I find them pretty spot on. Best if you record the activities all on the watch but it will pick up other Garmin activity devices well.
The trick with body battery is the first 25% drain faster, the middle slower and down to 5 the slowest. It does mean that not recording the activity and just having a high stress score can nuke the body battery number.
I have a forerunner 745 and wear it most of the time, I’ve noticed a couple of things:
That said, take the metrics with a big pinch of salt…
The recent updates (October?) to HRV have turned it from a nonsense metric to one that reflects my health/illness. I’m recovering from illness now and it is reminding me not to push back into training too quickly and make space for sleep - so yes, it has value.
The watch has literally no idea what life stress I am under though. I’ve recently had some of the worst days of my life and the stress/HRV scores didn’t even flicker from normal.
Funny…my BB will drain from the 90’s down to low 70’s in the morning just taking a shower and driving to work.
I feel it has sometimes put some feelings into perspective. I was always really pleased how well the trend correlated with my feelings but I guess that is usually after the fact. That in my opinion is the main problem.
Perhaps try to correlate it for a while and if indeed it works well when looking back, you can use it for more forward planning action, maybe?
Do you have to record your TR workouts on the Garmin unit before it factors into your body battery score? Or any of the fitness metrics?
If so how do you do that?
Just an interesting read as I was taking my routine morning HRV… Here is my two cents.
I use the Garmin 1040 Stress Score every morning immediately upon waking up. I use a chest strap and stand up for the three minutes as Garmin recommends. I also use two other devices I use the Elite HRV with Garmin chest strap and I use the Fitbit Versa with measures HRV during a 3 hour sleep period ( I think) but measures during sleep whereas the Garmin and ELITE HRV are at the moment/real time.
Here is what the I have found:
With Garmin 1040 Stress Test I have only had 2 Green readings after 3months of daily tracking. With Elite HRV and Fitbit fluctuates based on amount of TSS I have induced sleep or lack there off is probably a factor as well.
They appear to be accurate based on how I feel based on my personal workout stress the Fitbit and Elite HRV sync up closely Garmin does not as it has only given two Green readings. Snd If Garmin is Red and the other two show Green or Amber and don’t feel tired I will proceed with HIIT scheduled for the day.
I was condsidering a Garmin high end watch for biomarker tracking as I like Garmin and am active, However, I dont run anymore, and only do open water swimming in the summer and on the bike I use the 1040…So it’s a steep price just for some biotracking that I’m hearing may not be all that accurate.
I only went with Fitbit at the time because my wife wanted one and I did not want to have to learn two different systems plus I figured I could pass it off to my kid when her watch failed and then get the Garmin. But after reading this maybe not.
Final thoughts are biomarker tracking for me is good and keeps aware of what my body might be saying via the biomarkers vs how I actually feel. I recently did a three day MTB extreme marathon race and I had bad sleep Red HRV but felt okay each morning and had one of my best races ever… 
FYI this morning Garmin Stress Score was Amber 49, Fitbit was Red 29, and Elite HRV was Green 8. I had horrbile sleep 4:56hrs yesterday I did 48minutes TRX moderate in the morning and 1hr 50. Sweet Spot TR Outdoor Workout on the MTB basically a on fire roads and gravel flat. Ad I feel good or capable of doing the SS workout scheduled for today. But because my sleep is Red I’m also considering sitting on the couch**…FWIW
Good luck in 2023 
** I do have a huge Honey Do List that was largely neglected in the closing months of 2022 So the couch may not work out so well 
No, the stress and body battery measurements turn off during a workout (including by autodetect). Body battery and HRV stress are looking at how your body RESPONDS to that stress ie. initially sympathetic after a workout, then gradually turning parasympathetic over several hours as you recover.
So you would have to record the TR training session for the other training metrics?
In my experience dual recording allows both ecosystems to do what they want.
Synching any two different systems is convenient for some things but there will always be some kind of compromise.
Late to the party on this topic - I’m seeing way more accuracy with the Body Battery score via TSS / form metrics for me. TrainerRoad and or Training Peaks just can’t capture non cycling activities (walking, lawn work, pickleball, let alone sleep & HRV). I’m digging having Body Battery as a “tool”. Agree with previous post that its always best to default to how you feel.
Maybe it has improved, but on my Forerunner 245 it was absolutely useless–it had to be reset every couple of days to start registering normal numbers–it would by itself just tank and tell you you are low low low, but reset and you’re so very charged up. I got so disgusted I stiopped using the watch at all. You can have it to test if you like.
Beer a day? (Keeps body battery FLAT)
HRV, RHR, sleep score, body battery, etc will not tell you if your legs are sore/tired or if you are truly “recovered”, ready to train, etc. All of these algorithms are based on your heart rate throughout the day and/or sleep, processed, processed again, and then magically the software tells you what you want or don’t want to hear, and it may or may not correlate with how you actually feel or how well you may actually perform that day, or if you should try to perform or not. None of this can replace listening to your body and keeping a good training log, but maybe just maybe you’ll find some metrics that are useful for adding to your training log to help make decisions in the future.
Case in point, I did a lower body weight session two days ago and the last two days I’ve had a major case of DOMS. And yet my HRV is really high (70-80, which is good for me) and my RHR is really low (40 last night), stress levels low, body battery high, but it hurts to push anything above z1 (though I just did a really nice 45’ SST interval at 90% FTP this morning, just had to tell my legs to shut up the entire time).