Runner dilemma #34 Garmin Can Show How Much Battery My Body Has Left
Runner's Log | Runner's Dilemma #34
Garmin Can Show How Much Battery My Body Has Left
I trained for more than three months aiming for a sub-4 full marathon. But after 21km it got so hard that I kept alternating between walking and running, and I finally dropped out at 24km.
The club president, who paced me, kept going to 35km as a long slow distance (LSD) run.
Two days later I saw him again, and after we’d talked for a while, he said this.
I showed him how to find it in the Garmin app.
My body’s battery has really lost capacity.”
So I decided to take a closer look at this Garmin graph.
If you’re about to run a 10K, a half, or your first full marathon and wondering, “Can I make it to the end?”, it might be a useful reference.
I. How to View the Graph
After a run, you can check it in the Garmin Connect app in these steps.
- After the run, turn on Bluetooth on your phone. (This syncs the watch’s activity data to the app.)
- In the Connect app, tap the run you just finished.
- At the top, among Overview · Stats · Laps · Charts · Gear, tap Charts.
- Scroll down, and you’ll find the Stamina graph.
[Insert your Garmin app screenshot here: run record → Charts tab → Stamina graph — delete this note after adding the photo]
Two numbers appear above the graph.
| Label | Meaning |
|---|---|
| Initial potential | The value at the start of the activity |
| Final potential | The value at the end of the activity |
When I turn on the watch and start running right away, the initial potential is always 100%.
The graph above shows 94% instead of 100% because it comes after a 2km warm-up jog. I think the heat may have played a part, too. I lost 6% in just 2km…
This feature may not be available on every watch, so check first whether yours supports it.
II. Questions and Answers
| What I wondered | What I found |
|---|---|
| What is it? | Garmin’s Stamina feature. Think of it as a ‘body battery’ level that goes down the longer you run. |
| What is it calculated from? | It’s described as an estimate based on heart rate, pace, VO2max estimate, lactate threshold, recent training history, and so on. The exact formula isn’t public. |
| If 37% is left, can I run 37% more? | No, don’t read it that way. It doesn’t tell you the remaining distance. |
| Can I compare the president’s 64% with my 37%? | It’s described as being calculated against each person’s own VO2max and threshold estimates, so ranking people against each other is difficult. |
| Can I trust it? | I couldn’t find public data validating its accuracy. It’s safer to look at trends than at absolute numbers. |
III. Applying It to Myself
I put three types of my own runs side by side.
| Run | Pace | Initial → Final potential | How it felt |
|---|---|---|---|
| 35km LSD (three times) | 5:51–6:03 per km (paused the watch at 19km for a 10-minute fueling break; pace excludes the break) | 100% → around 42% (nearly identical all three times) | Got hard from 28km |
| Full marathon (dropped out at 24km; includes a 2km jog before the start) | Around 5:35 per km (through 21km) | 94% → 37% | Walked and ran after 21km; the battery ran out |
| Workout 8 days after the marathon (Oct 19) (3km jog at 7:00, then 16km) | 5:36 per km | 95% → 55% | Really hard |
The initial potential differed depending on whether the warm-up was included in the recording.
So when comparing final values, you also need to check the starting conditions.
Two things stood out.
First, the three 35km LSD runs ended at nearly the same final value.
(Sept 12: 5:55 · Sept 19: 5:51 · Sept 27: 6:03)
[Insert the three LSD Stamina graphs here — delete this note after adding the photo]
Second, on race day I ran about 20 seconds per km faster than my LSD pace, at 5:35, and ran out of battery at 24km. In Monday’s workout, running 5:36, I was down to 55% after just 16km.
[Insert the marathon vs. workout Stamina comparison here — delete this note after adding the photo]
My data looked consistent.
It seems that running faster drains the battery faster, and running slower drains it more slowly.
Of course, this is one person, one device, and three types of runs, so I can’t say it proves the metric is accurate.
There are other variables, like weather and how I felt that day.
The LSD runs also included a 10-minute break, while the race and Monday’s workout were run without stopping.
IV. My Own Plan to Track My Running Fitness
Like VO2max, I wondered whether this number could work as an indicator of how my body is changing.
So I plan to run at the same pace once a month.
| What to set | How |
|---|---|
| Pace & distance | Run the same pace for the same time (or distance), without stopping. (e.g., 5:50 per km for 90 minutes) |
| Warm-up | Start right away without a jog (starting at 100%), or always do the same warm-up. Including a warm-up in the recording changes the starting value. |
| Course & weather | The same course is ideal. Temperatures in Korea can change a lot within a month, so I’ll record the temperature each time and keep it in mind when reading the results. |
| What to record | Final potential (%), average heart rate, perceived exertion (1–10), temperature |
If the final value goes up at the same pace, that could be a hint that my body has changed. (It’s still just a hypothesis.)
This stamina number isn’t a validated indicator like VO2max, so I’ll treat it as ‘my own record’ rather than an accurate measurement.
V. If You Want to Try It Too
Looking at my records, I came up with this method.
- Run a set distance at a comfortable pace and see how far the final potential drops.
- Run the same distance at your target pace and see how much it drops, and at what distance.
- Put the two graphs side by side and compare.
That way, you might get a rough sense of whether you can hold your target pace to the end on race day, or whether you’d struggle midway and end up with a DNF.
And instead of a DNF, you could estimate in advance, “I’d need to slow down to about this pace to finish.”
Here are my records. (They’re from different conditions, so treat them as reference only.)
| Pace | Distance | Final potential |
|---|---|---|
| 5:51–6:03 | 35km | Around 42% (all three times) |
| Around 5:35 | At the 24km mark (5:35 through 21km, then walking and running) | 37% |
The distances differ, so this isn’t an exact comparison. Still, when my pace was 15–30 seconds per km faster, the battery dropped further over a much shorter distance.
What’s interesting is that when you raise your pace, the battery runs out quickly, and when you slow down, it lasts a bit longer.
From what I found, the feature is explained as estimating how fast your energy is used by comparing real-time heart rate and pace against your personal threshold. If so, it makes sense that the battery would drain faster as intensity rises. My records showed the same pattern. (But the conditions differed, so it isn’t direct proof.)
A few things to keep in mind.
- This graph is not a validated tool for predicting whether you’ll finish a race.
- I’m not aware of any evidence for a rule like “above X% left means you can finish.” So I suggest building your own record table and comparing against similar days.
- Running your full race distance at target pace in training is hard on the body. It’s safer to run only part of the distance and judge from that.
VI. Closing
From now on, I’ll run at the same pace once a month and record how this number changes.
I’ve done the Cheongnamdae 100km with the president. He paced me for more than 15 hours that day, and with 3km to go, he said this.
When I got to the finish and asked him about it, he said he’d run the last 3km at under 5:00 per km. On top of that, he drove for more than four hours on the way back.
Even then, I remember thinking, ‘Wow, his stamina is on another level.’
Now I wonder whether this graph could show more than running ability, and give a sense of overall fitness, too.
The president: 64% battery after 35km.
The president sleeps only a little, and he says he doesn’t feel tired even after hard work during the day.
If I don’t take a short nap after lunch, I’m half-asleep all day.
Somehow, I wonder if this graph shows a little of that difference.
Of course, as I said in Section II, this number is calculated against each person’s own baseline, so ranking people against each other is difficult, and I couldn’t find any data on how it relates to sleep or everyday fatigue. This is just my impression.
References: Garmin official manual (Real-Time Stamina) · the5krunner (Garmin Real-Time Stamina)
