Multi-Rail Battery Power Board

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1. 01

   Confirm the separated project scope

   Build this board only for the 3S UPS, 5 V rail, 3.3 V rail, J3 distribution, and J10 telemetry/control functions. Do not fit cellular-modem power parts, filament LED driver parts, PWM wiring, or a PCB-mounted master switch in this revision.

   • Tip: Keep the 18V_PRE and SYS_BUS labels distinct throughout schematic capture and assembly.

   • ⚠ 18V_PRE is mains-only charger input. SYS_BUS is the battery-backed 8.25–12.6 V rail; never connect 18 V to J3 SYS_BUS.
2. 02

   Install and inspect the field-input path

   Fit J1, the input-protection/EMI stage, ideal-diode stage, and SC8701 pre-regulator in power-flow order. Observe polarized TVS, electrolytic capacitors, MOSFET orientation, and all inductor markings.

   • Tip: Keep high-current and switching loops short during PCB layout and verify all footprints against final manufacturer data.

   • ⚠ Do not energize the board until the final input fuse, field wiring, and connector ratings are reconciled with the chosen input range and power rating.
3. 03

   Install the battery and charger section

   Fit the BQ25798 support circuit exactly from the verified reference design. Connect J2A directly to the protected 3S1P 18650 battery pack: PACK+ to BAT+ and PACK- to BAT-. The specified pack is 3.5 Ah, 11.1 V nominal, 12.6 V charge cutoff, 5 A continuous discharge, 10 A pulse discharge, and 3 A maximum charge, with a built-in BMS. Connect J2B only if the pack provides the required 10 kΩ NTC lead.

   • Tip: Check battery and NTC connector pin order before attachment.
   • Tip: Keep BQ25798 battery-discharge configuration at or below the pack’s 5 A continuous rating; the current architecture target remains 4.5 A continuous.
   • Tip: There is no battery kill jumper or PCB battery-disconnect jumper in this revision.

   • ⚠ Never short BAT+ to ground.
   • ⚠ Do not exceed the specified 3 A maximum battery charge current without an actual pack/cell manufacturer approval and thermal validation.
   • ⚠ The supplied pack specification must be confirmed against the exact purchased pack and its wiring before use.
4. 04

   Install the downstream rails and J3

   Fit the 5 V and 3.3 V buck stages and connect them to J3. J3 pins 2 and 3 are parallel SYS_BUS contacts; pins 5 and 6 are parallel returns.

   • Tip: Use matched harness conductors and verify terminal/current derating before relying on the parallel contacts for high current.

   • ⚠ Do not inject external power into J3 rails without a separately reviewed power-path design.
5. 05

   Install J10 protection and controller harness

   Fit the J10 protection/bias network and connect the seven-pin harness: 3.3 V, GND, SDA, SCL, BQ_INT, BQ_STAT, BQ_CE. Connect it to the external ESP32-S3 controller only after pin order is checked end-to-end.

   • Tip: The controller firmware keeps BQ_INT and BQ_STAT high impedance with no internal pull-ups.

   • ⚠ Do not connect legacy 10-pin PWM/cellular harnesses to this seven-pin interface.
6. 06

   Fit diagnostics and test pads

   Fit the five rail-present LEDs with one dedicated series resistor per LED: LED1 VIN_OK uses led_r_vin_1 (10 kΩ, 0805, at least 0.25 W); LED2 18V_PRE_OK uses led_r_18v_1 (8.2 kΩ); LED3 SYS_BUS_OK uses led_r_sys_1 (4.7 kΩ); LED4 5V_OK uses led_r_5v_1 (1.5 kΩ); and LED5 3V3_OK uses led_r_3v3_1 (680 Ω). Fit TP1–TP7 and the grounded M3 mounting provisions.

   • Tip: LED1’s 10 kΩ resistor is sized for the field-input domain and must be at least 0.25 W at a 36 V input.
   • Tip: Keep every resistor physically in series between its named rail and the corresponding LED-bank input.

   • ⚠ Do not substitute one common resistor value across 3.3 V, 5 V, 12.6 V, 18 V, and field-input rails.
   • ⚠ Confirm LED polarity and the selected LED forward voltage before fabrication.
7. 07

   Perform staged power-up

   Begin with a current-limited field supply and no battery. Verify VIN_FIELD, 18V_PRE, SYS_BUS, 5V_AUX, and 3V3_LOGIC at the labeled test pads. Then add the battery, verify charge behavior and seamless field-loss operation, and finally attach intended J3/J10 loads.

   • Tip: Record input current, rail voltages, temperature, and BQ25798 status at each stage.

   • ⚠ Do not claim 90 W operation until the final fuse, input wiring, thermal behavior, BQ25798 configuration, and load/charge allocation have been validated.

/*
  External ESP32-S3 controller for the Universal 3S UPS Power Board.
  J10 is a seven-pin telemetry/control harness only:
  3.3 V, GND, SDA, SCL, BQ_INT, BQ_STAT, BQ_CE.
  Cellular and filament-control functions are intentionally deferred to
  separate dedicated-power projects.
*/
#include <Arduino.h>
#include <Wire.h>

constexpr int I2C_SDA_PIN = 8;
constexpr int I2C_SCL_PIN = 9;
constexpr int BQ_INT_PIN = 4;
constexpr int BQ_STAT_PIN = 5;
constexpr int BQ_CE_PIN = 6;

void setup() {
  Serial.begin(115200);
  Wire.begin(I2C_SDA_PIN, I2C_SCL_PIN);

  // No MCU pull-ups: a USB-powered controller must not source the unpowered
  // power board through J10. Final line bias resides on the power board.
  pinMode(BQ_INT_PIN, INPUT);
  pinMode(BQ_STAT_PIN, INPUT);
  pinMode(BQ_CE_PIN, INPUT); // Leave CE high impedance until commanded.
}

void loop() {
  Serial.print("BQ_INT=");
  Serial.print(digitalRead(BQ_INT_PIN) == LOW ? "active" : "idle");
  Serial.print("  BQ_STAT=");
  Serial.println(digitalRead(BQ_STAT_PIN) == LOW ? "active" : "idle");
  delay(1000);
}

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