My Polar HRM is great – had it for a couple of years. Recently my heartbeat appears to have dropped dramatically (c100bpm instead of 180 on a fast run). Either that or the battery's on the way out!
The watch screen looks fine, no fading, so I guess it's the chest transmitter. But as far as I can see, that has no battery compartment.
So what's the likely problem and how do I fix it?
Cheers

Mine has, on the back, a round cover with a coin slot in it. Insert coin, turn, cover pops out, battery is revealed. CR2032. Two years is about right for a strap battery.

Older ones are sealed and a PITA to replace yourself (you'll find some DIY guides if you google but I gave up) – and Polar charge the price of a new (replaceable battery type) chest strap to replace the battery.

in my experiance straps not only need battery replacement, but actualy deteriate and wear out so loose their ability to work over time (all that salty dampness) – so I end up replacing my strap every couple of years out of need.
the new ones defo have easily replaceable batteries.