If I get the HRM will it impact the battery life of Forerunner?

Yes, power consumption of the ANT+ connection is less than running the OHR.

But given the battery life of the 945 is pretty massive anyway, I wouldn't have thought this was a necessity or significant advantage.

Keep in mind that if you use the Garmin Pay wallet your oHR sensor won't switch off even if you use an external HRM (it is used for on-wrist detection).

Even during exercise mode?

I'll have to check next time .. minimally I'd assume the frequency would go way down ...

Umm. This make me think, should I remove the wallet then. I don't really use it and if it has battery impact on activities why bother?

jmto said:

This make me think, should I remove the wallet then.

It's my understanding the sensor light stays on only if you enter your Garmin Pay pin code.  This is a safety precaution, so if the watch is removed, you must re-enter the pin to use Garmin Pay again.  It shouldn't keep the sensor on if the pin isn't entered unless something has changed in a past update.

An HRM-RUN will cause higher battery use on a Garmin device than a normal HR strap without running dynamics. To calculate stride length (amongst other things) the watch has to transmit speed data to the HRM-RUN (or RD pod) on ANT+. This will use more battery than using ANT+ in receive mode only.

tess said:

It's my understanding the sensor light stays on only if you enter your Garmin Pay pin code

Oh, good. So if I use it, then it's 24 hours on, or until I've taken it off or after that, it's not on that "am I worn in wrist?" -mode.

What data-screens are people using when running with an HRM-Run?

A theory: it's not the data from the RD data that's causing higher power draw but having a dynamic data field like GCT or balance.

Try the it with nothing but time and distance up on the screen, see how it fares.