When we are using a battery powered Arduino such as RC robots or Temperature Controller, we might want to check the battery voltage if it needs to be charged or replaced. It happens to me with my RC Panzer. Sometimes when my kids are about to run it, it moves very slow, low battery. Then they are disappointed and need to wait for charging time. I would rather had noticed this battery condition on the last run but I am too lazy to check it with multimeter.

Arduino Uno needs 5 volts power to run, then we need at least 7.4 volts to 9 volts battery. Since Arduino pins support only 5 volts maximum, then we need a Voltage Divider. It is simply made up of two resistors in series. To divide the voltage to half, we need two resistor with the same value. 1K to 20K resistors can be used, but the larger the resistance the lower the power consumed by the Voltage Divider. I am not that good in calculating such thing but that is what I summarize from sources I read. You can correct me if I am wrong and any better explanation to this is most welcome on the comment section.

Step 1: Bill of Materials

Arduino Uno or compatible.
Male pins (we need four pins only).
Two same value resistors (here I use 12K).
Two pin female connector. Photo shows three pin because that is what I have. Here I use power switch connector to motherboard instead :)
A battery (I use 7.4V lipo battery).
16x2 LCD with I2C adapter. Later on you can switch this to a red LED to indicate low battery at your desired voltage level.
A mini breadboard is optional for testing phase.

Step 2: Arduino Sketch

Well, I would like to upload this sketch to Arduino first before connecting it to a battery for testing. Uploading this will show you nothing before you connect all the parts needed for this project, but sooner or later you will still need to upload this sketch. I am not sure what will happen if you have power from usb and also from Vin at the same time. I guess it will be okay, Arduino designers must have think of this possibility and prevent this power conflict. But I will never try it on purpose and risking my Arduino to get burnt :P

In this instructable, I am not explaining about "how to get your LCD display works", but I will leave some links here (which I use) to get your LCD works through I2C connection:

// Print battery voltage
// to 16x2 LCD via I2C
// with Voltage Divider (2x 10K resistor)
/*
  Resistors are aligned in series.
  One end goes to Battery - and also to Arduino GND
  The other goes to Battery + and also to Arduino Vin
  The middle (connection between two resistors) goes to Arduino A0
*/

#include <Wire.h>
#include <LCD.h>
#include <LiquidCrystal_I2C.h>

#define I2C_ADDR    0x27 //Add your address here.  Find it from I2C Scanner
#define BACKLIGHT_PIN     3
#define En_pin  2
#define Rw_pin  1
#define Rs_pin  0
#define D4_pin  4
#define D5_pin  5
#define D6_pin  6
#define D7_pin  7
#define led_pin 13
LiquidCrystal_I2C lcd(I2C_ADDR,En_pin,Rw_pin,Rs_pin,D4_pin,D5_pin,D6_pin,D7_pin);

void setup()
{
  lcd.begin (16,2); //My LCD was 16x2
  lcd.setBacklightPin(BACKLIGHT_PIN,POSITIVE);
  lcd.setBacklight(HIGH);
  lcd.home (); // go home
  
  pinMode(led_pin, OUTPUT);
  digitalWrite(led_pin, LOW);
}

void loop()
{
  printVolts();
}
 
 void printVolts()
{
  int sensorValue = analogRead(A0); //read the A0 pin value
  float voltage = sensorValue * (5.00 / 1023.00) * 2; //convert the value to a true voltage.
  lcd.setCursor(0,0);
  lcd.print("voltage = ");
  lcd.print(voltage); //print the voltage to LCD
  lcd.print(" V");
  if (voltage < 6.50) //set the voltage considered low battery here
  {
    digitalWrite(led_pin, HIGH);
  }
}

Step 3: Circuit

The wire connections are simple as you can see on the images above.

Using 16x2 LCD and its I2C adapter:

Adapter GND to Arduino GND.
Adapter VCC to Arduino 5V.
Adapter SDA to Arduino A4 (or the pin next to AREF on Digital Pins side).
Adapter SCL to Arduino A5 (or the pin next to SDA, two pins from AREF on Digital Pins side).
Align two 10K resistors in series on breadboard.
Connect the middle of the resistors-in-series to Arduino A0.
Connect one end to Arduino GND and also to Battery - (negative).
Connect the other end to Arduino Vin and also to Battery + (positive). I think you should connect this one after you load the Arduino Sketch as I tell you my reason before.

Using a LED instead of LCD:

Connect LED anode (small piece inside) to Arduino D13.
Connect LED cathode (large piece inside) to GND (next to D13).
Align two 10K resistors in series on breadboard.
Connect the middle of the resistors-in-series to Arduino A0.
Connect one end to Arduino GND and also to Battery - (negative).
Connect the other end to Arduino Vin and also to Battery + (positive).

When you plug the battery to Arduino Vin, it should work right away showing the voltage of your battery on your 16x2 LCD because Arduino is powered by that battery. If it is not working, please re-check your connection or the battery you use might be lower than 5 volts needed by Arduino to power up. Please try another battery or check it with your voltmeter.

On my test with multimeter, the voltage shown on the LCD is slightly lower then the multimeter display. We are loosing around 0.05V to 0.15V on breadboard and Arduino circuits. But that is not a big problem for me (I don't know what about you), as long as I know whether my battery has enough power to run my robot. That's all.

Step 4: From Mini to Micro

Well, I don't want that "mini" breadboard goes along on my Panzer, then I make it "micro".

The first resistor : One end soldered to pin 1 from the left. The other end to pin 4 from the left.
The second resistor : One end soldered to pin 2 from the left. The other end to pin 4 from the left.
Soldered the connector inner pins to pin 1 and pin 2 from the left.
Put the black connector jacket on.
Pull out pin 3 from the left.

Well, now we have add-on to Arduino pin : GND, Vin, A0.

Step 5: Re-Connect and Run

Now re-connect the LCD and Battery, we have simpler connection without breadboard.

Adapter GND to Arduino GND.
Adapter VCC to Arduino 5V.
Adapter SDA to Arduino A4 (or the pin next to AREF on Digital Pins side).
Adapter SCL to Arduino A5 (or the pin next to SDA, two pins from AREF on Digital Pins side).
Battery - (negative) to Arduino GND (on our add-on pins).
Battery + (positive) to Arduino Vin (on our add-on pins).

On my test I lost 0.1V and pretty stable. Actually we lost only 0.05V on Arduino circuit. 7.79V is shown on my multimeter. Voltage Divider reduced it to half, that is 3.89V entering the A0 pin. The Arduino reads 3.84V. Then we double it to show the exact voltage back, that is 7.68V.

We can fix this in the sketch, but we need more data population to see the stable voltage lost. One more question is : "Is it my multimeter that is not accurate? Because I bought a cheap one."

Again, I don't mind that little difference as long as I know my battery is fit enough to run my RC toys :)