Feeding Robots Could Solve Energy Shortage Issue

Earlier this year in Beijing, a bipedal robot completed a half-marathon in just under 2 hours and 40 minutes—an impressive accomplishment given that many recreational runners would be proud of such a time. Yet the detail that the robot paused three times for battery swaps underscores a fundamental challenge in robotics: energy storage and endurance. While machines like Boston Dynamics’ Spot and Atlas exhibit animal‐like agility and mechanical efficiency, they cannot sustain operations for more than a couple of hours on battery power alone.

Limitation of Lithium-Ion Batteries
Most mobile robots rely on lithium-ion batteries, which currently achieve energy densities of around 0.25 kilowatt-hours per kilogram. In stark contrast, animals store nearly 9 kilowatt-hours per kilogram of energy as fat. For example, a sled dog carries roughly 68 kWh of energy—comparable to a fully charged mid-range electric vehicle—yet robots would need battery packs dozens of times larger in mass to match that level of endurance. Furthermore, each 25% improvement in lithium-ion performance typically requires around a decade of research and development, making rapid gains unlikely.

• Battery Weight vs. Mobility : Adding more batteries to extend runtime invariably increases a robot’s weight, which in turn demands more power to move. In highly mobile platforms, a delicate balance exists between payload, performance and endurance. Spot’s battery already comprises 16% of its total mass; doubling its energy storage would greatly impede its agility.
• Charging Limitations: In environments such as disaster sites or remote terrains, access to grid power is unreliable or non‐existent. Even fast-charging lithium-ion cells require tens of minutes to replenish, creating unacceptable downtime.

Impact on Real-World Applications
Short runtimes fundamentally constrain the tasks robots can perform:

• Search and Rescue: A search robot that can only operate for 90 minutes may not complete building scans or reach trapped victims.
• Agricultural Automation: A field robot that stops to recharge every hour cannot keep pace with harvest schedules or respond in time to weather changes.
• Industrial Support: In warehouses or manufacturing floors, frequent recharging interrupts workflows and increases labor costs for battery swapping logistics.

Next-Generation Batteries Offer Limited Relief
Researchers are exploring higher‐energy‐density chemistries such as lithium-sulfur and metal-air batteries, which in theory approach energy densities comparable to animal fat. When paired with highly efficient actuators, these systems could enable multi-day endurance for mobile robots. However, several obstacles remain:

• Rechargeability and Cycle Life: Many lithium-sulfur and metal-air designs degrade rapidly after a few cycles, making them impractical for extended use.
• Thermal Management: Fast charging strains battery cells, generates heat and often requires bulky, high-power charging stations. In the absence of reliable charging infrastructure, these improvements still leave robots unable to operate autonomously for hours on end.

Bioinspired “Robotic Metabolism” as an Alternative
Rather than relying solely on batteries, an emerging approach envisions robots that “eat” fuel—much like animals—through a synthetic metabolism. In biological systems, food is converted into chemical energy via digestion, circulation and cellular respiration. Robots could replicate this flow of energy using chemical reactors and fluidic networks:

• Chemical Reactors as “Stomachs”: Researchers are developing modules that convert aluminum or other high-energy metals into electricity when exposed to oxidizers. Such reactors operate like a mechanical stomach, transforming solid fuel into power on demand without needing an external outlet.
• Circulating Energy Fluids: Inspired by blood, fluid-based energy delivery systems circulate energy across the robot’s structure. In one prototype, a robotic fish tripled its energy density by replacing a conventional battery with a multifunctional fluid that circulated within its body. This fluid not only supplied power but also aided in temperature regulation and structural damping.

Advantages of a Synthetic Metabolic System

1. Extended Operation: By carrying compact, energy-dense fuel pellets or liquids, a robot could remain active for days rather than hours.
2. Distributed Storage: Instead of a single, heavy battery pack, energy could be stored in smaller reservoirs throughout the robot’s limbs and torso, reducing localized mass and improving balance.
3. Multi-Purpose Fluid: A circulating fluid could simultaneously act as coolant, structural damping medium and energy carrier—much as blood supports multiple functions in animals.

Potential Applications

• Search and Rescue in Hazardous Environments: A robot equipped with metal-fuel reactors could navigate collapsed buildings or forest fires for extended periods, refueling with compact metal cartridges carried by human responders or delivered by drones.
• Agricultural Monitoring and Harvesting: Long-endurance field robots could continuously scout crop health, tend to livestock or harvest produce over several days without returning to a charging station.
• Deep-Sea and Planetary Exploration: Robotic submarines or rovers powered by chemical fuels could survey remote ocean trenches or distant planets, where solar charging fails and recharging infrastructure is non-existent.

Challenges and Research Directions

• Fuel Handling and Safety: Storing and transporting high-energy metals or reactive chemicals on mobile robots introduces fire and corrosion risks. Researchers must develop safe enclosures and automated fuel-refill mechanisms.
• System Integration: Combining chemical reactors with electric motors, power electronics and onboard sensors requires new design paradigms. Fluidic circuits must remain sealed under dynamic loads, and refueling stations must be standardized.
• Cost and Scalability: Early prototypes demonstrate feasibility, but large-scale manufacturing of chemical reactors and fluidic networks remains expensive. Advances in additive manufacturing and materials science will be crucial to lowering costs.

Outlook on Robotic Endurance
Robotic enthusiasts have long celebrated breakthroughs in locomotion—robots that sprint, jump and climb. Yet without more effective energy systems, these machines cannot match the endurance of biological organisms. Synthetic metabolisms, combining high-energy fuels, circulating fluids and bioinspired design, represent one promising pathway. They could bypass the incremental improvements of battery technology and unlock robots capable of working alongside humans for full eight- to 12-hour shifts—or longer.

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If successful, feeding robots “energy pellets” or liquid fuel could transform autonomy across industries, enabling machines to assist in disaster relief, agriculture, logistics and exploration without constant recharging. The challenge will be to integrate these metabolic systems safely, efficiently and affordably. But once robots can truly “eat,” they may finally gain the staying power needed to operate in any environment—much like living creatures.

About the Author
James Pikul is an associate professor of mechanical engineering at the University of Wisconsin–Madison. His research focuses on energy systems for robotics and the development of next-generation power sources. He is funded by the Office of Naval Research and co-founded Metal Light Inc., a company developing metal-fuel reactors for autonomous systems.