SunSproutHub

Prototype hardware: charging works, validation incomplete

A first hub was fabricated and tested from commit 1645733. Charging, charge termination and running from the battery alone worked on it, and testing also found issues. The current design files fix them, but that revision has not been built yet, and not every feature has been bench-tested. Testing status lists each feature. You can build the tested prototype with its known issues, or the current design with its untested fixes.

Before you connect a battery:

• Set the cell chemistry in the firmware first. The charger comes up for a 4.2 V cell by default. The firmware sets the charge limit from cell_chemistry (LiFePO4 charges to 3.65 V), so set it to match your cell.
• Fit a 10 kΩ NTC thermistor on the battery, or set ts_ignore: true (turns temperature protection off). Otherwise the charger won't charge.
• Set the charge current in the firmware to suit your pack, and never above 2000 mA. The hardware input-current limit only restricts it loosely, so the firmware ceiling is what holds it.
• Check polarity. The reverse-polarity protection was changed after the prototype and the new design is untested on hardware.

A wrongly configured charger can damage a lithium cell. Build one board and bring it up on USB power with no battery before ordering a batch. See Getting started.

A compact power-management and connectivity board built around the ESP32-C5. It takes power from USB-C, a small DC/solar source, or a Li-ion/Li-Po battery, charges the battery with MPPT on the solar input, regulates a 3.3V system rail, and exposes two independent I2C buses for onboard and remote sensors.

Physical

• 6-layer PCB.
• External connectors: USB-C power input, screw-terminal DC/solar input, JST battery input, two STEMMA QT ports, two 8P8C (RJ45-style) differential-I2C jacks, and two thermistor connectors. Headers for GPIO expansion and VCC_2 injection are left unfitted. See Connectors; to find a part on the board, use the interactive BOM.
• Two LEDs: charge status (driven by the charger) and a firmware-controlled status LED.
• Three buttons: reset, boot-mode, and a wake button that brings the charger out of ship mode, or, held for about 10 seconds, hard power-cycles the board.
• U.FL connector for an external antenna.

Capabilities

Compute & radio

MCU	ESP32-C5-WROOM-1U
Wireless	Dual-band Wi-Fi 6, Bluetooth LE 5, 802.15.4. The SoC has a CAN FD controller, but no CAN transceiver is fitted.
Antenna	U.FL external connector

Power management

• Li-ion/Li-Po charging with JEITA temperature-qualified fast charge, using a thermistor input.
• Source selection between USB-C and the DC/solar input. Both can be connected at once.
• Fuel gauge for state of charge, with its own thermistor input.
• MPPT on the solar input, run by the charger itself with no firmware control loop.
• Measurements from the charger's 16-bit ADC: input voltage and current, battery voltage and current, and temperature, all readable over I2C.
• Switched 3.3V user rail that firmware can power down to save current.

Protection

• Reverse-polarity protection on the battery and DC/solar inputs. USB-C doesn't need one.
• ESD/TVS protection on the 8P8C jacks' differential I2C lines and the USB-C data lines, and a TVS on USB VBUS.
• No dedicated ESD protection on the STEMMA QT ports, the expansion headers, the debug UART, the thermistor inputs or the USB CC lines. If you run a cable from one of them out of the enclosure, add protection at that end.
• Separate interrupt lines from the charger and the fuel gauge, each on its own GPIO.

Interfaces

I2C: two independent buses

• Internal bus: always on, for the charger and fuel gauge, plus the Qwiic Sys STEMMA QT port.
• User bus: switchable. It feeds a PCA9615 differential buffer that drives the two 8P8C jacks, plus the Qwiic User STEMMA QT port. The unfitted expansion header also carries it.

The differential side is compatible with the SparkFun QwiicBus EndPoint: same PCA9615 buffer and 8P8C pinout, so a standard Ethernet patch cable carries I2C to an EndPoint, and Qwiic/STEMMA QT devices plug into it. This board has no termination resistors, so the far end must be an EndPoint. See Notes for how the buses differ.

Power

Input

Source	Connector	Notes
USB-C	USB-C receptacle	5V only, no USB-PD. The charger's USB source detection is disconnected by default (its two connecting resistors are left unpopulated), so the input limit comes from the hardware clamp; see Notes.
DC / small solar panel	Screw terminal marked SOLAR	Can be connected at the same time as USB-C.
Battery	JST PH 2-pin connector marked BATT + / BATT −	Single cell. Check pack polarity against the silkscreen before connecting.

Charging summary

Charger	BQ25798, buck-boost, dual-input
Cell configuration	1S, set in hardware by the PROG resistor
Chemistry	Li-ion / Li-Po and LiFePO4
Input voltage, either source	3.6 V to 24 V; 30 V absolute maximum
Solar panel sizing	Up to 24 V. Use the panel's cold-weather open-circuit voltage, which is above its rated Voc.
Input current limit	About 2.0–2.1 A (worst case 2.27 A), set in hardware. Firmware cannot exceed it.
Charge current	Firmware-set over I2C. Defaults to 1 A. Do not exceed 2 A.
Solar tracking	Open-circuit-voltage MPPT, run by the charger
Source selection	Both inputs may be connected at once; the charger selects between them
Temperature qualification	JEITA, via a thermistor on the CHGR_TS connector. With no thermistor fitted the charger suspends charging unless ts_ignore: true is set.

Output

Rail	Voltage	Description
System rail (always on)	3.3V	MCU, the internal I2C bus, and Qwiic Sys.
User rail (switched)	3.3V	The differential buffer and Qwiic User. Firmware-controlled, off at power-up.

Charging & power management

• Input current limit. About 2.0 A from USB-C and 2.1 A from a higher-voltage input, up to 2.27 A worst case. It limits the combined draw of the sources, not the battery charge rate. The charger sets it once at power-up from whichever source is present. Details are in the charger notes.
• Charging is enabled in hardware (CE is tied to ground), but without a thermistor the charger will not charge. Fit a 103AT-type 10 kΩ NTC on the CHGR_TS connector, or set ts_ignore: true in the firmware, which turns temperature protection off.
• Charge current is set over I2C and returns to 1 A on power-up, a watchdog timeout or a register reset. Set it for your pack's capacity, and never above 2000 mA: the input current limit only restricts it loosely, because in buck mode the charge current can exceed the input current.
• Ship mode and shutdown open the battery ship FET to disconnect the battery. They can only be entered with no adapter present; with USB or solar connected the charger ignores them.
• The WAKE button. Holding it about 1 s wakes the charger from ship mode; a brief tap does nothing. Holding it about 10 s hard power-cycles the board, with no firmware or I2C needed. The reset restarts the MCU and both 3.3V rails, and the fuel gauge too unless its FG_PWR jumper is set to the battery. It does not restart the charger itself.
• Firmware must set SFET_PRESENT (REG14, bit 7) at start-up. While it is 0, the charger locks out ship mode and shutdown.

Current consumption

Not yet characterized. Measurements will be added here once testing is complete.