Single-Cell Satellite EPS, a ESP32 project

This guide builds a complete electrical power system (EPS) for a single-cell satellite or remote device. The system prioritizes USB input power when available, falls back to a solar panel, and intelligently charges a protected 3.7 V lithium-ion battery while monitoring its temperature. All three power sources feed into a regulated 3.3 V output rail via a buck-boost converter, ensuring stable power delivery across a wide input voltage range.

The guide provides a complete wiring diagram, detailed parts list, and step-by-step assembly instructions organized by functional block: input protection and fusing, USB priority selection, battery charging with thermal monitoring, voltage regulation, and sensor integration. Included firmware demonstrates how to read battery and solar panel voltages using the ESP32's analog inputs, with divider ratios calibrated for accurate telemetry.

Wiring diagram

Gather all the parts

Qty	Component
1	USB Type-C receptacle, USB2.0 power-only sink The connector that accepts a 5 V USB-C power supply.
1	5 V unidirectional TVS diode for USB VBUS A surge clamp that protects the USB 5 V input from cable transients.
1	Resettable USB input fuse, 0.75 A hold A resettable fuse that limits fault current drawn from USB.
1	2-pin solar-panel connector The connector for the nominal 5 V, 100 mA maximum solar panel.
1	6 V unidirectional TVS diode for solar input A surge clamp across the solar input whose final voltage rating depends on the panel open-circuit voltage.
1	Resettable solar-input fuse, 0.10 A hold A resettable fuse that protects the panel wiring from a downstream fault.
1	TPS2121RUXR priority power multiplexer The dual-input ideal-diode power switch that gives USB input priority and blocks reverse current between inputs.
1	BQ25185DLHR 1-cell charger with power path The single-cell Li-ion/Li-Po charger that powers the system first and reduces charging when input power is limited.
1	Protected 3.7 V 1200 mAh single-cell Li-ion/Li-Po battery The protected rechargeable cell that stores energy and powers the prototype when no external input can meet the load.
1	10 kΩ NTC, B25/85 = 3435 K, battery-mounted A temperature-sensitive resistor attached to the battery so charging stops if the cell is too hot or too cold.
1	TPS63802DLAR 3.3 V buck-boost regulator The converter that holds the payload rail at 3.3 V as battery and power-path voltage change.
1	1x6 2.54 mm EPS output header The payload connector carrying regulated power, the two measured-voltage nets, and optional UART signals.
1	5.1 kΩ 1% USB-C CC resistor 5.1 kΩ The pull-down resistor that tells USB-C sources this is a power sink.
1	5.1 kΩ 1% USB-C CC resistor 5.1 kΩ The pull-down resistor that tells USB-C sources this is a power sink.
1	10.2 kΩ 1% resistor 10.2 kΩ Upper resistor that makes USB the TPS2121 priority input above its valid threshold.
1	5.00 kΩ 1% resistor 5.00 kΩ Lower resistor that sets the USB-priority threshold of the power multiplexer.
1	80.0 kΩ 1% resistor 80.0 kΩ The TPS2121 current-limit programming resistor; its typical limit is 1.5 A.
1	100 nF 16 V X7R ceramic capacitor 100 nF The TPS2121 soft-start capacitor that limits output inrush during switchover.
1	1 µF 10 V X7R ceramic capacitor 1 µF Local USB-input bypass capacitor at the power multiplexer.
1	1 µF 10 V X7R ceramic capacitor 1 µF Local solar-input bypass capacitor at the power multiplexer.
1	18.0 kΩ 1% resistor 18.0 kΩ The BQ25185 setting resistor for 4.2 V regulation and a 500 mA input-current limit.
1	1.00 kΩ 1% resistor 1.00 kΩ The BQ25185 setting resistor for a nominal 300 mA fast-charge current.
1	1 µF 25 V X7R ceramic capacitor 1 µF Input bypass capacitor placed directly at the BQ25185 input pin.
1	10 µF 10 V X7R ceramic capacitor 10 µF System-rail bypass capacitor placed directly at the BQ25185 SYS pin.
1	1 µF 10 V X7R ceramic capacitor 1 µF Battery-pin bypass capacitor placed directly at the BQ25185 BAT pin.
1	Coilcraft XFL4015-471ME, 0.47 µH power inductor 0.47 µH The low-resistance 5.4 A rated inductor used by the TPS63802 switching regulator.
1	10 µF 6.3 V X7R ceramic capacitor 10 µF Input bypass capacitor placed directly at the TPS63802 VIN pin.
1	22 µF 6.3 V X7R ceramic capacitor 22 µF Minimum regulated-output capacitor placed directly at the TPS63802 output pin.
1	511 kΩ 1% resistor 511 kΩ Upper TPS63802 feedback resistor that sets the regulator to 3.3 V.
1	91.0 kΩ 1% resistor 91.0 kΩ Lower TPS63802 feedback resistor that sets the regulator to 3.3 V.
1	220 kΩ 1% resistor 220 kΩ Upper battery monitor divider resistor.
1	100 kΩ 1% resistor 100 kΩ Lower battery monitor divider resistor.
1	100 nF 10 V X7R ceramic capacitor 100 nF Battery sense low-pass filter capacitor.
1	330 kΩ 1% resistor 330 kΩ Upper solar monitor divider resistor.
1	68.0 kΩ 1% resistor 68.0 kΩ Lower solar monitor divider resistor.
1	100 nF 16 V X7R ceramic capacitor 100 nF Solar sense low-pass filter capacitor.
1	Loop test point A probe point for checking protected USB input voltage.
1	Loop test point A probe point for checking protected solar input voltage.
1	Loop test point A probe point for checking battery voltage.
1	Loop test point A probe point for checking charger SYS voltage.
1	Loop test point A probe point for checking regulated 3.3 V output.

