Battery level indicator on an Arduino Uno

// Battery level indicator: an 18650 lithium-ion cell's voltage on A0, its
// charge on a 16x2 I2C LCD and a traffic light module on an Arduino Uno.
//
//   A0      the cell's plus; the cell's minus on GND
//   2, 3, 4 the traffic light module's G, Y and R
//   LCD     SDA -> A4    SCL -> A5    VCC -> 5 V    GND -> GND
//
// One 18650 is 4.2 V full and 3.0 V empty, under the Uno's 5 V reference, so
// it goes straight into A0: no divider. analogRead() counts 0 to 1023 for 0 to
// 5 V, so the voltage is the count times 5 / 1023. A cell above 5 V (two
// 18650s, a 9 V battery) needs a divider in front of A0 and the sum scaled up.
//
// The charge is a straight line from 3.0 V (0 %) to 4.2 V (100 %), which is
// how the simulated cell behaves. A real cell's voltage is flatter in the
// middle and falls fast at the ends, so on the bench a straight line reads a
// little low at first and drops quickly near empty; a real gauge measures
// the current in and out instead.
//
//   green   over 50 %
//   yellow  20 % to 50 %
//   red     under 20 %, blinking: charge it
//
// Press Run, then drag the charge slider on the battery.

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

const int BATTERY_PIN = A0;
const int GREEN_PIN = 2;
const int YELLOW_PIN = 3;
const int RED_PIN = 4;

const float EMPTY_V = 3.0;
const float FULL_V = 4.2;

LiquidCrystal_I2C lcd(0x27, 16, 2);
bool blink = false;

void setup() {
  pinMode(GREEN_PIN, OUTPUT);
  pinMode(YELLOW_PIN, OUTPUT);
  pinMode(RED_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Battery level indicator");
  lcd.init();
  lcd.backlight();
  lcd.print("18650 battery");
}

void loop() {
  // Average 16 readings to steady the last digit.
  long sum = 0;
  for (int i = 0; i < 16; i++) sum += analogRead(BATTERY_PIN);
  float volts = sum / 16.0 * 5.0 / 1023.0;

  int percent = (int)((volts - EMPTY_V) / (FULL_V - EMPTY_V) * 100.0 + 0.5);
  percent = constrain(percent, 0, 100);

  blink = !blink;
  digitalWrite(GREEN_PIN, percent > 50 ? HIGH : LOW);
  digitalWrite(YELLOW_PIN, percent >= 20 && percent <= 50 ? HIGH : LOW);
  digitalWrite(RED_PIN, percent < 20 && blink ? HIGH : LOW);

  lcd.setCursor(0, 1);
  lcd.print(volts, 2);
  lcd.print(" V  ");
  lcd.print(percent);
  lcd.print(" %    ");

  Serial.print(volts, 2);
  Serial.print(" V  ");
  Serial.print(percent);
  Serial.println(" %");
  delay(500);
}