Li-ion Battery Capacity Tester with Automatic Discharge Logging

The mAh printed on a Li-ion cell is a marketing number until it is measured. The real capacity of a cell is whatever it delivers when discharged at a defined current down to a defined cutoff voltage, and that number drifts with age, temperature and discharge rate — which is why battery labs test cells with a controlled discharge rather than trusting labels. Students usually have no way to perform that test: bench loads are expensive and crude stopwatch methods miss the cutoff moment entirely. This project fills the gap with a student-built capacity tester: an Arduino switches a calibrated resistive load onto the cell, samples voltage and current continuously, integrates charge over time (Coulomb counting), and stops the discharge at the configured cutoff voltage. The result is the true measured capacity of that specific cell, with a logged discharge curve the student can paste straight into the report.

1. The cell is placed in the spring-loaded holder; the tester reads the open-circuit voltage and the student selects the discharge current on the menu.
2. The Arduino switches the calibrated MOSFET load onto the cell, starting a constant-resistance discharge at approximately the selected current.
3. Voltage and current are sampled (approximately 4 Hz) and charge is integrated over time — the running Coulomb count is the delivered capacity.
4. The OLED shows live voltage, current, elapsed mAh and elapsed time while samples are appended to the SD-card CSV log.
5. When the cell voltage reaches the configured cutoff (3.0 V default), the load is switched off automatically and the final mAh/Wh totals are displayed.
6. The CSV log is opened in a spreadsheet to plot the discharge curve and compute the C-rate vs capacity relationship for the report.

Tech stack:

• Arduino Nano (ATmega328P)
• Calibrated resistive load bank + N-channel MOSFET switch
• INA219 current/voltage sensor module
• 0.96-inch OLED display
• SD-card module (CSV logging)
• 18650 spring-loaded cell holder, fuse, thermistor
• Arduino IDE (C/C++ firmware)

• [True capacity measurement] Coulomb counting — integrates measured current over the full discharge — reports the real mAh and Wh the cell delivered, not a datasheet copy.
• [Calibrated constant load] Resistive load bank switched by a MOSFET, sized for selectable discharge rates (0.5 A / 1 A nominal), so tests follow the standard constant-current-discharge method.
• [Configurable cutoff] Discharge stops automatically at the configured cutoff voltage (3.0 V default for Li-ion), protecting cells from over-discharge that would damage them.
• [Live OLED readout] Voltage, current, elapsed mAh, elapsed time and estimated remaining capacity update continuously during the test.
• [SD-card discharge logging] Timestamped voltage/current samples written to a CSV on an SD card; the log plots directly into the discharge curve for the report.
• [Cell-holder safety design] Spring-loaded 18650 holder, input fuse and reverse-polarity protection, plus firmware over-temperature guard using a thermistor on the load.
• [Multi-chemistry support] Configurable nominal voltage and cutoff so the same rig tests Li-ion, LiPo packs and NiMH packs within its voltage range.

• Working capacity-tester prototype (board, load bank, holder, display, SD module)
• Arduino firmware with menu, cutoff logic and CSV logging
• Wiring diagram and assembly documentation
• Load-calibration procedure (buyer runs it and records their own values)
• Project report PDF (battery fundamentals, Coulomb counting, test method, sample results template)
• PPT presentation for final review
• Viva Q&A preparation document