FR955 Solar HRV impact on Battery life and general questions about Battery life

I have no idea if the HRV setting makes a difference, but in general 24 hours are no problem for a „healthy“ 955. Settings:

• GPS only (UltraTrac is very inaccurate and therefore basically useless)
• backlight off (use your headlamp)
• bluetooth off
• no course navigation (or check course only occasionally, them stop navigation again)

Oh, I really wanted to use the navigation..... too bad

or does it even make sense to record HRV for an activity?

Or does the chest strap perhaps even record this so that I don't need to activate it on the watch?

TL;DR On my 1 1/2 year old FR955, the watch estimates roughly 40 hours on a full charge, for a running activity with GPS mode set to “GPS only”. You could always test your own battery usage with a 1-2 hour activity.

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Afaik, enabling Data Recording > Log HRV should have little to no impact on battery life. I believe the reason it’s disabled by default is due to the fact that it results in larger activity files, and older watches had very limited storage compared to newer watches. (Probably the same reason that Data Recording > Frequency defaults to Smart instead of Every Second). The 955 has plenty of storage, so that concern is irrelevant.

The other considerations, like bluetooth, the backlight, and gps mode are much more impactful. (I wouldn’t worry about disabling WLAN, although it wouldn’t hurt — the watch will only use WLAN if you try to manually sync, or after your activity ends, if you have wi-fi auto-sync enabled.)

The 955 spec sheet claims that GPS mode (without ultratrac) will last up to 42 hours if you don’t use music. It also claims that ultratrac will improve that number to 80 hours.

If I were you, I would test this claim out with a run of 1-2 hours. If you want your watch to last, say 30 hours (to give you a good margin of error), make sure that the battery only decreases about 3% per hour. You can also enable the battery life estimate which appears next to the battery meter on the first settings page, and the first page below the watchface:

Hold UP, select Power Manager, and enable Battery Percentage and Battery Estimates

For what it’s worth, on my FR955 (a year and half old, and used constantly), with bluetooth disabled and a running activity active (satellites set to All + Multiband), the estimate shows me 5h for 25%. So that should be good for 20 hours.

If I change the satellite setting to GPS Only, the estimate shoots up to 10h. If the estimate is believable, I should be able to get 40 hours on a full charge.

Funnily enough, enabling or disabling bluetooth makes no difference in the estimate, although I always have it disabled during activities nonetheless (I never run with my phone). I’m sure if you run with your phone and the watch is constantly syncing over bluetooth, there would be a definite battery impact.

You can also see detailed graphs of your recent battery usage with a 3rd-party Connect IQ app such as Battery Gauge.

JPM said:

if it is calculated afterwards in Garmin Connect or something

No, HRV comes from the HR sensor.

Notes:

- HRV is only logged if you use a chest strap (as you are), not if you use the wrist HR

- Connect won’t display or use the HRV data you logged, it’s only for 3rd-party apps or websites (such as https://www.runalyze.com)

The only reason to log the HRV (R-R intervals) in the fit file is if you later use a third party tool to analyze your HRV data for the activity. To my knowledge, all Garmin HRV analysis happens on the watch, so HRV recording doesn't change anything in that regard.

a bit off topic, since it is not your question. But consider a (Ultra) small powerbank for charging underway. If you are afraid you will not make the full 24 hours. I do not know if the activity wil stop once you plug in, but it might be an option to you.

this is indeed a good option, the activity will not stop. The lightest powerbank you can find (e.g. a cheap plastic giveaway thing with 2600 mah) will be perfectly fine.

I had already thought about this, after all you do take a break from time to time where you could recharge. A hike is different to an ultra run . It's also not a competition but a diy adventure. 

Has anyone ever tested whether an activity can be started while charging?

Otherwise I'll have to test it when I get back home.