I would no longer buy Polar: I have lying around an F5. Unfortunately, I can not (any longer) use them. Since the non-replaceable (!!!) battery of the transmitter belt is empty. What a nonsense! And for a new belt want the 34, - Euro (sending own expense) in exchange for the old belt! Ticking still right? That the non-interchangeability of the transmitter battery is at this F6 too.