I own a Forerunner 45S since a couple of months.
I enjoy the "Body Battery" feature, but recently it got stuck at 5% and it seemed not to change anymore regardless of my abundant periods of sleep and rest.
It might be that when the software algorithm fall into that pit of minimum BB it never can get out of it anymore (probably due to some bug).
I found that doing a "soft reset" works and restore the BB calculation to what's expected. But what is a "soft reset"?
In the forerunner 45 owner's manual it's called "Restarting the device" and is said:
Restarting the Device
If the device stops responding, you may need to restart it.
NOTE: Restarting the device may erase your data or settings.
1) Hold LIGHT for 15 seconds.
The device turns off.
2) Hold LIGHT for one second to turn on the device.
Notwithstanding the warning about possible data/settings loss I didn't experience any problem with my data/settings after the reset.
A beautiful and useful video showing how to do a soft reset (or a device restart) is here: https://youtu.be/E8onl2NxYgg?t=67 (the video is about the Garmin Fenix 6 but I think it applies to many other Garmin devices).
I hope this helps
