Battery-Powered Sensor: BME280 + MQTT + Deep Sleep for Months of Runtime

The goal

Put a sensor on a balcony or in a greenhouse with no power outlet and not touch the battery for months. The key is deep sleep: the ESP32 draws ~10 µA in this mode and only wakes up for 2–3 seconds to measure and send data.

Parts

• A board with low sleep current: FireBeetle ESP32, LOLIN D32, TinyPICO, or a bare ESP32-WROOM module with an HT7333/MCP1700 regulator. Regular DevKits with a USB-UART chip and AMS1117 draw 5–15 mA even while asleep!
• BME280 sensor (I²C) — temperature, humidity, pressure
• 18650 Li-Ion or 1000–3000 mAh Li-Po battery
• 2 × 100 kΩ voltage divider for battery measurement
• MQTT broker (Mosquitto, e.g. the Home Assistant add-on)

Wiring

Libraries

• PubSubClient (Nick O'Leary)
• Adafruit BME280 Library

Code

#include <WiFi.h>
#include <PubSubClient.h>
#include <Wire.h>
#include <Adafruit_BME280.h>

const char* WIFI_SSID  = "YOUR_SSID";
const char* WIFI_PASS  = "YOUR_PASSWORD";
const char* MQTT_HOST  = "192.168.1.10";
const int   MQTT_PORT  = 1883;
const char* MQTT_TOPIC = "home/balcony/sensor";

const uint64_t SLEEP_MINUTES = 10;
const int BATTERY_PIN = 35;           // via 100k/100k divider

RTC_DATA_ATTR uint32_t bootCount = 0; // survives deep sleep

WiFiClient net;
PubSubClient mqtt(net);
Adafruit_BME280 bme;

void goToSleep() {
  WiFi.disconnect(true);
  WiFi.mode(WIFI_OFF);
  esp_sleep_enable_timer_wakeup(SLEEP_MINUTES * 60ULL * 1000000ULL);
  esp_deep_sleep_start();
}

void setup() {
  unsigned long started = millis();
  Serial.begin(115200);
  bootCount++;

  if (!bme.begin(0x76)) {
    Serial.println("BME280 not found");
    goToSleep();
  }
  // Forced mode: the sensor measures once and goes back to sleep
  bme.setSampling(Adafruit_BME280::MODE_FORCED,
                  Adafruit_BME280::SAMPLING_X1,   // temperature
                  Adafruit_BME280::SAMPLING_X1,   // pressure
                  Adafruit_BME280::SAMPLING_X1,   // humidity
                  Adafruit_BME280::FILTER_OFF);
  bme.takeForcedMeasurement();
  float t = bme.readTemperature();
  float h = bme.readHumidity();
  float p = bme.readPressure() / 100.0F;
  float vbat = analogReadMilliVolts(BATTERY_PIN) * 2 / 1000.0;

  WiFi.mode(WIFI_STA);
  WiFi.begin(WIFI_SSID, WIFI_PASS);
  while (WiFi.status() != WL_CONNECTED) {
    if (millis() - started > 10000) {   // don't drain the battery if the router is down
      Serial.println("WiFi timeout");
      goToSleep();
    }
    delay(50);
  }

  mqtt.setServer(MQTT_HOST, MQTT_PORT);
  if (mqtt.connect("esp32-balcony")) {
    char payload[128];
    snprintf(payload, sizeof payload,
             "{\"t\":%.2f,\"h\":%.1f,\"p\":%.1f,\"vbat\":%.2f,\"boot\":%lu}",
             t, h, p, vbat, (unsigned long)bootCount);
    mqtt.publish(MQTT_TOPIC, payload, true);   // retained
    mqtt.loop();
    delay(50);
    mqtt.disconnect();
  }

  Serial.printf("Awake for %lu ms\n", millis() - started);
  goToSleep();
}

void loop() {}  // never reached

Battery life estimate

Average current ≈ 120 × 3 / 600 + 0.05 ≈ 0.65 mA. A 2000 mAh cell → ≈ 3000 h ≈ 4 months.

Wake faster (and live longer)

1. Static IP — WiFi.config(ip, gateway, subnet, dns) saves 0.5–1 s of DHCP.
2. Remember the router's channel and BSSID in RTC_DATA_ATTR and pass them to WiFi.begin(ssid, pass, channel, bssid) — another second off.
3. Publish less often: only send when the temperature changed by more than 0.2 °C (keep the previous value in RTC memory).
4. ESP-NOW instead of Wi-Fi (see that project) — the active phase drops to ~100 ms.

Home Assistant

Add MQTT sensors to configuration.yaml:

mqtt:
  sensor:
    - name: "Balcony temperature"
      state_topic: "home/balcony/sensor"
      unit_of_measurement: "°C"
      device_class: temperature
      value_template: "{{ value_json.t }}"
    - name: "Balcony battery"
      state_topic: "home/balcony/sensor"
      unit_of_measurement: "V"
      device_class: voltage
      value_template: "{{ value_json.vbat }}"

Troubleshooting

• Battery dies within a week — a board with USB-UART/AMS1117 or a power LED. Measure the sleep current with a multimeter.
• BME280 reads 2 °C high — self-heating; that's exactly why forced mode is used.
• Battery reading jumps around — add a 100 nF capacitor between GPIO 35 and GND.