diff --git a/include/devices.h b/include/devices.h
index a4e1124..fb7d813 100644
--- a/include/devices.h
+++ b/include/devices.h
@@ -12,8 +12,8 @@ auto roomSensor = Builder<TemperatureSensor>::instance(TEMP_SENSOR)
         .withArea("Basement")
         .withParent(gatewayDevice))
     .withValueTemplate("{{ value_json.sensor.temperature }}")
-    .addDiagnostic(new VoltageSensor{TEMP_SENSOR, "Battery voltage", "{{ value_json.sensor.diagnostic.voltage }}"})
-    .addDiagnostic(new BatterySensor{TEMP_SENSOR, "Battery level", "{{ ((value_json.sensor.diagnostic.voltage|float-2.5)|round(2)*100/2)|int }}"})
+    .addDiagnostic(createVoltageSensor(TEMP_SENSOR))
+    .addDiagnostic(createBatterySensor(TEMP_SENSOR, 2.5, 4.5))
     .build();
 
 auto tankSensor = Builder<Sensor>::instance(new Sensor{ "Level", OIL_SENSOR })
@@ -35,8 +35,8 @@ auto tankSensor = Builder<Sensor>::instance(new Sensor{ "Level", OIL_SENSOR })
         .withValueTemplate("{{ value_json.sensor.value }}")
         .build()
     )
-    .addDiagnostic(new VoltageSensor{OIL_SENSOR, "Battery voltage", "{{ value_json.sensor.diagnostic.voltage }}"})
-    .addDiagnostic(new BatterySensor{OIL_SENSOR, "Battery level", "{{ ((value_json.sensor.diagnostic.voltage|float-4.0)|round(2)*100/2.4)|int }}"})
+    .addDiagnostic(createVoltageSensor(OIL_SENSOR))
+    .addDiagnostic(createBatterySensor(OIL_SENSOR, 4.0, 6.4))
     .build();
 
 auto presenceTracker = Builder<BinarySensor>::instance(PRESENCE_SENSOR)
@@ -46,8 +46,8 @@ auto presenceTracker = Builder<BinarySensor>::instance(PRESENCE_SENSOR)
         .withModel("AtTiny85")
         .withParent(gatewayDevice))
     .withValueTemplate("{{ value_json.sensor.state }}")
-    .addDiagnostic(new VoltageSensor{PRESENCE_SENSOR, "Battery voltage", "{{ value_json.sensor.diagnostic.voltage }}"})
-    .addDiagnostic(new BatterySensor{PRESENCE_SENSOR, "Battery level", "{{ ((value_json.sensor.diagnostic.voltage|float-2.7)|round(2)*100/0.6)|int }}"})
+    .addDiagnostic(createVoltageSensor(PRESENCE_SENSOR))
+    .addDiagnostic(createBatterySensor(PRESENCE_SENSOR, 2.7, 3.3))
     .withOffDelaySeconds(5*60)
     .withDeviceClass("presence")
     .build();
diff --git a/include/rc_devices.h b/include/rc_devices.h
index f6da508..ebd37d0 100644
--- a/include/rc_devices.h
+++ b/include/rc_devices.h
@@ -71,3 +71,13 @@ struct EasyHomeSwitch : Switch {
         publisher(stateTopic, msg);
     }
 };
+
+VoltageSensor* createVoltageSensor(const char* id) {
+    return new VoltageSensor{id, "Battery voltage", "{{ value_json.sensor.diagnostic.voltage }}"};
+}
+
+BatterySensor* createBatterySensor(const char* id, float min, float max) {
+    char value_json[128];
+    snprintf(value_json, 128, "{{ ((value_json.sensor.diagnostic.voltage|float-%f)|round(2)*100/%f)|int }}", min, max - min);
+    return new BatterySensor{id, "Battery level", value_json};
+}