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.