Assemble it in 6 steps

1. Place the input protection parts

Place the USB-C connector and solar connector at the board edge. Put the USB TVS diode and USB resettable fuse beside the USB connector, and put the solar TVS diode and solar resettable fuse beside the solar connector. Keep each TVS diode's ground end short and wide to the ground plane so it can divert a surge before it reaches the EPS.

Do not connect a solar panel until its open-circuit voltage has been checked; an over-voltage panel can damage the input protection and charger.

2. Build the USB-priority selector

Place the TPS2121 beside the two protected inputs. Connect USB through the USB fuse to IN1 and solar through the solar fuse to IN2. Fit the 10.2 kΩ and 5.00 kΩ resistors from USB input to PR1 and then to ground, so a valid USB source wins. Fit the 80.0 kΩ ILIM resistor and 100 nF SS capacitor to ground, and place both 1 µF input capacitors directly beside the IC.

Keep IN1, IN2, OUT, and their bypass capacitors in a tight group; these are the high-current paths.

3. Wire the battery charger and temperature check

Place the BQ25185 close to the battery connector. Connect the protected cell positive lead to BAT and the negative lead to GND. Attach the 10 kΩ NTC tightly to the battery pouch or cell holder, then wire it from TS/MR to ground. Fit the 18.0 kΩ and 1.00 kΩ setting resistors to ground and place the 1 µF, 10 µF, and 1 µF capacitors directly beside IN, SYS, and BAT.

Make sure battery positive and negative are not swapped — reverse battery wiring can damage the charger even when the cell itself is protected.
Do not charge a swollen, punctured, or unlabelled lithium cell.

4. Build the regulated 3.3 V rail

Place the 0.47 µH inductor, 10 µF input capacitor, 22 µF output capacitor, and the 511 kΩ / 91.0 kΩ feedback resistors immediately around the TPS63802. Connect SYS to VIN and tie EN high to SYS. Tie MODE to ground for low-load power-save operation. Route the switching loop between L1, L2, and the inductor as short and wide copper traces.

