18650 battery capacity tester with discharge graph and SD log (ESP32-S3)

// 18650 capacity tester — ESP32-S3-DevKitC-1 + INA219 + 2.8" ILI9341 + microSD + relay.
// The cell discharges through a 5 ohm / 10 W load while the INA219 measures volts and amps.
// The S3 adds up mAh and mWh every second, draws the discharge curve on the TFT, logs a CSV
// to the microSD card and opens the relay at 3.00 V so the cell is never over-discharged.
// Wiring: INA219 SDA 8 / SCL 9 (default Wire), VIN+ from cell +, VIN- to relay COM.
// SPI (FSPI defaults): SCK 12, MOSI 11, MISO 13 (SD card only). TFT CS 10, DC 4, RESET 5.
// SD CS 6. Relay IN 7 (active-LOW). Start button GPIO2 to GND.
#include <Wire.h>
#include <SPI.h>
#include <SD.h>
#include <Adafruit_INA219.h>
#include <Adafruit_GFX.h>
#include <Adafruit_ILI9341.h>

constexpr int TFT_CS = 10, TFT_DC = 4, TFT_RST = 5, SD_CS = 6, RELAY = 7, BTN = 2;
constexpr float CUTOFF_V = 3.00;  // stop here (under load); 2.75 V is the absolute floor for most cells
constexpr float MIN_START_V = 3.30;

Adafruit_INA219 ina;
Adafruit_ILI9341 tft(TFT_CS, TFT_DC, TFT_RST);
enum State { IDLE, RUNNING, DONE } state = IDLE;
bool sdOk = false;
File logFile;
uint32_t startMs = 0, lastSample = 0;
double mAh = 0, mWh = 0;
float vOpen = 0, rInt = 0;
int lastX = -1, lastY = -1;

void relay(bool on) { digitalWrite(RELAY, on ? LOW : HIGH); }  // active-LOW module

void header(const char* msg, uint16_t col) {
  tft.fillRect(0, 0, 320, 24, col);
  tft.setTextColor(ILI9341_BLACK);
  tft.setTextSize(2);
  tft.setCursor(6, 4);
  tft.print(msg);
}

void axes() {
  tft.fillRect(0, 120, 320, 120, ILI9341_BLACK);
  tft.drawRect(30, 124, 286, 110, ILI9341_DARKGREY);
  tft.setTextSize(1);
  tft.setTextColor(ILI9341_DARKGREY);
  tft.setCursor(0, 124);
  tft.print("4.2V");
  tft.setCursor(0, 226);
  tft.print("3.0V");
  lastX = lastY = -1;
}

void numbers(float v, float mA, uint32_t secs) {
  tft.fillRect(0, 28, 320, 90, ILI9341_BLACK);
  tft.setTextColor(ILI9341_WHITE);
  tft.setTextSize(3);
  tft.setCursor(6, 32);
  tft.printf("%.3f V", v);
  tft.setCursor(170, 32);
  tft.printf("%4.0fmA", mA);
  tft.setTextColor(ILI9341_GREEN);
  tft.setCursor(6, 64);
  tft.printf("%.0f mAh", mAh);
  tft.setTextSize(2);
  tft.setTextColor(ILI9341_LIGHTGREY);
  tft.setCursor(6, 96);
  tft.printf("%.2f Wh  %02lu:%02lu:%02lu  R %.0fmO", mWh / 1000.0, secs / 3600, secs / 60 % 60, secs % 60, rInt * 1000);
}

