Battery Capacity Testing: Build a Discharge Tester

That 18650 cell claiming 3000mAh – is it genuine or a relabelled 800mAh cell? The only way to know is to test it. A battery discharge tester drains the cell at a known rate while measuring voltage and current, giving you the true capacity. This guide shows you how to build one using an Arduino and a few common components available in India.

Why Test Battery Capacity

India’s electronics market is flooded with counterfeit and relabelled lithium cells. A ₹50 cell claiming 9900mAh might actually deliver only 500mAh. Testing every cell before using it in a project protects you from wasted money and dangerous battery packs. Mismatched cells in a pack cause uneven discharge, reducing pack life and creating safety risks.

How Discharge Testing Works

A discharge tester works by drawing a constant current from the battery while monitoring its voltage. When the voltage drops to the safe minimum (typically 2.8V for Li-ion), the test stops. The total mAh delivered is calculated by multiplying current (in amps) by time (in hours).

Formula: Capacity (mAh) = Discharge Current (mA) x Time (hours)

For example, if a cell delivers 0.5A for 4.2 hours before hitting 2.8V, its capacity is 500mA x 4.2h = 2100mAh.

Components Needed

You will need the following components:

• Arduino Nano or Uno
• INA219 current/voltage sensor module
• Power MOSFET (IRLZ44N or IRF540) for load control
• Power resistors (2-5 ohm, 10W or higher) as discharge load
• 18650 battery holder
• OLED display (optional, for real-time readout)
• Cooling fan (recommended for sustained testing)

Building the Discharge Tester Circuit

The circuit is straightforward:

1. Connect the 18650 cell positive terminal through the INA219 high-side to the drain of the MOSFET.
2. Connect a power resistor between the MOSFET source and battery negative.
3. The INA219 measures both voltage across the cell and current through the load.
4. Arduino controls the MOSFET gate via a digital pin (through a 100-ohm resistor).
5. Arduino reads INA219 over I2C (SDA/SCL) and calculates running mAh total.

The power resistor value determines discharge current. For a 1A discharge from a fully charged 4.2V cell: R = V/I = 4.2/1 = 4.2 ohms. A 4.7-ohm 10W resistor is suitable. The resistor will get hot during testing, so mount it with adequate airflow.

Arduino Code for Capacity Measurement

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

Adafruit_INA219 ina219;

float totalMah = 0;
float cutoffVoltage = 2.8;  // Safe minimum for Li-ion
unsigned long lastTime = 0;
bool testing = false;

void setup() {
  Serial.begin(9600);
  ina219.begin();
  pinMode(7, OUTPUT);      // MOSFET gate control
  digitalWrite(7, LOW);     // Start with load OFF

  Serial.println("Battery Discharge Tester");
  Serial.println("Send 'S' to start, 'X' to stop");
}

void loop() {
  if (Serial.available()) {
    char cmd = Serial.read();
    if (cmd == 'S' || cmd == 's') startTest();
    if (cmd == 'X' || cmd == 'x') stopTest();
  }

  if (testing) {
    float voltage = ina219.getBusVoltage_V();
    float current_mA = ina219.getCurrent_mA();

    unsigned long now = millis();
    float elapsed_h = (now - lastTime) / 3600000.0;
    totalMah += current_mA * elapsed_h;
    lastTime = now;

    Serial.print("V: "); Serial.print(voltage, 3);
    Serial.print("  I: "); Serial.print(current_mA, 1);
    Serial.print("mA  Total: "); Serial.print(totalMah, 1);
    Serial.println("mAh");

    if (voltage  0.5) {
      stopTest();
      Serial.print("TEST COMPLETE! Capacity: ");
      Serial.print(totalMah, 0);
      Serial.println(" mAh");
    }
    delay(1000);
  }
}

void startTest() {
  totalMah = 0;
  lastTime = millis();
  testing = true;
  digitalWrite(7, HIGH);  // Turn on load
  Serial.println("Test started...");
}

void stopTest() {
  testing = false;
  digitalWrite(7, LOW);   // Turn off load
  Serial.println("Test stopped.");
}

Interpreting Test Results

After testing, compare your results:

• Above 80% of rated capacity: Cell is genuine and in good condition.
• 60-80% of rated capacity: Cell is used or slightly degraded but still usable for low-drain applications.
• Below 60% of rated capacity: Cell is either fake, old, or damaged. Do not use in multi-cell packs.
• Below 30% of rated capacity: Cell is counterfeit. Dispose safely.

For pack building, match cells within 5% of each other’s capacity. Test all cells from the same batch and group them accordingly.

Tips for Accurate Testing

• Let cells rest for at least 1 hour after charging before testing.
• Test at room temperature (25°C ± 5°C) for consistent results.
• Use a consistent discharge rate (0.2C to 0.5C is standard for capacity testing).
• Run the test twice and average the results for accuracy.
• Keep a spreadsheet of results if building battery packs – you need matched cells.