The Garmin body battery is a universal measure that collects data across many different health statistics that is then presented as one aggregated piece of information. For example, throughout the day Garmin watches are tracking your various activities-steps, HRV, your stress level both while awake and asleep, and numerous other health stats. To track this, Garmin uses your HRV and then compares this across numerous factors, one of which is stress. The stress rating given in Garmin connect is then able to be read and compared against what the overall battery rating is. If your stress is high, the battery rating will be low, which means you need to pace yourself to prevent your battery from bottoming out by the end of the day.