// One point per minute; the x axis covers 286 minutes (enough for a 3000 mAh cell at 0.7 A).
void plot(float v, uint32_t secs) {
  int x = 31 + min<uint32_t>(secs / 60, 284);
  int y = 232 - (int)((constrain(v, 3.0f, 4.2f) - 3.0f) / 1.2f * 106);
  if (lastX >= 0) tft.drawLine(lastX, lastY, x, y, ILI9341_YELLOW);
  lastX = x;
  lastY = y;
}

void startTest() {
  vOpen = ina.getBusVoltage_V();
  if (vOpen < MIN_START_V) {
    header("Cell too low / missing", ILI9341_ORANGE);
    Serial.printf("Not starting: %.2f V (need %.2f V)\n", vOpen, MIN_START_V);
    return;
  }
  relay(true);
  delay(2000);  // let the voltage settle under load
  float vLoad = ina.getBusVoltage_V();
  float a = ina.getCurrent_mA() / 1000.0;
  rInt = a > 0.05 ? (vOpen - vLoad) / a : 0;  // includes holder and wire resistance
  mAh = mWh = 0;
  startMs = lastSample = millis();
  if (sdOk) {
    char name[24];
    for (int i = 1; i < 1000; i++) {
      snprintf(name, sizeof name, "/cell_%03d.csv", i);
      if (!SD.exists(name)) break;
    }
    logFile = SD.open(name, FILE_WRITE);
    if (logFile) logFile.printf("# open %.3f V, internal %.0f mOhm\nseconds,volts,mA,mAh\n", vOpen, rInt * 1000);
    Serial.printf("Logging to %s\n", name);
  }
  state = RUNNING;
  header("Discharging...", ILI9341_CYAN);
  axes();
  Serial.printf("Start: open %.3f V, loaded %.3f V, %.0f mA, R %.0f mOhm\n", vOpen, vLoad, a * 1000, rInt * 1000);
  Serial.println("seconds,volts,mA,mAh");
}

void setup() {
  Serial.begin(115200);
  pinMode(RELAY, OUTPUT);
  relay(false);
  pinMode(BTN, INPUT_PULLUP);
  Wire.begin(8, 9);
  if (!ina.begin()) Serial.println("INA219 not found at 0x40 - check SDA 8 / SCL 9");
  ina.setCalibration_32V_2A();
  SPI.begin(12, 13, 11);
  tft.begin(40000000);
  tft.setRotation(1);
  tft.fillScreen(ILI9341_BLACK);
  sdOk = SD.begin(SD_CS, SPI, 20000000);
  if (!sdOk) Serial.println("No microSD card - testing without a log file");
  header("Insert cell, press START", ILI9341_WHITE);
  axes();
}

void loop() {
  static bool lastBtn = HIGH;
  bool b = digitalRead(BTN);
  if (b == LOW && lastBtn == HIGH) {
    delay(30);
    if (state == RUNNING) {  // a second press stops the test early
      relay(false);
      state = DONE;
      header("Stopped by button", ILI9341_ORANGE);
      if (logFile) logFile.close();
    } else startTest();
  }
  lastBtn = b;

  if (millis() - lastSample < 1000) return;
  uint32_t dt = millis() - lastSample;
  lastSample = millis();
  float v = ina.getBusVoltage_V();
  float mA = state == RUNNING ? ina.getCurrent_mA() : 0;
  uint32_t secs = state == IDLE ? 0 : (lastSample - startMs) / 1000;
  if (state == RUNNING) {
    mAh += mA * dt / 3600000.0;
    mWh += mA * v * dt / 3600000.0;
    Serial.printf("%lu,%.3f,%.0f,%.1f\n", secs, v, mA, mAh);
    if (logFile && secs % 10 == 0) {
      logFile.printf("%lu,%.3f,%.0f,%.1f\n", secs, v, mA, mAh);
      logFile.flush();
    }
    if (secs % 60 == 0) plot(v, secs);
    static uint8_t low = 0;
    low = v < CUTOFF_V ? low + 1 : 0;
    if (low >= 3) {  // three readings in a row, so one noisy sample doesn't end the test
      relay(false);
      state = DONE;
      header("Done - cell rested", ILI9341_GREEN);
      if (logFile) {
        logFile.printf("# capacity %.0f mAh, %.2f Wh\n", mAh, mWh / 1000.0);
        logFile.close();
      }
      Serial.printf("Cut-off at %.3f V: %.0f mAh, %.2f Wh in %lu s\n", v, mAh, mWh / 1000.0, secs);
    }
  }
  if (state != DONE) numbers(v, mA, secs);
}

