Solar Battery & Power Logger

Overview

Building something solar-powered? This logger tells you whether your panel and battery are big enough. The ET-S07 INA228 measures the load's voltage, current, power and accumulated energy with 20-bit precision, while the ET-S40 fuel gauge reports battery percentage and charge rate.

It's built on the same ET-P02 + ET-P03 power stage as the solar weather station, so you can measure that build — or any 3.3 V project — before deploying it.

• Load voltage, current, power and energy (Wh)
• Battery percentage and %/hour charge rate
• Charging and solar status on the ET-P02 LEDs
• CSV over Serial every second

What you need

ElectroTrical boards

ET-S07INA228 85 V Power MonitorINA228 20-bit power and energy monitor

AED 59.00Out of stock

ET-S40MAX17048 LiPo Fuel GaugeMAX17048 LiPo fuel gauge

AED 19.00Out of stock

ET-P02BQ24074 Solar / USB-C ChargerSolar / USB-C charger with power path

AED 49.00Out of stock

ET-P03TPS63802 Buck-Boost 3.3 V3.3 V buck-boost for the ESP32 and load

AED 39.00Out of stock

Boards total: AED 166.00

These boards are being manufactured. Open a board's page to see when it's back, or ask us about a kit.

You'll also need

• ESP32-S3 DevKitC-1 (or any ESP32-S3 board) with a USB-C data cable. Not sold by ElectroTrical.
• Qwiic / STEMMA QT cables (JST SH 4-pin) to chain the boards, or jumper wires
• 6 V solar panel, 2–5 W
• 1S LiPo with JST-PH connector
• Your test load (or the project you want to measure)

Wiring

The INA228 sits on the high side: current flows SOURCE+ → shunt → LOAD+. Addresses: INA228 0x40, MAX17048 0x36. Note that the ESP32 itself is part of the measured load here.

Build it

1. 01

   Set up the Arduino IDE

   Install Arduino IDE 2, add the Espressif ESP32 board package (Boards Manager → “esp32” by Espressif), select “ESP32S3 Dev Module” and turn on USB CDC On Boot so Serial prints over USB. Install the libraries listed above the sketch from Library Manager.

2. 02

   Bench test on USB

   First power the ESP32 from USB and feed SOURCE+ from a bench supply through a resistor load. Check the current matches your multimeter.

3. 03

   Build the solar path

   Wire the full power path. Disconnect USB from the ESP32 — it now runs from the ET-P03 and its own consumption is included in the reading.

4. 04

   Let the fuel gauge learn

   The MAX17048 gives a usable percentage within a few seconds, and becomes more accurate after a full charge/discharge cycle.

5. 05

   Log for a day

   Copy the Serial output into a spreadsheet (or use any serial logger) and compare energy in during the day against energy used at night.

Arduino code

Written for the ESP32-S3 DevKitC-1 with the Espressif Arduino core. Install these libraries from the Arduino Library Manager:

• Adafruit MAX1704X

#include <Wire.h>
#include <Adafruit_MAX1704X.h>

// ---- INA228 on the ET-S07: 15 mΩ shunt, ±10.9 A range ----
const uint8_t INA = 0x40;
const float SHUNT_OHM   = 0.015;
const float MAX_AMPS    = 10.0;
const float CURRENT_LSB = MAX_AMPS / 524288.0;          // 2^19

Adafruit_MAX17048 gauge;

void inaWrite16(uint8_t reg, uint16_t v) {
  Wire.beginTransmission(INA);
  Wire.write(reg); Wire.write(v >> 8); Wire.write(v & 0xFF);
  Wire.endTransmission();
}
uint64_t inaRead(uint8_t reg, uint8_t bytes) {
  Wire.beginTransmission(INA);
  Wire.write(reg);
  Wire.endTransmission(false);
  Wire.requestFrom(INA, bytes);
  uint64_t v = 0;
  for (uint8_t i = 0; i < bytes; i++) v = (v << 8) | Wire.read();
  return v;
}
int32_t signed20(uint64_t raw24) {                      // 20-bit value in bits 23..4
  int32_t v = (int32_t)(raw24 >> 4);
  return (v & 0x80000) ? v - 0x100000 : v;
}

void setup() {
  Serial.begin(115200);
  Wire.begin(8, 9);

  inaWrite16(0x00, 0x8000);                             // reset
  delay(2);
  inaWrite16(0x01, 0xFB6B);                             // continuous, 1052 us, 64 averages
  inaWrite16(0x02, (uint16_t)(13107.2e6 * CURRENT_LSB * SHUNT_OHM));  // SHUNT_CAL

  if (!gauge.begin()) Serial.println("MAX17048 not found (0x36)");
  Serial.println("ms,volts,amps,watts,wh,battery_pct,pct_per_hr");
}

void loop() {
  float volts = (inaRead(0x05, 3) >> 4) * 195.3125e-6;
  float amps  = signed20(inaRead(0x07, 3)) * CURRENT_LSB;
  float watts = inaRead(0x08, 3) * 3.2 * CURRENT_LSB;
  float wh    = inaRead(0x09, 5) * 16 * 3.2 * CURRENT_LSB / 3600.0;

  Serial.printf("%lu,%.3f,%.4f,%.3f,%.4f,%.1f,%.2f\n",
                millis(), volts, amps, watts, wh,
                gauge.cellPercent(), gauge.chargeRate());
  delay(1000);
}

Troubleshooting

Take it further

• Write the log to flash or upload it over Wi-Fi
• Add a second INA228 (address 0x41) on the solar input to measure energy harvested
• Size your next project's battery from a week of real data