DIY 18650 Li-Ion Battery Capacity and Discharge Testing Meter using Arduino

This DIY testing meter operates by controlled discharge of an 18650 Li-ion battery through a power resistor while measuring voltage, current, and temperature. At the heart of the system is an Arduino Nano, which oversees the entire testing cycle. To draw a precise and adjustable current from the battery, the circuit employs an IRLZ44N MOSFET acting as an electronic load. The Arduino uses pulse-width modulation (PWM) passed through a signal conditioning network featuring an LM358 op-amp, 100nF capacitors, and 10K resistors to accurately drive the gate of the MOSFET. A 0.5-ohm, 5-watt high-power resistor safely dissipates the energy discharged from the battery as heat. Current flow is monitored across this power path, while a 10K NTC thermistor monitors the temperature of the setup to prevent thermal runaway. Voltage levels across the battery terminals are continuously tracked by the analog-to-digital converter (ADC) inputs of the Arduino Nano. Real-time metrics such as current voltage, calculated capacity, and operational status are rendered onto an SSD1306 0.96-inch OLED display using I2C communication. User inputs, including start and stop commands, are handled by two tactile switches, while a barrel DC connector provides external power to the control circuit. The entire analog and digital layout is constructed on a perf-board with supportive filtering provided by a 220uF capacitor and 1M and 4.7K resistors.