Connectors

Ten external connectors, deliberately spread across distinct connector families by role — so a solar source can't be physically mis-plugged into the battery input, and the two temperature-probe inputs use a field-terminable connector rather than requiring a pre-terminated probe.

The thermistor inputs use JST PA, not the JST PH of the battery input J4, precisely so a cell can never be plugged into a TS sense pin. Both are 2.0 mm pitch; the housings do not intermate.

Ref	Type	Purpose	Mating part
USBC1	USB-C receptacle	USB-C power input	Any standard USB-C cable
CN5	2.54mm screw terminal	DC / small solar panel input	Bare-wire or ferrule termination
J4	JST PH, 2-pin	Battery input	Single-cell Li-ion/Li-Po pack with a JST-PH cable — check polarity against the silkscreen before connecting
J6	JST SH, 4-pin (STEMMA QT)	Internal I2C bus — for sensors that should share the always-on power-management bus	Any STEMMA QT / Qwiic cable
J203	JST SH, 4-pin (STEMMA QT)	User I2C bus — for sensors on the switched, differential-buffer-side bus	Any STEMMA QT / Qwiic cable
J201 / J202	8P8C (RJ45-style)	Differential I2C — connects to a SparkFun QwiicBus EndPoint for remote sensors	Standard Ethernet patch cable
J2	2.54mm header (not fitted)	Optional external supply for the VCC_2 rail on the 8P8C jacks — see Notes	Standard 2.54mm header strip, hand-soldered
J302	JST PA, 2-pin (S02B-PASK-2)	Charger thermistor input	Semitec 103AT-11 NTC thermistor (10kΩ @ 25°C), mounted on the battery pack — see below
J401	JST PA, 2-pin (S02B-PASK-2)	Fuel-gauge thermistor input	A second, separate Semitec 103AT-11 NTC thermistor (10kΩ @ 25°C), also mounted on the pack
J7 / J8	2.54mm header (not fitted)	Spare/expansion GPIO breakout, plus the debug UART	Standard 2.54mm header strip, hand-soldered

Why two separate thermistors (J302 and J401)

The charger and fuel gauge each apply their own excitation/bias to whatever's wired to their thermistor pin. Sharing a single NTC between both active bias circuits would give both ICs a bad reading, so each gets its own thermistor — both mounted at the same physical location on the battery cell.

Terminating your own thermistor probe

J302 and J401 are JST PA 2-pin side-entry headers (S02B-PASK-2). To make up a probe cable:

Part	Number	Notes
Housing	PAP-02V-S	2-circuit, latching
Crimp contact	SPHD-001T-P0.5	AWG 28–22, insulation O.D. 0.76–1.5 mm
Thermistor	Semitec 103AT-11	10 kΩ @ 25 °C, B25/85 = 3435 K, 600 mm insulated lead

The PA series is rated −40 °C to +105 °C and latches. Both matter for a probe that lives on a battery pack: an intermittent TS connection reads as out-of-range and suspends charging, and the connector should not be the narrowest part of the temperature range it is qualifying.

Do not substitute the Semitec 103AT-2. It is the same element electrically, but it ships as a bare bead on 17 mm solid 42-alloy leads — too short for a crimp barrel, not ductile enough to form a gas-tight crimp, and with no insulation for the support wings to grip. If you only have 103AT-2 parts, solder them to a 24–26 AWG stranded pigtail, heatshrink the joint, and crimp the pigtail instead.

A 3D-printer hotend thermistor will not work here, despite the familiar-looking connector. Those are 100 kΩ, B ≈ 3950 parts; the R305/R306 divider is sized for 10 kΩ. Borrow the connector ecosystem, not the part.

Why two STEMMA QT ports

J6 and J203 sit on genuinely independent I2C buses (see Pins & Signals) — one always-on and dedicated to the board's own power-management ICs, the other switched and shared with the differential-buffer path. Plugging a sensor into one vs. the other has real electrical consequences; see Notes.

Controls

Three buttons, all on the top side:

Ref	Label	Function
SW1	RESET	Pulls the MCU's EN pin low. Resets the board.
SW2	BOOT	Hold during a reset to enter firmware download mode.
SW3	WAKE	Pulls the charger's QON pin low — it does not go to the MCU. The hold time picks the action: a short press (~1s) wakes the charger from ship mode, bringing the board back after it has shut down to preserve the battery. Holding for ~10 seconds triggers a full system power reset — the charger opens the ship FET, actively pulls the SYS rail down, and restores it after ~350ms, restarting the MCU and both 3.3V rails. That path is pure hardware, so it works even with no firmware running. The charger and the fuel gauge stay powered through it.

Headers you fit yourself

J7 and J8 are on the board but not factory-fitted. They're excluded from both the bill of materials and the pick-and-place file, so the assembler neither sources nor places them — the pads are simply left bare for you to solder a standard 2.54mm header strip onto.

They break out the free GPIOs and the debug UART. Several of the exposed pins are strapping pins, so check Pins & Signals before loading one down at reset.