make all possible functions const

lib/Battery/battery.h

@@ -48,14 +48,14 @@ class Battery : public Component {
          * 
          * @return double in Volt
          */
-        double getBatteryVoltage();
+        double getBatteryVoltage() const;
 
         /**
          * @brief Get the charge level of the battery
          * 
          * @return uint8_t charge level in percent
          */
-        uint8_t getBatteryPercent() { return this->batteryPercent; }
+        uint8_t getBatteryPercent() const { return this->batteryPercent; }
 
         /**
          * @brief Checks if the battery is low.
@@ -68,7 +68,7 @@ class Battery : public Component {
          * @return true if the battery is low
          * @return false if the battery is high
          */
-        bool isBatteryLow(double voltage);
+        bool isBatteryLow(double voltage) const;
 
         bool isNewValue();
 
@@ -79,7 +79,7 @@ class Battery : public Component {
         void calculateBatteryPercent();
         void readAdcToBuf();
         void initBuffer();
-        uint16_t getBufAvg();
+        uint16_t getBufAvg() const;
 
         static const uint8_t bufferSize = 30;