrast did not fit properly and thus could no longer be read off the clock, only when I turned on the light I could read the display more.
After some Internet research it turned out that many people had this problem after they had themselves replaced the battery.
The clock (Polar RS800CX PTE Premium Edition) is zZt. Polar for repair, I'm waiting for the bill; (