Do not run the battery-sense or solar-sense traces through the inductor area; switching noise can make the readings inaccurate.

5. Add the monitor dividers and output header

Fit the 220 kΩ over 100 kΩ battery divider and its 100 nF capacitor, then route its midpoint to BAT_SENSE and ESP32 GPIO34. Fit the 330 kΩ over 68.0 kΩ solar divider and its 100 nF capacitor, then route its midpoint to SOLAR_SENSE and ESP32 GPIO35. Connect the output header's 3V3 and GND pins to the regulated rail and ground, and connect UART_TX to GPIO17 and UART_RX to GPIO16.

GPIO34 and GPIO35 are input-only pins, which is exactly what these voltage measurements need.

6. Fit and use the test points

Fit the five loop test points on protected USB input, protected solar input, battery positive, SYS, and 3.3 V. Before connecting the ESP32 payload, check each point with a multimeter: battery must never exceed 4.2 V while charging and the output must be close to 3.3 V.

Do not short a test-point loop with a probe tip; a lithium battery can supply enough current to overheat traces or wires.

Review all connections

1. Connections between "usb_c" and "ESP32"

Function	usb_c	ESP32
`power`	VBUS → Resettable USB input fuse, 0.75 A hold IN	EXT
`ground`	GND	GND
`ground`	SHIELD	GND
`digital`	CC1 → 5.1 kΩ 1% USB-C CC resistor 1	EXT
`digital`	CC2 → 5.1 kΩ 1% USB-C CC resistor 1	EXT

2. Connections between "r_cc1" and "ESP32"

Function	r_cc1	ESP32
`ground`	2	GND

3. Connections between "r_cc2" and "ESP32"

Function	r_cc2	ESP32
`ground`	2	GND

4. Connections between "usb_esd" and "ESP32"

Function	usb_esd	ESP32
`power`	A → Resettable USB input fuse, 0.75 A hold IN	EXT
`ground`	K	GND

5. Connections between "usb_ptc" and "ESP32"

Function	usb_ptc	ESP32
`power`	OUT → TPS2121RUXR priority power multiplexer IN1	EXT

6. Connections between "c_mux_usb" and "ESP32"

Function	c_mux_usb	ESP32
`power`	1 → TPS2121RUXR priority power multiplexer IN1	EXT
`ground`	2	GND

7. Connections between "tp_vbus" and "ESP32"

Function	tp_vbus	ESP32
`power`	TP → TPS2121RUXR priority power multiplexer IN1	EXT

8. Connections between "solar_in" and "ESP32"

Function	solar_in	ESP32
`power`	PV+ → Resettable solar-input fuse, 0.10 A hold IN	EXT
`ground`	PV-	GND

9. Connections between "solar_esd" and "ESP32"

Function	solar_esd	ESP32
`power`	A → Resettable solar-input fuse, 0.10 A hold IN	EXT
`ground`	K	GND

10. Connections between "solar_ptc" and "ESP32"

Function	solar_ptc	ESP32
`power`	OUT → TPS2121RUXR priority power multiplexer IN2	EXT

11. Connections between "c_mux_solar" and "ESP32"

Function	c_mux_solar	ESP32
`power`	1 → TPS2121RUXR priority power multiplexer IN2	EXT
`ground`	2	GND

12. Connections between "tp_solar" and "ESP32"

Function	tp_solar	ESP32
`power`	TP → TPS2121RUXR priority power multiplexer IN2	EXT

13. Connections between "power_mux" and "ESP32"

Function	power_mux	ESP32
`power`	OUT → BQ25185DLHR 1-cell charger with power path IN	EXT
`ground`	GND	GND
`analog`	PR1 → 10.2 kΩ 1% resistor 2	EXT
`ground`	CP2	GND
`ground`	OV1	GND
`ground`	OV2	GND
`analog`	ILIM → 80.0 kΩ 1% resistor 1	EXT
`analog`	SS → 100 nF 16 V X7R ceramic capacitor 1	EXT

