IoT Battery Health and State-of-Charge Monitor for LiFePO4 Packs

Lead-acid and lithium battery packs in solar installations, e-rickshaws and UPS systems die early when nobody watches their health: a single weak cell drags the whole 4S string down, over-discharge sulfates the pack, and the owner discovers the damage only when the load dies. A proper battery monitor turns the invisible visible — current in and out, per-cell voltage spread, temperature and cumulative cycle count — so the user can act before a cell is destroyed. This project builds that monitor as a working prototype: an ESP32 with an INA219 current sensor sits in series with a 12V 100Ah LiFePO4 pack, balance taps report each cell's voltage, an NTC watches temperature, and the firmware integrates current over time (coulomb counting) to report state-of-charge, corrected at rest by the LiFePO4 open-circuit voltage curve. A relay provides an armed over-temperature cutoff. All of it is visible on a local web dashboard and a 16×2 LCD, making the sensing chain — sensor, calibration, algorithm, display — completely demonstrable.

1. The INA219 is wired in series with the pack on the high side; balance taps from the 4S string are divided to ADC-safe levels for per-cell voltage.
2. The ESP32 samples bus voltage and current (typically 1 Hz), computes pack voltage and applies a two-point calibration from the buyer's own measurement procedure.
3. Current samples are integrated over time to track charge in/out; when the pack rests, SoC snaps to the LiFePO4 open-circuit-voltage lookup table.
4. Each cell voltage is compared against the group mean; a spread above the warning threshold flags the cell on the dashboard and LCD.
5. Cycle counting accumulates full-equivalent cycles; SoH is estimated as remaining usable capacity versus the 100 Ah design rating.
6. The NTC thermistor reading drives the over-temperature logic — crossing 60 °C de-energises the relay output and latches a FAULT state until reset.
7. All readings are served on the ESP32-hosted CellScope dashboard over Wi-Fi and mirrored to the 16×2 LCD.

Tech stack:

• ESP32 (Arduino framework)
• INA219 current sensor · NTC thermistor
• 4S balance taps with resistor dividers
• 5V relay module (over-temperature cutoff)
• 16×2 LCD (I2C)
• HTML/CSS/JavaScript local web dashboard
• Coulomb counting + OCV recalibration algorithm
• 12V 100Ah LiFePO4 4S pack (demonstration unit)

• [Coulomb-counted SoC] The firmware integrates INA219 current over time for state-of-charge, recalibrated against the LiFePO4 open-circuit voltage curve whenever the pack rests.
• [4S cell-balance view] Individual cell voltages from balance taps are shown with the max spread in millivolts, so an unbalanced cell is visible before it causes damage.
• [INA219 current sensing] High-side bidirectional sensing measures both charge and discharge current with the sensor's ±3.2 A / 0.1 mA resolution range (datasheet).
• [State-of-health estimate] Cycle count plus capacity-fade tracking against the pack's 100 Ah rating yields an SoH percentage graded GOOD / DEGRADED.
• [Over-temperature cutoff] An NTC on cell 2 feeds a comparator-driven relay output that disconnects the load at a configurable temperature limit (60 °C default).
• [CellScope web dashboard] Local Wi-Fi dashboard with SoC ring, pack voltage, current, temperature, 24-hour charge/discharge chart and a cell-balance panel.
• [16×2 LCD status] Live voltage, SoC, current and temperature on the hardware itself, so the rig is demonstrable without a browser.

• Working hardware prototype (ESP32, INA219, taps, NTC, relay, LCD) with the demonstration pack
• Complete ESP32 firmware source with calibration procedure
• CellScope local web dashboard (single-file app)
• Wiring diagram and component list with part numbers
• Buyer-run current/voltage calibration procedure
• Project report PDF (coulomb counting, OCV curve, calibration, test procedure)
• PPT presentation for final review
• Viva Q&A preparation document (SoC methods, sensor accuracy, safety limits)

• This is a monitor, not a certified BMS: it does not balance cells or manage charging — it observes and cuts off. The report and page state this plainly.
• SoC accuracy of ±3–5% is a design target after the buyer's own calibration run; no figure is claimed as measured before that run.
• The demonstration pack is a 12V 100Ah unit; adapting the divider ratios and current range to other pack sizes needs the documented rescaling steps.
• Coulomb counting drifts between rests; long continuous cycling without rest periods reduces accuracy, documented as a limitation.
• The relay cutoff is a prototype-grade disconnect, not a safety-rated contactor for high-current traction use.
• Wi-Fi dashboard is local-network only; no cloud logging in the base build (logged to serial / future scope).