How to Make a Simple Arduino Battery Monitor for Under

Making a battery monitor with an Arduino is an easy and inexpensive way to keep track of your battery's voltage and avoid draining it too low. With just a few basic components, you can build a simple yet effective Arduino battery monitor for under $5.

What You Will Need

To build your own Arduino battery monitor, you will need just a few parts:

That's it! These basic components are all you need to build your own Arduino battery monitor on the cheap.

How a Voltage Divider Works

The key component of the battery monitor is the voltage divider circuit. This lowers the battery voltage to a safe level that can be handled by the Arduino's analog input pins.

A voltage divider uses two resistors to divide a voltage into a lower value. It works by creating a voltage drop across each resistor, proportional to their resistance.

For example, if R1 is 10 kΩ and R2 is 20 kΩ, the voltage will be divided by 3, because 20k / (10k + 20k) = 0.66. So if the input voltage is 9V, the output would be 9V * 0.66 = 6V.

We can use this to convert a 3-7V battery voltage into the 0-5V range usable by the Arduino.

Choosing Resistor Values

To choose resistor values for the voltage divider, we first need to know the max voltage we want to measure. For a standard 3.7V LiPo battery, 4.2V is a full charge, while 3V is completely discharged.

To convert 4.2V into the Arduino's 0-5V range, we can divide by 1.2 to give us a max voltage of 5V/1.2 = 4.17V.

Using the voltage divider formula, if R1 is 10k and R2 is 24k, the output will be:

Vout = Vin * (R2 / (R1 + R2))

Vout = 4.2V * (24k / (10k + 24k)) = 4.17V

This scales the max battery voltage to the Arduino's input range.

Adding Protection with a Zener Diode

The Arduino's analog inputs can safely handle 0-5V, but they can be damaged by higher voltages. To protect from overvoltage, we add a Zener diode.

A Zener diode conducts electricity once the voltage exceeds its rated breakdown voltage. We can use one with a 5.1V breakdown, so any voltage over 5.1V will be shunted through the diode instead of the Arduino.

The Zener diode ensures the Arduino only sees 0-5V from the battery pack.

Building the Circuit

With the components selected, we can now build the circuit:

This divides the battery voltage while protecting the Arduino from overvoltage.

Here is a circuit diagram showing how it is wired together:

Reading the Voltage with the Arduino

With the circuit built, we can now use the Arduino to read the divided battery voltage on the analog input pin.

The Arduino has a 10-bit analog-to-digital converter, giving 1024 steps between 0-5V. So each step equals ~5mV.

By reading the analog input value, we can convert it back into the actual battery voltage:

int sensorValue = analogRead(A0); // Read analog voltage
float volt = sensorValue * (5.0 / 1023.0) * 4; // Convert back to battery voltage

This converts the 0-1023 analog reading back into the 0-4.2V battery range.

We can display the voltage using Serial.print() to help monitor battery drain.

Adding a Voltage Display

For a better visual display, we can add an LED bar graph module. These use LEDs to display a voltage value and are easy to wire up.

Simply connect power, ground, and the voltage monitor signal to the module. Then calibrate it for the 0-4.2V range. Now the battery voltage will be displayed with a neat LED bar indicator.

Adding a display makes it easy to monitor the battery state at a glance.


Building a DIY Arduino battery monitor is an easy and inexpensive project using just a few basic parts. The voltage divider reduces the battery voltage to a safe level for the Arduino, while the Zener diode provides overvoltage protection.

With a simple sketch, the Arduino can read the voltage and display it on a serial monitor, LCD screen, or LED bar graph. This allows you to keep tabs on your battery's state of charge and avoid draining it completely.

So if you need an easy way to monitor your project's battery life, an Arduino voltage divider circuit is a great option requiring minimal components for under $5!