Garmin correlates stress level with HRV and body battery, regardless of whether the HRV is measured post workout or not.

Are you mistaking heart rate for heart rate variability?

Heart rate - weather recovery heart rate measured or not - will be higher than resting heart rate after a workout.

Heart rate variability will be low during and after workout. High HRV - high variance in inter heart beat times - means low stress.

Regarding body battery: recovering after a workout does not simply mean being relaxed. So after workout the body recovers and that takes "resources" - hypercompensation.

Whether you feel alright in the evening or not, your body was put under stress during the workout and first recovering phase also uses up resources. During that time you will have higher metabolisms happening and that indeed puts your body under stress...

It's a good point that the body takes up more resources during the post-run recovery, thereby having an increased metabolism and resulting in a higher stress. 

However, what I meant about the stress, HRV, and body battery was somewhat different though. Garmin seems to correlate all of these three metrics regardless of the context: whether a person is in a post-workout state, or during a long and mostly restful day, for instance. I think it would have been more accurate to differentiate between these situations, instead of applying the same formula indiscriminately.

Based on the account here, HRV is higher after an intense workout. It makes sense: the body expects you to exert more effort, because it doesn't realize that you had already completed the workout and are going to recover and take it easy for the rest of the day. So every slight physical exertion, such as getting up to get a glass of water, results in a significant jump in heart rate. This in turn implies that the HRV is also higher: your body is more limber and being ready to ramp it up quick, in an anticipation that you are about to resume an intense physical activity.

Unless I am misunderstanding it, Garmin applies the same formula to translate high HRV to a high stress level. However, such a high stress level for hours after a workout is hardly entirely analogous to a high stress in case when one is agitated, or nervous, but overall sedentary. As I pointed out above, it's also counter-intuitive that the body battery keeps plummeting for hours after workout, because Garmin also transcribes high stress level into a drain on the body battery. 

Ideally Garmin would refine their body battery metric, in such a way that it wouldn't keep going down while all I am doing is sitting in a chair, occasionally getting up to have some water or visit a bathroom and such (or even just stretch out).

I think you are misunderstanding the HRV and HR changes.

HRV is defined on R-R intervals or beat to beat times. So a jump in HR does not mean higher HRV.

Look at the graph under HRV Training in your link. The grey area is your workout, the yellow is that resource using recovery, the green is overcompensation. So directly after workout your HRV will remain low and gradually rise.

And also mind that the timeframes may differ from your thinking. A hard workout can push your metabolism high and HRV low for up to 48 hours.

HRV is a quite universal concept, it does not need to take the source of lower variability into account.

Try pushing for some subsequent days really hard and you will see that your stress level will rise and body bat will decrease day after day. Also not doing hard workouts for a week but having stressful days will let you perform worse on workouts.

For me it works really well and reflects quite good what I am able to perform

Thanks for your response. I think the culprit in question here is the body battery, because this is the metric I wanted to discuss originally, and HRV came up mostly since this is the main measurement which Garmin seems to incorporate into their calculation of the body battery metric.

In the end it's kind of naively self-serving: Garmin calculation works out in such a way that if I run in the morning, and then rest (physically) for the remainder of the day, my body battery will be at shockingly low 25 by the time I go to sleep.

On one hand, this is accurate in the sense that I am hardly in the absolute best shape to repeat another long and productive run in the evening of such a day (despite resting for the bulk of the day). On the other hand, it's counter-intuitive, because apart from the low workout potential at that moment I feel rather ok. The latter aspect of it is what I was referring to; while I think you were mostly addressing the former one.

But even then, what still concerns me regarding the accuracy of the body battery metric is that it seems to indicate that I am more fit to work out 3 hours after completing the long run than 8 hours after! This is hardly a correct assessment, at least not in my case: the more I have rested after the last workout, the better shape I am in to go for another run. So the original point stands: Garmin should think harder about how to calculate the body battery, instead of merely tie it up to the stress via the HRV, ignoring the context and all.

On a related note, i have been ill but recovering. Last nightt, i had low HR, lower than typical stress and in-range HRV. Yet my body battery only charged 30 points to ~60. Doesn't compute….

As a result. It's 13:00 and my Body Battery reading indicates that i am ready to bottom out. 

Is this a glitch in the algorithm or is there an explanation for this?