Add battery level example

source/_components/sensor.mqtt.markdown

@@ -12,7 +12,7 @@ ha_category: Sensor
 ---
 
 
-This MQTT sensor implementation uses the MQTT message payload as the sensor value. If messages in this state_topic are published with *RETAIN* flag, the sensor will receive an instant update with last known value. Otherwise, the initial state will be undefined.
+This `mqtt` sensor platform uses the MQTT message payload as the sensor value. If messages in this `state_topic` are published with *RETAIN* flag, the sensor will receive an instant update with last known value. Otherwise, the initial state will be undefined.
 
 To use your MQTT sensor in your installation, add the following to your `configuration.yaml` file:
 
@@ -24,7 +24,7 @@ sensor:
   name: "MQTT Sensor"
   qos: 0
   unit_of_measurement: "°C"
-
+  value_template: '{{ value_json.value }}'
 ```
 
 Configuration variables:
@@ -33,3 +33,29 @@ Configuration variables:
 - **name** (*Optional*): The name of the sensor. Default is 'MQTT Sensor'. 
 - **qos** (*Optional*): The maximum QoS level of the state topic. Default is 0.
 - **unit_of_measurement** (*Optional*): Defines the units of measurement of the sensor, if any.
+- **value_template** (*Optional*): Defines a template to extract a value from the payload.
+
+## {% linkable_title Examples %}
+
+In this section you find some real life examples of how to use this sensor.
+
+### {% linkable_title Get battery level %}
+
+If you are using the [Owntracks](components/device_tracker.owntracks/) and enable the reporting of the battery level then you can use a MQTT sensor to keep track of your battery. A regular MQTT message from Owntracks looks like this: 
+
+```bash
+owntracks/tablet/tablet {"_type":"location","lon":7.21,"t":"u","batt":92,"tst":144995643,"tid":"ta","acc":27,"lat":46.12}
+```
+
+Thus the trick is extract the battery level from the payload.
+
+```yaml
+# Example configuration.yml entry
+sensor:
+  platform: mqtt
+  state_topic: "owntracks/tablet/tablet"
+  name: "Battery Tablet"
+  unit_of_measurement: "%"
+  value_template: '{{ value_json.batt }}'
+```
+