If it helps, I have the Vivosmart 4 and Body Battery is the least useful feature I've ever seen on a watch. It tells you nothing useful about your training or recovery status and you're better off standing up and deciding whether you feel tired and/or achy or just using common sense.
what did you get? do you find it more useful the Polar recovery pro than the body battery from Garmin?
As far as I know, body battery uses HRV, which is very accurate in predicting your overall status. Just like recovery pro from Polar ;)