Add internal battery measurement example (#1470)

* Add internal battery measurement example

* Use existing hwCPUVoltage()

* Make debug output prettier

Author: mfalkvidd

examples/BatteryPoweredSensorInternal/BatteryPoweredSensorInternal.ino

@@ -0,0 +1,74 @@
+/*
+   The MySensors Arduino library handles the wireless radio link and protocol
+   between your home built sensors/actuators and HA controller of choice.
+   The sensors forms a self healing radio network with optional repeaters. Each
+   repeater and gateway builds a routing tables in EEPROM which keeps track of the
+   network topology allowing messages to be routed to nodes.
+
+   Created by Henrik Ekblad <henrik.ekblad@mysensors.org>
+   Copyright (C) 2013-2020 Sensnology AB
+   Full contributor list: https://github.com/mysensors/MySensors/graphs/contributors
+
+   Documentation: http://www.mysensors.org
+   Support Forum: http://forum.mysensors.org
+
+   This program is free software; you can redistribute it and/or
+   modify it under the terms of the GNU General Public License
+   version 2 as published by the Free Software Foundation.
+
+ *******************************
+
+   DESCRIPTION
+
+   This is an example that demonstrates how to report the battery level for a sensor
+   using the internal measurement method.
+   Instructions for measuring battery capacity are available here:
+   http://www.mysensors.org/build/battery
+
+*/
+
+// Enable debug prints to serial monitor
+#define MY_DEBUG
+
+// Enable and select radio type attached
+#define MY_RADIO_RF24
+//#define MY_RADIO_NRF5_ESB
+//#define MY_RADIO_RFM69
+//#define MY_RADIO_RFM95
+
+#include <MySensors.h>
+
+uint32_t SLEEP_TIME = 900000;  // sleep time between reads (seconds * 1000 milliseconds)
+int oldBatteryPcnt = 0;
+#define FULL_BATTERY 3 // 3V for 2xAA alkaline. Adjust if you use a different battery setup.
+
+void setup()
+{
+}
+
+void presentation()
+{
+  // Send the sketch version information to the gateway and Controller
+  sendSketchInfo("Battery Meter", "1.0");
+}
+
+void loop()
+{
+  // get the battery Voltage
+  long batteryMillivolts = hwCPUVoltage();
+  int batteryPcnt = batteryMillivolts / FULL_BATTERY / 1000.0 * 100 + 0.5;
+#ifdef MY_DEBUG
+  Serial.print("Battery voltage: ");
+  Serial.print(batteryMillivolts / 1000.0);
+  Serial.println("V");
+  Serial.print("Battery percent: ");
+  Serial.print(batteryPcnt);
+  Serial.println(" %");
+#endif
+
+  if (oldBatteryPcnt != batteryPcnt) {
+    sendBatteryLevel(batteryPcnt);
+    oldBatteryPcnt = batteryPcnt;
+  }
+  sleep(SLEEP_TIME);
+}