Monitoring battery power

in your code you need to check this value e.g. once per hour or so

some code inspiration

unsigned long previous = 0;

void loop()
{
  if (millis() - previous > (3600L * 1000L))  // or other timing
  previous = millis();
  {
    int voltage = analogRead(A0) * 12 * somefactor /1024;  // somefactor to be determined
    // write to display
    if (voltage < 10 )  
    {
       // alarm!!!
    }
  }
  // additional things here
}