measure battery voltage

src/main.cpp

@@ -69,6 +69,7 @@ struct __attribute__((packed)) sensor_readings_t {
   float uvb = NAN;
   float temperature_max = NAN; // °C
   float temperature_min = NAN; // °C
+  int battery = 0;
   uint32_t lastUpdate = 0;
 } sensor_readings;
 
@@ -180,6 +181,8 @@ void getSensorMeasurements() {
     }
   }
 
+  sensor_readings.battery = analogRead(_VBAT);
+
   sensor_readings.lastUpdate = millis();
 }
 
@@ -458,6 +461,8 @@ void printValues() {
     Serial.println(sensor_readings.pm10);
   }
 
+  Serial.print("Battery: "); Serial.println(sensor_readings.battery);
+
   Serial.print("Free HEAP: "); Serial.println(ESP.getFreeHeap());
 
 }
@@ -512,6 +517,12 @@ void sendValues() {
     delay(10);
   }
 
+  {
+    String topic_battery = String("thomas/sensor/") + ota.getMAC() + String("/battery");
+    mqtt.publish(topic_battery.c_str(), (sensor_readings.battery/4096.0)*2*3.42, "%.2f");
+    delay(10);
+  }
+
 }
 
 /**
@@ -529,6 +540,12 @@ void setup()
   // initialize LED digital pin as an output.
   pinMode(LED_BUILTIN, OUTPUT);
 
+  pinMode(_VBAT, INPUT);
+  analogReadResolution(12);
+  analogSetAttenuation(ADC_11db);
+  adcAttachPin(_VBAT);
+  adcStart(_VBAT);
+
   // initialize e-paper display
   SPI.begin(18, 19, 23, TFT_CS);
   display.init();