Bunker battery gauge on an Arduino Uno

// Bunker battery gauge: how much is left in the battery, and how long will it
// last at this load? An INA219, a traffic light and a piezo on an Uno.
//
//   INA219    VIN+ <- battery +, VIN- -> the load -> battery -, on I2C (A4, A5)
//   traffic light   green -> A0, yellow -> A1, red -> A2
//   piezo     pin 9 and GND
//
// The traffic light is the gauge you read from across the room: green above
// 50 percent, yellow from 20 to 49, red below 20. The Serial Monitor has the
// numbers, once a second.
//
// The battery's minus, the INA219's GND and the Uno's GND are the same wire.
// The load stands in for the bunker's lamps and radio. Slide the battery's
// charge down in the properties panel and watch the percent and the time left
// fall; make the load heavier and the time left shortens faster than the
// percent does.
//
// # Volts are not a fuel gauge, but they are close enough
//
// A lithium cell sits at 4.2 V full and 3.0 V empty, but not in a straight
// line: it drops quickly at the top, lies almost flat through the middle and
// falls off a cliff at the bottom. So the sketch keeps a table of eleven
// points, one every ten percent, finds the two the reading is between and
// slides along the line joining them. That is linear interpolation, and it is
// how most battery gauges that have no coulomb counter work.
//
// The reading is taken under load, which pulls the battery's terminal volts
// down a little, so the gauge reads slightly low while the lights are on.
// That is honest: the battery really is working harder.
//
// # A smoothed current
//
// Loads flicker. Dividing the charge left by a current that jumps about gives
// a time left that jumps about as well. The sketch keeps an exponential moving
// average: each second the average moves one eighth of the way to the new
// reading. One line, no array, and a single noisy sample barely shows.

#include <Wire.h>
#include <Adafruit_INA219.h>

const uint8_t PIEZO = 9;
const uint8_t GREEN_LED = A0;
const uint8_t YELLOW_LED = A1;
const uint8_t RED_LED = A2;
const int CAPACITY_MAH = 2000;  // the battery's label
const int LOW_PERCENT = 20;

// Millivolts at 0, 10, 20 ... 100 percent for one lithium cell.
const int CURVE[11] = {3000, 3300, 3450, 3550, 3620, 3680, 3750, 3830, 3930, 4050, 4200};

Adafruit_INA219 ina;

float averageMa = 0;
bool firstReading = true;
unsigned long lastRead = 0;
unsigned long lastBeep = 0;
bool warned = false;

int percentFor(int mv) {
  if (mv <= CURVE[0]) return 0;
  if (mv >= CURVE[10]) return 100;
  int i = 0;
  while (mv >= CURVE[i + 1]) i++;
  // how far between point i and point i + 1, in tenths of a step
  long into = (long)(mv - CURVE[i]) * 10 / (CURVE[i + 1] - CURVE[i]);
  return i * 10 + (int)into;
}

void setup() {
  Serial.begin(115200);
  pinMode(PIEZO, OUTPUT);
  pinMode(GREEN_LED, OUTPUT);
  pinMode(YELLOW_LED, OUTPUT);
  pinMode(RED_LED, OUTPUT);
  ina.begin();
  Serial.println("bunker battery gauge ready");
}

void loop() {
  unsigned long now = millis();
  if (now - lastRead < 1000 && !firstReading) return;
  lastRead = now;

  int mv = (int)((ina.getBusVoltage_V() + ina.getShuntVoltage_mV() / 1000.0) * 1000.0);
  float ma = ina.getCurrent_mA();
  if (ma < 0) ma = -ma;
  if (firstReading) {
    averageMa = ma;
    firstReading = false;
  } else {
    averageMa += (ma - averageMa) / 8.0;
  }

  int percent = percentFor(mv);
  long leftMah = (long)CAPACITY_MAH * percent / 100;
  long minutes = averageMa > 1 ? (long)(leftMah * 60.0 / averageMa) : 0;

  Serial.print("battery ");
  Serial.print(percent);
  Serial.print("% ");
  Serial.print(mv);
  Serial.print(" mV ");
  Serial.print((int)averageMa);
  Serial.print(" mA ");
  Serial.print(minutes / 60);
  Serial.print("h");
  Serial.print(minutes % 60);
  Serial.println("m left");

  bool low = percent < LOW_PERCENT;
  digitalWrite(GREEN_LED, percent >= 50);
  digitalWrite(YELLOW_LED, percent >= LOW_PERCENT && percent < 50);
  digitalWrite(RED_LED, low);
  if (low && !warned) {
    warned = true;
    Serial.println("LOW BATTERY: save the lights for the night");
  }
  if (!low) warned = false;
  if (low && now - lastBeep > 5000) {
    lastBeep = now;
    tone(PIEZO, 880, 200);
  }
}