My HRM was not working on my walk today because I really doubt my heart rate was 47 bpm on my walk ;). Im thinking it's the battery.
My question is: do I only need to change the battery in the watch part, or is there something in the strap part that needs to be replaced?
I changed the battery in my strap last week after 460 hours of use. It did the trick!
I would try just changing the watch battery first. My old hrm strap battery lasted 3 years, the watch one however needed to be replaced about every 3 months ( 1-2hr 6 days a week in use)
My current HRM Polar FT60 is about 2 years old and I still haven't had to change the strap battery, but did have to get a new strap about 6 months ago cause I noticed the old one had a snag on the electrode and that is why it wasn't working
Wash the HRM in warm water inspect the electrodes and check the watch to see if the "battery" is on - at least on my Polars when the watch battery is low it warns you.