14. Connections between "r_pr1_hi" and "ESP32"

Function	r_pr1_hi	ESP32
`analog`	1 → TPS2121RUXR priority power multiplexer IN1	EXT

15. Connections between "r_pr1_lo" and "ESP32"

Function	r_pr1_lo	ESP32
`analog`	1 → TPS2121RUXR priority power multiplexer PR1	EXT
`ground`	2	GND

16. Connections between "r_mux_ilim" and "ESP32"

Function	r_mux_ilim	ESP32
`ground`	2	GND

17. Connections between "c_mux_ss" and "ESP32"

Function	c_mux_ss	ESP32
`ground`	2	GND

18. Connections between "charger" and "ESP32"

Function	charger	ESP32
`ground`	GND	GND
`power`	BAT → Protected 3.7 V 1200 mAh single-cell Li-ion/Li-Po battery B+	EXT
`ground`	/CE	GND
`analog`	ISET → 1.00 kΩ 1% resistor 1	EXT
`analog`	ILIM/VSET → 18.0 kΩ 1% resistor 1	EXT
`power`	SYS → TPS63802DLAR 3.3 V buck-boost regulator VIN	EXT

19. Connections between "battery" and "ESP32"

Function	battery	ESP32
`ground`	B-	GND

20. Connections between "battery_ntc" and "ESP32"

Function	battery_ntc	ESP32
`analog`	TS → BQ25185DLHR 1-cell charger with power path TS/MR	EXT
`ground`	GND	GND

21. Connections between "r_chg_iset" and "ESP32"

Function	r_chg_iset	ESP32
`ground`	2	GND

22. Connections between "r_chg_ilimvset" and "ESP32"

Function	r_chg_ilimvset	ESP32
`ground`	2	GND

23. Connections between "c_chg_in" and "ESP32"

Function	c_chg_in	ESP32
`power`	1 → BQ25185DLHR 1-cell charger with power path IN	EXT
`ground`	2	GND

24. Connections between "c_sys" and "ESP32"

Function	c_sys	ESP32
`power`	1 → BQ25185DLHR 1-cell charger with power path SYS	EXT
`ground`	2	GND

25. Connections between "c_bat" and "ESP32"

Function	c_bat	ESP32
`power`	1 → BQ25185DLHR 1-cell charger with power path BAT	EXT
`ground`	2	GND

26. Connections between "tp_bat" and "ESP32"

Function	tp_bat	ESP32
`power`	TP → BQ25185DLHR 1-cell charger with power path BAT	EXT

27. Connections between "tp_sys" and "ESP32"

Function	tp_sys	ESP32
`power`	TP → TPS63802DLAR 3.3 V buck-boost regulator VIN	EXT

28. Connections between "c_buck_in" and "ESP32"

Function	c_buck_in	ESP32
`power`	1 → TPS63802DLAR 3.3 V buck-boost regulator VIN	EXT
`ground`	2	GND

29. Connections between "buckboost" and "ESP32"

Function	buckboost	ESP32
`ground`	GND	GND
`ground`	AGND	GND
`digital`	EN → BQ25185DLHR 1-cell charger with power path SYS	EXT
`ground`	MODE	GND
`power`	VOUT → 1x6 2.54 mm EPS output header 3V3	EXT

30. Connections between "l_buck" and "ESP32"

Function	l_buck	ESP32
`power`	1 → TPS63802DLAR 3.3 V buck-boost regulator L1	EXT
`power`	2 → TPS63802DLAR 3.3 V buck-boost regulator L2	EXT

31. Connections between "c_3v3" and "ESP32"

Function	c_3v3	ESP32
`power`	1 → 1x6 2.54 mm EPS output header 3V3	EXT
`ground`	2	GND

