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KAIST’s RAIBO2 quadruped finished the full 42.195 km Sangju Dried Persimmon Marathon in November 2024, completing it in 4 hours, 19 minutes and 52 seconds without a battery change or recharge. The reason it could do that is now documented: a paper in Nature, published on 23 September 2026, argues that the range came from reducing energy losses across the whole machine, not from a bigger battery alone.
The run and the explanation are separate events, two years apart. The marathon took place in 2024. The engineering account was published in 2026. Below is what was measured on the course, how the robot used less energy, what each reported number means, and where the evidence stops.
What happened on the course
RAIBO2 was officially entered in the Sangju Dried Persimmon Marathon so that its split times would be recorded, according to KAIST’s announcement. The robot covered the full 42.195 km course in a single run. KAIST says it is the first quadruped robot to complete the full course of an actual marathon event; that is KAIST’s own claim, and the sources cited here do not independently verify the “first.”
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This was a measured outdoor event. It was not a range estimate from a laboratory test, and that distinction matters when comparing it with other robots.
Why the explanation arrived in 2026
The peer-reviewed account is “A quadruped robot designed to complete a marathon on a single battery charge,” by Lee, Youm, Park and colleagues, published in Nature on 23 September 2026. Nature Portfolio’s plain-language summary and KAIST’s institutional announcement both describe it. When you cite the achievement, give the two dates separately: the run happened in November 2024, and the paper was published in September 2026.
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How the robot used less energy
The central argument is about system design. Range did not come from swapping in a larger battery. It came from finding and reducing losses throughout the robot. KAIST describes a coordinated design of the mechanical structure, electrical systems, actuation and locomotion control, with a loss analysis that covered the whole machine rather than one component.
Lighter legs with less friction and inertia
According to Yonhap’s report, the legs were designed to reduce joint friction and inertia. Less mass moving at the joints means less energy spent on every stride, which compounds over more than 40 kilometres.
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Control that avoids wasted motion
Nature Portfolio says movement rules were reinforced through machine learning. Yonhap reports that the reinforcement-learning controller reduced foot slipping and slowed the foot before it touched the ground. That slowing cuts collision losses, the energy lost each time a foot hits the surface hard.
Motor-driving electronics
Nature Portfolio identifies optimized motor-driving circuits as one of the main hardware changes. The summary does not give component-level efficiency figures, so the contribution of the electronics cannot be separated from the other changes using the reported numbers.
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Recovering energy on downhill sections
Yonhap reports that the motors operated as generators on downhill stretches, converting some mechanical energy back into electricity. The sources do not state how much energy this recovered over the course, so treat it as a real mechanism with an unquantified share of the total saving.
The numbers and what they mean
The figures below come from different sources and carry different conditions. The table keeps each value attached to its source and qualification.
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| Measure | Reported value | Source and date | What it does and does not mean |
|---|---|---|---|
| Course | 42.195 km, full marathon | KAIST, 2026 | Sangju Dried Persimmon Marathon, November 2024, single run. |
| Completion time | 4 h 19 min 52 s | KAIST, 2026 | Time for the single official run. |
| Average speed | 2.64 m/s | Nature Portfolio summary, 2026 | Dividing 42.195 km by the completion time gives about 2.71 m/s. The summary does not explain the basis for 2.64 m/s, so quote it with its label. |
| Elevation change | 286 m | Nature Portfolio summary, 2026 | Total change along the course. |
| Energy used | 1,280 Wh | Nature Portfolio summary, 2026 | Energy consumed during the marathon. |
| Battery capacity | 2,016 Wh (journal); 1,930 Wh (Yonhap) | Nature Portfolio summary, 2026; Yonhap | The two sources disagree. See the battery section below. |
| Total cost of transport (TCOT) | 0.25 for RAIBO2; 0.37 human benchmark | Nature, 2026 | Energy normalized by body weight and distance. Lower means less energy per unit of transport under this metric. It is the paper’s own comparison. |
| Projected extra travel | About 25 km | Authors’ estimate, per Nature Portfolio summary, 2026 | Extrapolated from the 2,016 Wh capacity. It is not a second course result. |
| Range per charge versus existing quadrupeds | About three times | Paper’s comparison, per Nature Portfolio summary, 2026 | The summary does not establish that comparator robots were tested under matched conditions. |
Why the battery figures differ
The journal summary gives a capacity of 2,016 Wh. Yonhap reports 1,930 Wh and says the marathon used 66% of that capacity, which it translates into a maximum range of about 65 km. Yonhap’s 66% is consistent with its 1,930 Wh base. Against the journal’s 2,016 Wh, the same 1,280 Wh of energy works out to about 63%.
When you describe the paper, use the 2,016 Wh figure and its 25 km projection, or say explicitly that Yonhap gives a different capacity and a different derived range. Do not average or mix the two.
What the 25 km figure is
The roughly 25 km of additional travel is what the authors estimate remains from the battery after the marathon, based on the 2,016 Wh capacity. It answers the question “how much further could it go,” but it was not measured by running the extra distance.
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Professor Jemin Hwang, as quoted by KAIST, framed the result this way: “This study goes beyond demonstrating how far a quadruped robot can run. Using actual marathon data, we demonstrated how the entire robot must be designed to achieve both high locomotion performance and energy efficiency. We will connect the world-class performance achieved in the laboratory to products that anyone can use reliably in industrial environments.” The final sentence is KAIST’s English rendering of the statement.
What the result does not show
- It is one prototype and one recorded run. The sources describe a single marathon completion, not a series of repeated events or a population of robots.
- The range comparison is the paper’s. The “about three times” figure is the authors’ comparison with existing quadrupeds. The sources do not show that every comparator was tested on the same course, terrain, speed or payload.
- The 25 km figure is an extrapolation. It depends on the stated battery capacity, which the two sources report differently.
- It is not a product. KAIST describes Raion Robotics, a faculty startup from Jemin Hwangbo’s lab, as contributing manufacturing technology and developing commercialization and mass-production methods for quadrupeds meant for extended operation in real environments. The sources do not establish that RAIBO2 is currently offered for sale.
How to compare range claims for any quadruped
Before comparing RAIBO2 with another robot’s range figure, check these points:
Quick Recap
- Battery capacity in watt-hours, and whether the figure is nameplate or usable.
- Course, terrain and elevation change, since a downhill-heavy route differs from a flat test.
- Average speed and how it was calculated.
- Payload carried during the test.
- Whether the range was measured on a run or extrapolated from capacity.
- Whether energy is normalized, such as with TCOT, or reported only as distance.
The Bottom Line
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