Power and Energy

Every motor that spins, every sensor that reads, every calculation the controller performs — all of it costs energy. Energy is the most fundamental constraint in robotics. An engineer can design the most capable robot in the world, but if it runs out of power after two minutes, it is not useful. Understanding where energy comes from, how much each subsystem uses, and how to stretch the available supply is a core skill in robotics engineering.

The Battery: Energy Storage on Board

Most mobile robots carry their energy in batteries. A battery stores chemical energy and converts it to electrical energy on demand. Three key numbers describe a battery's usefulness for a robot. Voltage (measured in volts, V) is the electrical pressure that drives current through the circuit. A small hobby robot might run on a 7.4 V lithium-polymer battery, while a large warehouse robot might need 48 V. Capacity (measured in milliamp-hours, mAh, or amp-hours, Ah) tells you how much total charge the battery holds. A 5,000 mAh battery can supply 5,000 milliamps for one hour, or 2,500 milliamps for two hours, or 1,000 milliamps for five hours. Discharge rate (the C rating) tells you how fast you can draw current without damaging the battery. A 2,000 mAh battery rated at 20C can safely deliver 40,000 mA (40 A) in a short burst — essential for robots with powerful motors that demand large current spikes when they start.

Power Budgets

A power budget is a table that lists every subsystem in the robot and how much current it draws. To build a power budget, you list each component, find its current draw in milliamps from its datasheet, and add them all up. That sum tells you the total current the battery must supply and, combined with battery capacity, how long the robot can run. For example: four motors drawing 800 mA each (3,200 mA total) + a controller drawing 150 mA + four sensors drawing 20 mA each (80 mA) + miscellaneous wiring losses (70 mA) = 3,500 mA total. A 7,000 mAh battery at that draw rate lasts about two hours. Double the drive speed and the motors might draw twice as much current — now you only get one hour. Power budgets force engineers to make tradeoffs. Want a longer-running robot? Use more efficient motors, reduce sensor polling rates, put the controller to sleep between cycles, or carry a heavier battery (which then the motors have to carry, increasing their load — another tradeoff).

Beyond Batteries

Not every robot runs on batteries alone. Robots tethered to a building — like robotic arms on an assembly line — draw power directly from the electrical grid through a cable. This gives them essentially unlimited power but limits where they can go. Solar panels charge batteries on robots deployed in outdoor environments for long periods, like the Perseverance Mars rover, which supplements its radioisotope thermoelectric generator with solar cells during favorable seasons. Some experimental robots harvest energy from the environment — vibrations, heat gradients, or even digesting organic material — but these approaches remain mostly research curiosities for now.