Connectors
The external connectors, plus the unfitted headers, are 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, precisely
so a cell can never be plugged into a TS sense pin. Both are 2.0 mm pitch; the housings do
not intermate.
| Connector | Type | Purpose | Mating part |
|---|---|---|---|
| USB-C port | USB-C receptacle | Power input, firmware flashing and log output | Any standard USB-C cable |
Solar input (SOLAR) | 2.54mm screw terminal | DC / small solar panel input. Pin 1 is + (marked on the silkscreen), pin 2 is ground | Bare-wire or ferrule termination |
Battery (BATT + / BATT −) | JST PH, 2-pin | Single-cell battery input. Pin 1 is BATT+, pin 2 is BATT− | Single-cell Li-ion/Li-Po/LiFePO4 pack with a JST-PH cable. Check polarity against the silkscreen before connecting |
STEMMA QT port (Qwiic Sys) | JST SH, 4-pin | Internal I2C bus (3.3 V from the always-on rail), for sensors that should share the always-on power-management bus | Any STEMMA QT / Qwiic cable |
STEMMA QT port (Qwiic User) | JST SH, 4-pin | User I2C bus (3.3 V from the switched rail), for sensors on the switched bus that also feeds the differential buffer | Any STEMMA QT / Qwiic cable |
Two 8P8C jacks (J201, J202) | 8P8C (RJ45-style) | Differential I2C, to a SparkFun QwiicBus EndPoint for remote sensors | Standard Ethernet patch cable |
VCC_2 injection header (VCC_2 INJ, J2; not fitted) | 2.54mm header, 3-pin | Optional external supply for the VCC_2 rail on the 8P8C jacks (pin 2 VCC_2, pin 3 its return), plus an always-on 3V3_SYS pin (pin 1) for powering other hardware. See Notes | Standard 2.54mm header strip, hand-soldered |
Charger thermistor (CHGR_TS) | JST PA, 2-pin (S02B-PASK-2) | Charger thermistor input (pin 1 ground, pin 2 the charger's TS sense node) | Semitec 103AT-11 NTC thermistor (10kΩ @ 25°C), mounted on the battery pack. See below |
Fuel-gauge thermistor (FG_TS) | JST PA, 2-pin (S02B-PASK-2) | Fuel-gauge thermistor input (pin 1 the gauge's 2.5 V bias output, pin 2 its TS sense node) | A second, separate Semitec 103AT-11 NTC thermistor (10kΩ @ 25°C), also mounted on the pack |
Expansion headers (J7, J8; not fitted) | 2.54mm header | Spare/expansion GPIO breakout, plus the debug UART | Standard 2.54mm header strip, hand-soldered |
To see where each connector sits on the board, open the interactive BOM. Names in parentheses are printed on the board.
Pinouts
Pin 1 is the pin marked on the board's silkscreen. Check it against the silkscreen before connecting anything.
Battery (JST PH) and solar input (screw terminal)
| Pin | Battery | Solar input |
|---|---|---|
| 1 | BATT+ (VBAT_RAW) | + DC input |
| 2 | BATT− (battery return) | Ground |
Qwiic Sys (internal bus) and Qwiic User (user bus) STEMMA QT ports (JST SH, Qwiic order)
| Pin | Qwiic Sys | Qwiic User |
|---|---|---|
| 1 | Ground | Ground |
| 2 | 3.3 V (3V3_SYS, always on) | 3.3 V (3V3_USER, switched) |
| 3 | SDA_INT | SDA_USER |
| 4 | SCL_INT | SCL_USER |
Differential I2C jacks J201 / J202 (8P8C, same pinout on both)
| Pin | Signal | Pin | Signal |
|---|---|---|---|
| 1 | DSCL− | 5 | Ground |
| 2 | DSCL+ | 6 | VCC_2 return (GRN_N) |
| 3 | VCC_2 (GRN_P) | 7 | DSDA− |
| 4 | VCC_1 (3V3_USER) | 8 | DSDA+ |
VCC_2 INJ header, J2 (unfitted): pin 1 3V3_SYS (always-on 3.3 V), pin 2 VCC_2, pin 3 the VCC_2 return (GRN_N). Pin 1 is next to pin 2 so a jumper joins them, which puts the always-on rail on VCC_2 (cut JP2 first).
Expansion headers J7 and J8 (unfitted, 9 pins each)
| Pin | J7 | J8 |
|---|---|---|
| 1 | 3V3_USER | 3V3_USER |
| 2 | Ground | Ground |
| 3 | GPIO0 | GPIO26 |
| 4 | GPIO1 | GPIO25 |
| 5 | GPIO7 | U0TXD (GPIO11) |
| 6 | SDA_USER | U0RXD (GPIO12) |
| 7 | SCL_USER | GPIO24 |
| 8 | Ground | GPIO27 |
| 9 | 3V3_USER | GPIO5 |
Several of these GPIOs are strapping pins. See Pins & Signals before loading them at reset. The thermistor connectors are described below.
Why two separate thermistors
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.
The two connectors are not wired the same way. Pin 1 of the CHGR_TS connector is ground, but pin 1 of the FG_TS connector is
the gauge's own 2.5 V bias output, so never short FG_TS pin 1 to ground. The thermistor itself
has no polarity.
Terminating your own thermistor probe
The CHGR_TS and FG_TS connectors 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 charger's thermistor divider is sized for 10 kΩ. Borrow the connector ecosystem, not the part.
Why two STEMMA QT ports
Qwiic Sys and Qwiic User 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:
| Silkscreen label | Function |
|---|---|
RESET | Pulls the MCU's EN pin low. Resets the board. |
BOOT | Hold during a reset to enter firmware download mode. |
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
The expansion headers 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.