High stamina but low body battery?

Body battery is a derived metric showing how well you recovered after last nights sleep (probably looking at HR, HRV, length of sleep, phases, maybe pulsox, not sure).
Stamina is an indicator of how well you can perform at any given moment during an ongoing activity. If your "recovery time" is 0 at start of the activity, stamina starts with 100% and decreases with efforts below and above threshold.

I assume stamina and BB are not related.

"The Body Battery feature works by continuously analyzing combinations of heart rate, heart rate variability (HRV) and movement data while you wear your device. The goal of this analysis is to identify meaningful physiological states and to describe the impact they have on your body’s energy levels. At a high level, this means documenting if you are awake or asleep, when you are physically active and tracking stress levels during periods of inactivity."

https://www.garmin.com/en-US/garmin-technology/health-science/body-battery/

For each 24h period, the algorithm gives you positive points or negative points of body battery based on how much time you were in these "meaningful physiological states". The maximum and minimum of body battery will fluctuate but the metric will always be displayed between 0 and 100. We have seen cases where it was clear that the BB value exceeded 100 for a while, before stress levels took it down below 100 during the days. 

" However, as your fitness level improves, exercise and stressful experiences have a smaller relative impact on your body, and your resiliency tends to improve."

Garmin doesn't explain the details of the calculation and normalization, but say that increasing your level of activity will tend to give you more battery potential. My guess is that RMSSD HRV fluctuations and variance in time is combined with movement data. You could imagine that LO-MED-HI stress level combined with LO-MED-HI activity level define some of these "physiological states". By looking at the variability of stress and activity over a period of time, a normalized range could be calculated.

Stamina is different at least in 2 ways.

First, it is activity based. If there is no recovery need, the stamina is always 100% to start with).

Second, the stamina seems to be based on the capacity of your body to reach an aerobic and anaerobic peak EPOC.  It uses VO2 Max, Rest HR, HR/HRV combine with pace/power effort data. The principle seems similar to dFRC or W' defined as your 'anaerobic" capacity. At least the Garmin stamina behavior during a workout and the graph is very similar to dFRC in WKO5 software.

"Stamina [...] considers both general fatigue accumulation and resource depletion together with the unique performance limitations associated with sustained high-intensity exertion above your anaerobic threshold. "

https://www.garmin.com/en-US/blog/outdoor/introducing-the-garmin-real-time-stamina-feature/

If your Body Battery is low/average/high, it could be because your are low/average/high on energy or because the range is being learned and adjusted, or both. Simultaneously, your stamina at the beginning of the workout can be 100% or less if you had a prior aerobic/anaerobic activity that day. Because stamina is based on your training and performance history, the estimated stamina depletion might not reflect your RPE, which is influenced by your energy levels (reflected by BB).