// 18650 capacity tester — ESP32-S3-DevKitC-1 + INA219 + 2.8" ILI9341 + microSD + relay.
// The cell discharges through a 5 ohm / 10 W load while the INA219 measures volts and amps.
// The S3 adds up mAh and mWh every second, draws the discharge curve on the TFT, logs a CSV
// to the microSD card and opens the relay at 3.00 V so the cell is never over-discharged.
// Wiring: INA219 SDA 8 / SCL 9 (default Wire), VIN+ from cell +, VIN- to relay COM.
// SPI (FSPI defaults): SCK 12, MOSI 11, MISO 13 (SD card only). TFT CS 10, DC 4, RESET 5.
// SD CS 6. Relay IN 7 (active-LOW). Start button GPIO2 to GND.
#include <Wire.h>
#include <SPI.h>
#include <SD.h>
#include <Adafruit_INA219.h>
#include <Adafruit_GFX.h>
#include <Adafruit_ILI9341.h>

constexpr int TFT_CS = 10, TFT_DC = 4, TFT_RST = 5, SD_CS = 6, RELAY = 7, BTN = 2;
constexpr float CUTOFF_V = 3.00;  // stop here (under load); 2.75 V is the absolute floor for most cells
constexpr float MIN_START_V = 3.30;

Adafruit_INA219 ina;
Adafruit_ILI9341 tft(TFT_CS, TFT_DC, TFT_RST);
enum State { IDLE, RUNNING, DONE } state = IDLE;
bool sdOk = false;
File logFile;
uint32_t startMs = 0, lastSample = 0;
double mAh = 0, mWh = 0;
float vOpen = 0, rInt = 0;
int lastX = -1, lastY = -1;

void relay(bool on) { digitalWrite(RELAY, on ? LOW : HIGH); }  // active-LOW module

void header(const char* msg, uint16_t col) {
  tft.fillRect(0, 0, 320, 24, col);
  tft.setTextColor(ILI9341_BLACK);
  tft.setTextSize(2);
  tft.setCursor(6, 4);
  tft.print(msg);
}

void axes() {
  tft.fillRect(0, 120, 320, 120, ILI9341_BLACK);
  tft.drawRect(30, 124, 286, 110, ILI9341_DARKGREY);
  tft.setTextSize(1);
  tft.setTextColor(ILI9341_DARKGREY);
  tft.setCursor(0, 124);
  tft.print("4.2V");
  tft.setCursor(0, 226);
  tft.print("3.0V");
  lastX = lastY = -1;
}

void numbers(float v, float mA, uint32_t secs) {
  tft.fillRect(0, 28, 320, 90, ILI9341_BLACK);
  tft.setTextColor(ILI9341_WHITE);
  tft.setTextSize(3);
  tft.setCursor(6, 32);
  tft.printf("%.3f V", v);
  tft.setCursor(170, 32);
  tft.printf("%4.0fmA", mA);
  tft.setTextColor(ILI9341_GREEN);
  tft.setCursor(6, 64);
  tft.printf("%.0f mAh", mAh);
  tft.setTextSize(2);
  tft.setTextColor(ILI9341_LIGHTGREY);
  tft.setCursor(6, 96);
  tft.printf("%.2f Wh  %02lu:%02lu:%02lu  R %.0fmO", mWh / 1000.0, secs / 3600, secs / 60 % 60, secs % 60, rInt * 1000);
}

