ModBee MPPT Charger

Solar MPPT battery charger for 2S to 4S lithium packs, built on the TI BQ25798 buck-boost charger and an ESP32-C3 microcontroller. It is configured and monitored from a browser over Wi-Fi.

ModBee MPPT Charger

Features

BQ25798 charging. Buck-boost charger with charge voltage and current set from the web interface.
Two inputs.
- PV input with the BQ25798's VOC-sampling MPPT.
- A second DC input (5-24 V).
- Automatic selection between them through external FET pairs.
Battery. 2S to 4S Li-ion, Li-Po or LiFePO4. JEITA temperature control from a 10 kΩ NTC.
Load output. From the charger's SYS rail (NVDC power path): it runs from the inputs when present and from the battery otherwise.
Protection. TVS diodes and RC snubbers on the inputs, battery and load output (schematic rev 0.02).
Web interface. Live dashboard, settings and register-level debug page, served from the board's own Wi-Fi access point.
Unattended operation. Settings are stored on the board and re-applied at every boot, and charging resumes automatically after any power loss.
Expansion. RS485 transceiver with switchable termination, two Grove I2C ports and four spare GPIO on a header.
Packaging. A 35 mm DIN-rail enclosure, 118 × 42 × 45 mm.

Specifications

Item	Value
Charger	TI BQ25798, buck-boost, I2C address 0x6B
Microcontroller	ESP32-C3FH4: RISC-V up to 160 MHz, 4 MB flash, Wi-Fi 4, Bluetooth 5 LE, chip antenna
POWER input	5 V to 24 V DC
PV input	12 V nominal panel, Voc 24 V max, 3 A, 50 W
Battery	2S to 4S, 6 V to 18 V, charge current up to 3 A
Load output	3 A, at battery voltage (SYS rail)
Logic supply	3.3 V synchronous buck from SYS or USB
Interfaces	USB-C (programming, serial console), RS485, 2 × Grove I2C (3.3 V), GPIO header
Indicators	WS2812B RGB status LED (SYS), charger status LED (CHG)
Buttons	WIFI, RESET, BOOT, QON

There is no reverse-polarity protection on the power terminals. Reversed wiring can damage the charger IC and the battery-side capacitors, so check polarity before connecting.

Connection order

The charger is enabled in hardware so it can run unattended. Connect the battery first, so the ESP32-C3 applies your saved battery settings before any charging source is present.

Connecting	Disconnecting
1. Battery	1. Load
2. PV and/or POWER input	2. PV and/or POWER input
3. Load	3. Battery

The reasons for this order, and what to avoid, are in docs/HARDWARE.md.

Connections

Quick start

git clone https://github.com/mod-bee/modbee-mppt-ti-c3.git
cd modbee-mppt-ti-c3
pio run -t upload      # firmware
pio run -t uploadfs    # web interface

Connect the battery.
Join the ModbeeMPPT Wi-Fi network and open http://192.168.4.1/.
- It is an open network with no password.
- If the network is not visible, press WIFI.
On Settings, set the battery type, cell count, charge voltage and current. The default is 3S Li-Po, 12.6 V, 1.0 A.
Connect the panel or DC supply, then the load.

Full instructions are in docs/GETTING_STARTED.md.

Firmware status

Feature	Status
Charging, MPPT, dual input, telemetry	Implemented
Web dashboard, settings and debug pages	Implemented
Settings and energy statistics stored on the board	Implemented
Wi-Fi timeout, WIFI button, light sleep	Implemented
Modbus RTU over RS485	Planned
I2C target (slave) telemetry over Grove	Planned
Bluetooth LE	Not implemented

Documentation

Document	Content
Getting Started	Build, flash, wiring, first configuration, troubleshooting
Hardware	Ratings, connectors, connection order, schematic walkthrough, pin map
Software	Firmware architecture, start-up sequence, settings file, WebSocket protocol
API Reference	`ModbeeMpptAPI` methods and examples
Schematics	Rev 0.01 and rev 0.02 (PDF)

Repository layout

src/            Application entry point (main.cpp)
lib/ModbeeMPPT/ Charger controller, API, settings, web server, power saving
lib/bq25798/    BQ25798 register driver
lib/...         Third-party libraries (vendored)
data/           Web interface, uploaded to LittleFS
docs/           Documentation
schematics/     Schematic PDFs
images/         Board renders

Contributing

Issues and pull requests are welcome. Areas where help is useful:

Modbus RTU telemetry over the RS485 port
I2C target telemetry on the Grove port
Additional battery profiles and web interface improvements

Test changes that affect charging on real hardware with a fused battery.

License

This project is licensed under the GNU Affero General Public License v3.0. See LICENSE.

Third-party code in lib/ keeps its own license:

ArduinoJson, FastLED, ESP32_SoftWire: MIT
ESPAsyncWebServer, AsyncTCP: LGPL-3.0
BQ25798 driver: BSD. Derived from the Adafruit BQ25798 library by Limor Fried/Ladyada for Adafruit Industries.

Disclaimer

This is a lithium battery charger. Incorrect settings or wiring can damage batteries and cause fire. Use cells with their own protection (BMS), fuse the battery connection, and verify the settings against your battery's datasheet before leaving the charger unattended.