Robots need a steady source of electrical energy to run motors, sensors, lights, and controller boards. The power source affects how long a robot can run, how fast it can move, and how safe it is to use. Educational robots often use battery packs because they are portable and easy to replace or recharge.
Choosing the right power source is an engineering decision that balances voltage, current, capacity, size, cost, and safety.
Different robot jobs need different power options. AA alkaline batteries are simple but may drain quickly in motor-heavy robots, while NiMH packs are rechargeable and handle repeated classroom use well. Lithium-ion and LiPo batteries store more energy for their size, but they require careful charging and protection from damage.
Wall power works well for stationary robots, and solar panels can extend runtime for outdoor rovers when sunlight is available.
Understanding Robot Power Sources
A battery label gives only part of the story. A robot does not draw the same current every second. Motors need a large current when they first start, when the robot climbs, or when a wheel is blocked.
A stalled motor can draw several times its normal running current. This can make the battery voltage dip for a moment. That dip is called voltage sag.
If the voltage falls too far, a controller board may reset even when the battery still has stored energy. Students often see this when a robot works on a stand but restarts as soon as it drives on the floor. Loose wires, weak connectors, and thin cables can cause a similar problem because they add electrical resistance.
Capacity in amp hours tells how much charge a pack can supply, but it does not fully describe the energy available to a robot. A pack with a higher voltage delivers more energy for the same amp hour rating. Energy is often compared in watt hours.
This combines voltage with capacity. Battery cells can be connected in series to raise the pack voltage. In a series pack, the voltage of the cells adds while the amp hour capacity stays like that of one cell.
Cells can be connected in parallel to increase capacity. In a parallel pack, capacity adds while voltage stays the same. Real battery packs need matched cells and protection circuits, especially when lithium cells are used.
Robots need a safe path for electricity from the battery to each part. Motors can create electrical noise when their brushes switch current. This noise can interfere with sensors and microcontrollers.
A well designed robot may use a motor driver, a voltage regulator for the controller, and capacitors near sensitive electronics. The regulator changes battery voltage into a steady level that a board can use. It wastes some energy as heat, depending on its design.
A fuse or resettable protection device helps limit damage during a short circuit. Switches and connectors must be rated for the current the robot can draw. A small classroom connector may heat up if it carries the current of a large drive motor.
Runtime estimates are starting points, not promises. A robot that spends most of its time waiting or rolling slowly can run much longer than one that accelerates often. Carrying a load, driving on carpet, turning sharply, and using bright lights all raise current draw.
Cold weather can reduce the useful capacity of many batteries. Solar panels are especially variable because their output changes with light angle, clouds, and shade. Testing gives the most useful answer.
Students can measure current during different tasks, record how battery voltage changes, and calculate an average. They should stop using a pack that becomes unusually hot, swollen, damaged, or difficult to charge. Safe charging equipment and the correct charger setting matter as much as the battery itself.
Key Facts
- Electrical power is found with P = V * I, where P is power in watts, V is voltage in volts, and I is current in amperes.
- Battery capacity is often measured in mAh, and 2000 mAh = 2 Ah.
- Approximate runtime in hours is runtime = capacity in Ah / current in A.
- Example: a 7.4 V 2000 mAh battery powering a 1 A robot lasts about 2 hours because 2 Ah / 1 A = 2 h.
- Higher voltage can make motors spin faster or provide more torque only if the motor and controller are rated for that voltage.
- LiPo and lithium-ion batteries have high energy density, while alkaline AA and NiMH packs are often safer and simpler for beginners.
Vocabulary
- Voltage
- Voltage is the electrical push that moves charge through a circuit, measured in volts.
- Current
- Current is the rate at which electric charge flows through a circuit, measured in amperes.
- Capacity
- Capacity tells how much charge a battery can supply over time, often measured in milliamp-hours.
- Energy density
- Energy density is the amount of stored energy compared with a battery's size or mass.
- Power regulator
- A power regulator is a circuit that changes or stabilizes voltage so robot parts receive the correct supply.
Common Mistakes to Avoid
- Using the highest voltage battery available is wrong because motors, controllers, and sensors can overheat or fail if their voltage rating is exceeded.
- Treating mAh as power is wrong because mAh measures stored charge, while power depends on both voltage and current using P = V * I.
- Ignoring motor current spikes is wrong because motors can draw much more current when starting, turning, or climbing than when rolling smoothly.
- Charging lithium-ion or LiPo batteries with the wrong charger is wrong because these batteries need specific charging control to prevent damage, overheating, or fire.
Practice Questions
- 1 A robot uses a 6 V battery pack and draws 0.5 A while driving. What electrical power does it use?
- 2 A 7.4 V 2000 mAh battery powers a robot that draws 1 A on average. Estimate the runtime in hours, then find the average power in watts.
- 3 A classroom robot stays on a table all day for demonstrations. Explain why wall power might be better than a battery, and name one disadvantage of using wall power.