32. Connections between "tp_3v3" and "ESP32"

Function	tp_3v3	ESP32
`power`	TP → 1x6 2.54 mm EPS output header 3V3	EXT

33. Connections between "r_fb_top" and "ESP32"

Function	r_fb_top	ESP32
`analog`	1 → TPS63802DLAR 3.3 V buck-boost regulator VOUT	EXT
`analog`	2 → TPS63802DLAR 3.3 V buck-boost regulator FB	EXT

34. Connections between "r_fb_bottom" and "ESP32"

Function	r_fb_bottom	ESP32
`analog`	1 → TPS63802DLAR 3.3 V buck-boost regulator FB	EXT
`ground`	2	GND

35. Connections between "output_header" and "ESP32"

Function	output_header	ESP32
`ground`	GND	GND
`analog`	BAT_SENSE	GPIO 34
`analog`	SOLAR_SENSE	GPIO 35
`uart`	UART_TX	GPIO 17
`uart`	UART_RX	GPIO 16

36. Connections between "r_bat_top" and "ESP32"

Function	r_bat_top	ESP32
`analog`	1 → BQ25185DLHR 1-cell charger with power path BAT	EXT
`analog`	2 → 1x6 2.54 mm EPS output header BAT_SENSE	EXT

37. Connections between "r_bat_bottom" and "ESP32"

Function	r_bat_bottom	ESP32
`ground`	2	GND
`analog`	1 → 1x6 2.54 mm EPS output header BAT_SENSE	EXT

38. Connections between "c_bat_sense" and "ESP32"

Function	c_bat_sense	ESP32
`ground`	2	GND
`analog`	1 → 1x6 2.54 mm EPS output header BAT_SENSE	EXT

39. Connections between "r_solar_top" and "ESP32"

Function	r_solar_top	ESP32
`analog`	1 → TPS2121RUXR priority power multiplexer IN2	EXT
`analog`	2 → 1x6 2.54 mm EPS output header SOLAR_SENSE	EXT

40. Connections between "r_solar_bottom" and "ESP32"

Function	r_solar_bottom	ESP32
`ground`	2	GND
`analog`	1 → 1x6 2.54 mm EPS output header SOLAR_SENSE	EXT

41. Connections between "c_solar_sense" and "ESP32"

Function	c_solar_sense	ESP32
`ground`	2	GND
`analog`	1 → 1x6 2.54 mm EPS output header SOLAR_SENSE	EXT

Deploy the firmware

main.cppOpen in Schematik

#include <Arduino.h>

constexpr int BAT_SENSE_PIN = 34;
constexpr int SOLAR_SENSE_PIN = 35;
constexpr float ADC_FULL_SCALE_COUNTS = 4095.0f;
constexpr float ADC_REFERENCE_VOLTS = 3.3f;
// Divider ratios: battery = 100k/(220k+100k); solar = 68k/(330k+68k).
constexpr float BATTERY_DIVIDER_RATIO = 100.0f / 320.0f;
constexpr float SOLAR_DIVIDER_RATIO = 68.0f / 398.0f;

float readScaledVoltage(int pin, float dividerRatio) {
  const uint32_t raw = analogRead(pin);
  const float adcVoltage = (static_cast<float>(raw) * ADC_REFERENCE_VOLTS) / ADC_FULL_SCALE_COUNTS;
  return adcVoltage / dividerRatio;
}

void setup() {
  Serial.begin(115200);
  analogReadResolution(12);
  analogSetPinAttenuation(BAT_SENSE_PIN, ADC_11db);
  analogSetPinAttenuation(SOLAR_SENSE_PIN, ADC_11db);
}

void loop() {
  const float batteryVoltage = readScaledVoltage(BAT_SENSE_PIN, BATTERY_DIVIDER_RATIO);
  const float solarVoltage = readScaledVoltage(SOLAR_SENSE_PIN, SOLAR_DIVIDER_RATIO);

  Serial.print("BAT_SENSE: ");
  Serial.print(batteryVoltage, 3);
  Serial.print(" V, SOLAR_SENSE: ");
  Serial.print(solarVoltage, 3);
  Serial.println(" V");

  delay(1000);
}