Short answer
Body Battery is a 0–100 estimate of your energy reserve, built by Firstbeat from heart rate variability. When HRV shows parasympathetic dominance — during sleep and calm rest — the battery charges; when it shows sympathetic activity — exercise, stress, illness, alcohol, caffeine — it drains. It is not measuring energy, glycogen or muscle fatigue; it is measuring your nervous system’s state and calling it energy. That makes it honest about sleep, alcohol and illness, quietly wrong about hard sessions in the heat, and blind to whether your legs are actually recovered.
Body Battery is the Garmin number people talk about like a fact: “I was on 15 by lunchtime.” It is worth knowing what the 15 is, because the metaphor of a battery is doing a lot of work that the physiology does not.
What it is built from
The model comes from Firstbeat, the Finnish analytics company Garmin acquired, and it is described in their white paper on stress and recovery from 24-hour HRV. The idea is simple. High HRV with a low heart rate indicates the parasympathetic branch of the nervous system is in charge — rest and recovery — and the battery charges. Low HRV with an elevated heart rate indicates the sympathetic branch — effort, stress, arousal — and it drains. Activity drains it faster in proportion to intensity. Sleep is where almost all the charging happens, which is why the number you wake up to matters most.
Three things follow from that. It is an HRV metric wearing a different outfit. It cannot see anything that does not change heart rate variability. And it is reactive, not predictive — it reports the state you are in, not the state you will be in at 6pm.
Where it is honest
Sleep quality. A poor night reliably charges less. If you wake on 40 instead of 85, something happened, and the battery is usually right about that.
Alcohol. Two drinks in the evening flatten the overnight charge with impressive consistency, because alcohol keeps the sympathetic system active for hours after the last glass. It is one of the clearest demonstrations of the model working.
Illness. The battery often refuses to charge a day or two before you feel a cold, for the same reason HRV drops: the immune response is a stressor.
Cumulative stress. A week of poor sleep, hard training and long days shows up as a battery that never gets above 60. That pattern is the useful one.
Body Battery measures your nervous system and calls it energy. Most of the time the two agree. Runners live in the cases where they don’t.
Where it misleads runners
Heat drains it like a hard session
An easy run in a 20°C dew point elevates heart rate and suppresses HRV, so the battery drops as if you did intervals. Nothing about your legs is tired. The dew point did it.
Caffeine and a busy morning
Both raise sympathetic tone. A double espresso and a stressful commute can take 20 points off before you have done anything physical.
It cannot see muscle damage
Downhill running, a race, a heavy strength session. Your quads are wrecked for three days; your HRV can be back to normal in one. The battery says 90. The stairs disagree.
It cannot see fuel
Low glycogen after a long run without eating properly is exactly the “no energy” people mean, and it does not appear in HRV until the stress of underfuelling has gone on for days. How many carbs runners need is the number the battery is missing.
Sitting still charges it
A calm afternoon on the couch reads as recovery whether or not you are recovering. A stressed afternoon of desk work reads as drain. Neither has anything to do with tomorrow’s session.
Body Battery vs Training Readiness
They share inputs and are often confused. Body Battery is an all-day energy estimate for general wellness, and it drains with the errands you run. Training Readiness is a morning score built specifically for the question of whether to train hard, and it adds HRV status, acute load and recovery time that Body Battery does not use. If you are deciding about a session, Readiness is the more relevant number; is Training Readiness accurate? covers its own limits. If you are trying to understand why you feel flat at 3pm, Body Battery is the one to look at.
How to use it
Read the morning value and the overnight charge, not the number at 4pm. The morning value is a clean read of sleep and yesterday’s recovery; everything after it is contaminated by coffee and meetings. Watch the week: a battery that wakes lower each day is telling you something the individual days are not. And when the number and your legs disagree, believe your legs — the battery cannot see them.
Kovr sees what Body Battery can’t: the food, the drinks and the air.
Body Battery drops after a hot run and after a heavy one, and cannot tell you which. Kovr can: it reads the dew point every run happened in, logs what you ate and drank, and reads them together with your sleep and HRV. A flat afternoon becomes “70g short on carbs after yesterday’s long run” instead of a number with no cause.
Download Kovr on the App StoreFree with a 14-day trial · Garmin, Apple Watch & Oura
Sources cited
- Firstbeat Technologies (2014). Stress and Recovery Analysis Method Based on 24-hour Heart Rate Variability (white paper). Firstbeat Technologies Ltd — the HRV-based stress and recovery model Body Battery is built on
- Buchheit, M. (2014). Monitoring training status with HR measures: do all roads lead to Rome?. Frontiers in Physiology — why HRV-derived metrics cannot separate their causes
- Miller, D.J., Sargent, C. & Roach, G.D. (2022). A validation of six wearable devices for estimating sleep, heart rate and heart rate variability in healthy adults. Sensors — overnight HR and HRV accuracy of consumer wearables
- Ralevski, E., Petrakis, I. & Altemus, M. (2019). Alcohol and heart rate variability: a systematic review. Alcoholism: Clinical and Experimental Research — the reliable suppression of HRV by alcohol
Frequently asked questions
How does Garmin Body Battery work?
It is an HRV-based model from Firstbeat. When heart rate variability shows the parasympathetic system in charge — sleep and calm rest — the 0–100 battery charges; when it shows sympathetic activity — exercise, stress, caffeine, alcohol, illness — it drains, faster with higher intensity. It measures nervous-system state, not energy or fuel.
Is Garmin Body Battery accurate?
It is reliable about the things that change HRV — sleep quality, alcohol, illness, cumulative stress — and misleading about the things that do not: muscle damage, glycogen, and hot runs that drain it like hard ones. Use the morning value as a trend and trust your legs when they disagree.
Why is my Body Battery low after an easy run?
Most often heat or humidity: a high dew point elevates heart rate and suppresses HRV during an easy run, so the model treats it like a hard session. Dehydration, caffeine and a stressful morning have the same effect.
Why does my Body Battery not charge overnight?
Alcohol is the most common cause, followed by illness, a late heavy meal, poor sleep and an unusually hard evening session. Each keeps the sympathetic system active into the night so the model sees little recovery.
What is the difference between Body Battery and Training Readiness?
Body Battery is an all-day energy estimate that drains with daily life. Training Readiness is a morning score built for deciding whether to train hard, and adds HRV status, acute training load and recovery time. Use Readiness for session decisions and Body Battery to understand a flat afternoon.
Should I skip a run if my Body Battery is low?
Not because of the number alone. Check what drained it: if it was a hot run, coffee or a busy morning, run. If it was a night that would not charge with no obvious cause, ease off and consider whether you are getting sick.