Measure (Voltage) using (Arduino uno) only !!

code:

const float referenceVolts = 5.0; // the default reference on a 5-volt

const int batteryPin = A0; // battery is connected to analog pin 0
void setup()
{
Serial.begin(9600);
}

// ██████  ███████ ██ ███████ ███████     ██████  ██████  ███    ███ 
//██  ████ ██      ██ ██      ██         ██      ██    ██ ████  ████ 
//██ ██ ██ ███████ ██ █████   █████      ██      ██    ██ ██ ████ ██ 
//████  ██      ██ ██ ██      ██         ██      ██    ██ ██  ██  ██ 
// ██████  ███████ ██ ███████ ██      ██  ██████  ██████  ██      ██ 

void loop()
{
int val = analogRead(batteryPin); // read the value from the sensor
float volts = (val / 1023.0) * referenceVolts; // calculate the ratio

Serial.println(volts); // print the value in volts
}

// ██████╗ ███████╗██╗███████╗███████╗    ██████╗ ██████╗ ███╗   ███╗
//██╔═████╗██╔════╝██║██╔════╝██╔════╝   ██╔════╝██╔═══██╗████╗ ████║
//██║██╔██║███████╗██║█████╗  █████╗     ██║     ██║   ██║██╔████╔██║
//████╔╝██║╚════██║██║██╔══╝  ██╔══╝     ██║     ██║   ██║██║╚██╔╝██║
//╚██████╔╝███████║██║███████╗██║  ██╗   ╚██████╗╚██████╔╝██║ ╚═╝ ██║
// ╚═════╝ ╚══════╝╚═╝╚══════╝╚═╝  ╚═╝   ╚═════╝ ╚═════╝ ╚═╝      ╚═╝

 

referance: Arduino_Cookbook_3rd_Edition_-_Nicholas_Robert_Weldin