// One point per minute; the x axis covers 286 minutes (enough for a 3000 mAh cell at 0.7 A).
void plot(float v, uint32_t secs) {
  int x = 31 + min<uint32_t>(secs / 60, 284);
  int y = 232 - (int)((constrain(v, 3.0f, 4.2f) - 3.0f) / 1.2f * 106);
  if (lastX >= 0) tft.drawLine(lastX, lastY, x, y, ILI9341_YELLOW);
  lastX = x;
  lastY = y;
}

void startTest() {
  vOpen = ina.getBusVoltage_V();
  if (vOpen < MIN_START_V) {
    header("Cell too low / missing", ILI9341_ORANGE);
    Serial.printf("Not starting: %.2f V (need %.2f V)\n", vOpen, MIN_START_V);
    return;
  }
  relay(true);
  delay(2000);  // let the voltage settle under load
  float vLoad = ina.getBusVoltage_V();
  float a = ina.getCurrent_mA() / 1000.0;
  rInt = a > 0.05 ? (vOpen - vLoad) / a : 0;  // includes holder and wire resistance
  mAh = mWh = 0;
  startMs = lastSample = millis();
  if (sdOk) {
    char name[24];
    for (int i = 1; i < 1000; i++) {
      snprintf(name, sizeof name, "/cell_%03d.csv", i);
      if (!SD.exists(name)) break;
    }
    logFile = SD.open(name, FILE_WRITE);
    if (logFile) logFile.printf("# open %.3f V, internal %.0f mOhm\nseconds,volts,mA,mAh\n", vOpen, rInt * 1000);
    Serial.printf("Logging to %s\n", name);
  }
  state = RUNNING;
  header("Discharging...", ILI9341_CYAN);
  axes();
  Serial.printf("Start: open %.3f V, loaded %.3f V, %.0f mA, R %.0f mOhm\n", vOpen, vLoad, a * 1000, rInt * 1000);
  Serial.println("seconds,volts,mA,mAh");
}

void setup() {
  Serial.begin(115200);
  pinMode(RELAY, OUTPUT);
  relay(false);
  pinMode(BTN, INPUT_PULLUP);
  Wire.begin(8, 9);
  if (!ina.begin()) Serial.println("INA219 not found at 0x40 - check SDA 8 / SCL 9");
  ina.setCalibration_32V_2A();
  SPI.begin(12, 13, 11);
  tft.begin(40000000);
  tft.setRotation(1);
  tft.fillScreen(ILI9341_BLACK);
  sdOk = SD.begin(SD_CS, SPI, 20000000);
  if (!sdOk) Serial.println("No microSD card - testing without a log file");
  header("Insert cell, press START", ILI9341_WHITE);
  axes();
}

void loop() {
  static bool lastBtn = HIGH;
  bool b = digitalRead(BTN);
  if (b == LOW && lastBtn == HIGH) {
    delay(30);
    if (state == RUNNING) {  // a second press stops the test early
      relay(false);
      state = DONE;
      header("Stopped by button", ILI9341_ORANGE);
      if (logFile) logFile.close();
    } else startTest();
  }
  lastBtn = b;

  if (millis() - lastSample < 1000) return;
  uint32_t dt = millis() - lastSample;
  lastSample = millis();
  float v = ina.getBusVoltage_V();
  float mA = state == RUNNING ? ina.getCurrent_mA() : 0;
  uint32_t secs = state == IDLE ? 0 : (lastSample - startMs) / 1000;
  if (state == RUNNING) {
    mAh += mA * dt / 3600000.0;
    mWh += mA * v * dt / 3600000.0;
    Serial.printf("%lu,%.3f,%.0f,%.1f\n", secs, v, mA, mAh);
    if (logFile && secs % 10 == 0) {
      logFile.printf("%lu,%.3f,%.0f,%.1f\n", secs, v, mA, mAh);
      logFile.flush();
    }
    if (secs % 60 == 0) plot(v, secs);
    static uint8_t low = 0;
    low = v < CUTOFF_V ? low + 1 : 0;
    if (low >= 3) {  // three readings in a row, so one noisy sample doesn't end the test
      relay(false);
      state = DONE;
      header("Done - cell rested", ILI9341_GREEN);
      if (logFile) {
        logFile.printf("# capacity %.0f mAh, %.2f Wh\n", mAh, mWh / 1000.0);
        logFile.close();
      }
      Serial.printf("Cut-off at %.3f V: %.0f mAh, %.2f Wh in %lu s\n", v, mAh, mWh / 1000.0, secs);
    }
  }
  if (state != DONE) numbers(v, mA, secs);
}