Agrymrat - the watch is easy: you'll notice a battery indicator starts appearing when there's like 10% battery left. It will still track HR, but you'll know it's getting really low when you try to turn the backlight on and the display goes blank. If the display is constantly fading out, the battery is about kaput.
Figuring it out with the stap is a little more ambiguous. My experience is when the strap has a weak battery it will send funky readings to the watch. For instance, one second it will show a HR of 150, then 75, then 225 and back to 150. I actually have two straps now, so I can compare performance between them as well.
One more note on the straps. I've found the rigid strap has longer battery life than the comfort strap. I think this is because the straps transmit so long as the contact points are wet. The rigid strap dries pretty quickly, whereas the soft strap can remain damp all day. Just my 2 cents.
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