Get Arduino Battery Status

batteryStatus.ino

// Function created to obtain chip's actual Vcc voltage value, using internal bandgap reference
// This demonstrates ability to read processors Vcc voltage and the ability to maintain A/D calibration with changing Vcc
// Now works for 168/328 and mega boards.
// Thanks to "Coding Badly" for direct register control for A/D mux
// 1/9/10 "retrolefty"

int battVolts;   // made global for wider avaliblity throughout a sketch if needed, example a low voltage alarm, etc

void setup(void)
{
    Serial.begin(38400);
    Serial.print("volts X 100");
    Serial.println( "\r\n\r\n" );

    delay(100);
}

void loop(void)
{
    battVolts=getBandgap();  //Determins what actual Vcc is, (X 100), based on known bandgap voltage
    
    Serial.print("Battery Vcc volts =  ");
    Serial.println(battVolts);
    Serial.print("Analog pin 0 voltage = ");
    Serial.println(map(analogRead(0), 0, 1023, 0, battVolts));
    Serial.println();    
    
    delay(1000);
}

int getBandgap(void) // Returns actual value of Vcc (x 100)
{
    // For 168/328 boards
    const long InternalReferenceVoltage = 1056L;  // Adjust this value to your boards specific internal BG voltage x1000

    // REFS1 REFS0          --> 0 1, AVcc internal ref. -Selects AVcc external reference
    // MUX3 MUX2 MUX1 MUX0  --> 1110 1.1V (VBG)         -Selects channel 14, bandgap voltage, to measure
    ADMUX = (0<<REFS1) | (1<<REFS0) | (0<<ADLAR) | (1<<MUX3) | (1<<MUX2) | (1<<MUX1) | (0<<MUX0);

    delay(50);  // Let mux settle a little to get a more stable A/D conversion
    
    // Start a conversion  
    ADCSRA |= _BV( ADSC );
    
    // Wait for it to complete
    while( ( (ADCSRA & (1<<ADSC)) != 0 ) );
    
    // Scale the value
    int results = (((InternalReferenceVoltage * 1024L) / ADC) + 5L) / 10L; // calculates for straight line value 
    
    return results;
}