see: https://medium.com/p/315dd25e0c3b/edit

_35_ProMini_StepDownConverter_DRV8825_StepperMotor_BatMon_03.ino

#define DIR 2
#define STEP 3
#define BATT A0

#define M0 4
#define M1 5
#define M2 6
#define _TRUE 1
#define _FALSE 0


int dPause    =  100;
int dirLevel  = HIGH;

int battery_voltage;
byte low_bat = 0;

/* in production comment debug = _TRUE */

boolean debug = _TRUE;
//boolean debug = _FALSE;


void setup() {

  pinMode(DIR, OUTPUT);
  pinMode(STEP, OUTPUT);
  pinMode(M0, OUTPUT);
  pinMode(M1, OUTPUT);
  pinMode(M2, OUTPUT);

  pinMode( BUTTON, INPUT );

  if (debug) {

    Serial.begin(9600);
    Serial.println( "BATT Test: Stop Range level 8v < -- > 10v5" );
    Serial.println( "BATT Output Must be above > 1050" );

  }

}

void loop()
{
  /*
    Load the battery voltage to the battery_voltage variable.
    Resistor voltage divider => (3.3k + 3.3k)/2.2k = 2.5
    12.5V equals ~5V @ Analog 0.
    12.5V equals 1023 analogRead(0).
    1250 / 1023 ~= 1.222
    The variable battery_voltage holds 1050 if the battery voltage is 10.5V.
    DEBUG: Please, to debug batt voltage keep USB TTL cable connected:
    when the Arduino is transmitting the USB TTL adapter will receive.
    If battery_voltage is below 10.5V and higher than 8.0V
    Turn on the led if battery voltage is to low
    Set the low_bat variable to 1 - for future routine
  */
  
  battery_voltage = analogRead(BATT) * (1250 / 1023.);

  if (debug) {
    Serial.println(battery_voltage);
    delay(2000);
  }

  if (battery_voltage < 1050 && battery_voltage > 800) {
    digitalWrite(13, HIGH);
    low_bat = 1;
  }

  delay(10);

  /* motor  FULL throttle */

  digitalWrite(M0, LOW );
  digitalWrite(M1, LOW  );
  digitalWrite(M2, LOW  );

  digitalWrite( DIR, dirLevel );

  /* Motor  rotates if there is enough battery power.
    Otherwise the LED 13 lights indicating that the battery is discharged */

  if (!low_bat)
  {
    stepGo();
  }

}

void stepGo() {

  digitalWrite(STEP, HIGH);
  delayMicroseconds(dPause);
  digitalWrite(STEP, LOW);
  delayMicroseconds(dPause);

}