Powering your robot

Learn

What you’ll learn

• Common battery types and their voltages
• What mAh means and how to estimate run time
• Why motors and microcontrollers need careful power wiring

Battery types

Type	Voltage per cell	Rechargeable	Good for
Alkaline AA/AAA	1.5 V	No	First projects, cheap and safe
NiMH AA	1.2 V	Yes	Classroom robots, safe and reusable
Li-ion 18650	3.7 V nominal (4.2 V full)	Yes	Longer run times
LiPo pack	3.7 V per cell (2S = 7.4 V)	Yes	Drones, fast robots; needs care
9 V block	9 V	Some	Low-current only; bad for motors
USB power bank	5 V	Yes	Powering a Raspberry Pi or Arduino

Caution

Lithium batteries need the right charger and careful handling. Re-read Safety in robotics before using them.

Capacity: mAh

Battery capacity is measured in milliamp-hours (mAh). A 2,000 mAh battery could, in theory, supply 2,000 mA for 1 hour, or 500 mA for 4 hours.

Run time ≈ capacity (mAh) ÷ current draw (mA)

Example: a robot drawing 400 mA from a 2,000 mAh pack → 2,000 ÷ 400 = about 5 hours. In real life expect roughly 60–80% of that, because batteries lose voltage as they drain and motors draw extra when starting.

Motors are power-hungry

• An LED: ~15 mA. An Arduino: ~50 mA. One small motor: 200 mA–1 A+, and much more when it stalls (is blocked).
• When motors start, they pull a sudden burst of current. The battery voltage dips, and a microcontroller on the same supply can reset or behave strangely.

The golden rules

1. Never power motors from a microcontroller’s pins or its 5 V pin. Use a motor driver board and feed it from the battery.
2. Connect all grounds together (battery −, motor driver GND, Arduino GND). Without a common ground, signals have no reference and nothing works.
3. Consider separate batteries for motors and logic, or add a capacitor near the motor driver to smooth dips.

Getting the right voltage

• Arduino UNO: 5 V via USB, or 7–12 V into the barrel jack/VIN (its onboard regulator drops it to 5 V).
• 3.3 V boards (ESP32, Pico): never feed 5 V into their input pins.
• Voltage regulators / buck converters: small boards that step a higher voltage (e.g. 7.4 V) down to 5 V efficiently.

A typical beginner robot power plan

 6× AA (≈9 V) ──┬──▶ Motor driver (VM) ──▶ 2 DC motors
                │
                └──▶ Arduino VIN  (regulated to 5 V on board)

 GND ───────────── all GNDs joined ──────────────

Examples

Example: planning a robot car’s battery

Part	Current
2 × gear motors (driving)	2 × 250 mA = 500 mA
Arduino UNO	50 mA
Ultrasonic + line sensors	30 mA
Total	580 mA

With a 2000 mAh pack: 2000 ÷ 580 ≈ 3.4 h in theory, about 2–2.5 h in practice.

Example: why the robot resets

Motors start and pull 1.5 A for a moment. The battery voltage dips from 6 V to 4 V, below what the Arduino’s regulator needs, so the board resets. Fix: a separate battery for the Arduino, or a bigger battery plus a large capacitor (e.g. 470 µF) near the motor driver.

Practice

Run timeEasy

A 1200 mAh pack powers a robot drawing 400 mA. How long will it run, in theory? 1200 ÷ 400 = 3 hours (expect about 2 in real life).

Pick the packMedium

Your Arduino needs 7–12 V on VIN and your motors want about 6–9 V. Which AA pack works for both? 6 × AA (about 9 V with alkalines, 7.2 V with NiMH). It’s in range for VIN and for the motor driver.

Fix the resetsHard

List three different fixes for an Arduino that resets every time the motors start. Fresh or bigger batteries; separate batteries for logic and motors (grounds joined); a large capacitor across the motor driver supply; start motors gradually instead of at full power.

Quick check

1. A 1,500 mAh battery powers a robot drawing 300 mA. Roughly how long will it run (in theory)?

0.2 hours5 hours450 hours1,800 hours

2. Why shouldn’t you power a motor straight from an Arduino pin?

Motors need much more current than a pin can supplyMotors only run on ACArduino pins are 100 VIt is fine to do this

3. Your Arduino resets every time the motors start. A likely cause is…

The code is too longVoltage dips because motors and Arduino share a weak supplyThe LED is backwardsToo many comments

4. Why must all the grounds be connected together?

To make the robot heavierSo every part shares the same 0 V referenceTo double the voltageThey must be kept separate

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