Solar battery monitor with an INA3221 on an Arduino Uno

// Solar battery monitor: one INA3221 watches three places in a small solar
// power system at once, the panel, the battery and the load, and a 20x4 I2C
// LCD shows each one's volts and current and whether the battery is charging.
//
//   INA3221    VCC 5V, GND, SDA A4, SCL A5, address 0x40 (A0 to GND)
//   channel 1  the panel:   panel + -> IN1+, IN1- -> TP4056 IN+
//   channel 2  the battery: TP4056 BAT+ -> IN2+, IN2- -> the 18650's +
//   channel 3  the load:    IN2+ jumpered to IN3+, IN3- -> the lamp
//   LCD        20x4 with an I2C backpack at 0x27: SDA, SCL, VCC 5V, GND
//   Every - and GND joins: the panel, the TP4056, the cell, the lamp and the Uno.
//
// HOW IT WORKS
//   Each channel has a 0.1 ohm shunt in series. The INA3221 measures the few
//   millivolts across it (40 uV a step) and the voltage on its IN- side to
//   ground (8 mV a step); the library divides the shunt voltage by the shunt
//   resistance for the current. Channel 2 is signed: plus while the TP4056
//   charges the cell, minus while the cell carries the load.
//
//   The TP4056's output feeds both the cell and the load, so in sunshine the
//   panel runs the lamp and charges the cell with what is left, and in the
//   dark the cell runs the lamp on its own.
//
// WHAT TO CHANGE
//   SHUNT_OHMS    0.1 on the common purple and CJMCU boards; some have 0.01
//   IDLE_MA       below this the battery counts as neither charging nor in use

#include <Wire.h>
#include <LiquidCrystal_I2C.h>
#include "INA3221.h"

INA3221 ina(0x40);
LiquidCrystal_I2C lcd(0x27, 20, 4);

const float SHUNT_OHMS = 0.1;
const float IDLE_MA = 10;

const uint8_t SOLAR = 0;   // the library numbers the channels 0 to 2
const uint8_t BATTERY = 1;
const uint8_t LOAD = 2;

void showChannel(uint8_t row, const char *name, float volts, float mA, bool sign) {
  char v[8];
  char line[21];
  dtostrf(volts, 5, 2, v);
  int ma = (int)(mA + (mA < 0 ? -0.5 : 0.5));
  if (sign) snprintf(line, sizeof line, "%-6s%sV %+5dmA ", name, v, ma);
  else snprintf(line, sizeof line, "%-6s%sV %5dmA ", name, v, ma);
  lcd.setCursor(0, row);
  lcd.print(line);
}

void setup() {
  Serial.begin(115200);
  Wire.begin();
  lcd.init();
  lcd.backlight();
  if (!ina.begin()) {
    Serial.println("INA3221 not found at 0x40: check SDA, SCL and VCC");
    lcd.print("No INA3221 at 0x40");
    while (1);
  }
  for (uint8_t ch = 0; ch < 3; ch++) ina.setShuntR(ch, SHUNT_OHMS);
  Serial.println("solar monitor ready");
}

void loop() {
  float solarV = ina.getBusVoltage(SOLAR);
  float solarMa = ina.getCurrent_mA(SOLAR);
  float battV = ina.getBusVoltage(BATTERY);
  float battMa = ina.getCurrent_mA(BATTERY);
  float loadV = ina.getBusVoltage(LOAD);
  float loadMa = ina.getCurrent_mA(LOAD);

  showChannel(0, "Solar", solarV, solarMa, false);
  showChannel(1, "Batt", battV, battMa, true);
  showChannel(2, "Load", loadV, loadMa, false);

  const char *state = "Idle";
  if (battMa > IDLE_MA) state = "Charging";
  else if (battMa < -IDLE_MA) state = "On battery";
  char watts[8];
  dtostrf(solarV * solarMa / 1000.0, 4, 2, watts);
  char line[21];
  snprintf(line, sizeof line, "%-10s Sun%sW", state, watts);
  lcd.setCursor(0, 3);
  lcd.print(line);

  Serial.print("solar ");
  Serial.print(solarV, 2);
  Serial.print(" V ");
  Serial.print(solarMa, 0);
  Serial.print(" mA  battery ");
  Serial.print(battV, 2);
  Serial.print(" V ");
  Serial.print(battMa, 0);
  Serial.print(" mA  load ");
  Serial.print(loadV, 2);
  Serial.print(" V ");
  Serial.print(loadMa, 0);
  Serial.print(" mA  ");
  Serial.println(state);

  delay(1000);
}