diff --git a/README.md b/README.md
index ef58762..ed166a5 100644
--- a/README.md
+++ b/README.md
@@ -39,3 +39,12 @@ updates.push.domain.tld {
 5. In the android app, change server to the autoconnect endpoint (here `push.domain.tld`).
 
 [Autopush]: https://github.com/mozilla-services/autopush-rs
+
+## Developpers
+
+It is possible to configure a few things with build config:
+
+| Name | Description | Default |
+|------|-------------|---------|
+| DEFAULT_API_URL | Define the API Url used by default | `"https://push.services.mozilla.com"` |
+| URGENCY | To add support for urgency requirement depending on the battery level | `false` until this is supported by the main server |
diff --git a/app/build.gradle.kts b/app/build.gradle.kts
index a012315..57adfb5 100644
--- a/app/build.gradle.kts
+++ b/app/build.gradle.kts
@@ -23,6 +23,7 @@ android {
 
         // buildConfigField("String", "DEFAULT_API_URL", "\"http://10.0.2.2:8088\"")
         buildConfigField("String", "DEFAULT_API_URL", "\"https://push.services.mozilla.com\"")
+        buildConfigField("Boolean", "URGENCY", "false")
     }
 
     buildFeatures {
diff --git a/app/src/main/java/org/unifiedpush/distributor/sunup/api/MessageSender.kt b/app/src/main/java/org/unifiedpush/distributor/sunup/api/MessageSender.kt
index 5d3dcf0..1a1246e 100644
--- a/app/src/main/java/org/unifiedpush/distributor/sunup/api/MessageSender.kt
+++ b/app/src/main/java/org/unifiedpush/distributor/sunup/api/MessageSender.kt
@@ -1,9 +1,11 @@
 package org.unifiedpush.distributor.sunup.api
 
 import android.content.Context
+import android.util.Log
 import okhttp3.WebSocket
 import org.unifiedpush.distributor.sunup.api.data.ClientMessage
 import org.unifiedpush.distributor.sunup.services.RestartWorker
+import org.unifiedpush.distributor.sunup.utils.TAG
 
 object MessageSender {
     private var websocket: WebSocket? = null
@@ -20,6 +22,7 @@ object MessageSender {
     }
 
     fun send(context: Context, message: ClientMessage) {
+        Log.d(TAG, "Sending ${message.serialize()}")
         synchronized(this) {
             websocket?.let {
                 message.send(it)
diff --git a/app/src/main/java/org/unifiedpush/distributor/sunup/api/ServerConnection.kt b/app/src/main/java/org/unifiedpush/distributor/sunup/api/ServerConnection.kt
index 58ae852..b4aabef 100644
--- a/app/src/main/java/org/unifiedpush/distributor/sunup/api/ServerConnection.kt
+++ b/app/src/main/java/org/unifiedpush/distributor/sunup/api/ServerConnection.kt
@@ -75,6 +75,9 @@ class ServerConnection(private val context: Context, private val releaseLock: ()
             ServerMessage.Ping -> onPing(ws)
             is ServerMessage.Register -> onRegister(message)
             is ServerMessage.Unegister -> onUnregister(message)
+            is ServerMessage.Urgency -> {
+                Log.d(TAG, "Urgency status=${message.status}")
+            }
         }
     }
 
diff --git a/app/src/main/java/org/unifiedpush/distributor/sunup/api/data/ClientMessage.kt b/app/src/main/java/org/unifiedpush/distributor/sunup/api/data/ClientMessage.kt
index 586b57c..be7891d 100644
--- a/app/src/main/java/org/unifiedpush/distributor/sunup/api/data/ClientMessage.kt
+++ b/app/src/main/java/org/unifiedpush/distributor/sunup/api/data/ClientMessage.kt
@@ -85,6 +85,25 @@ sealed class ClientMessage {
     @SerialName("nack")
     class Nack(val code: UInt?, val version: String) : ClientMessage()
 
+    @Serializable
+    @SerialName("urgency")
+    class MinUrgency(val min: Urgency) : ClientMessage()
+
+    @Serializable
+    enum class Urgency {
+        @SerialName("very-low")
+        VeryLow,
+
+        @SerialName("low")
+        Low,
+
+        @SerialName("normal")
+        Normal,
+
+        @SerialName("high")
+        High
+    }
+
     /**
      *     Ping,
      */
diff --git a/app/src/main/java/org/unifiedpush/distributor/sunup/api/data/ServerMessage.kt b/app/src/main/java/org/unifiedpush/distributor/sunup/api/data/ServerMessage.kt
index 0c1c0be..ad94c01 100644
--- a/app/src/main/java/org/unifiedpush/distributor/sunup/api/data/ServerMessage.kt
+++ b/app/src/main/java/org/unifiedpush/distributor/sunup/api/data/ServerMessage.kt
@@ -25,7 +25,7 @@ sealed class ServerMessage {
      */
     @Serializable
     @SerialName("hello")
-    class Hello(val uaid: String?, val status: UInt, val use_webpush: Boolean) : ServerMessage()
+    data class Hello(val uaid: String?, val status: UInt, val use_webpush: Boolean) : ServerMessage()
 
     /**
      *     Register {
@@ -38,7 +38,7 @@ sealed class ServerMessage {
      */
     @Serializable
     @SerialName("register")
-    class Register(val channelID: String, val status: UInt, val pushEndpoint: String) : ServerMessage()
+    data class Register(val channelID: String, val status: UInt, val pushEndpoint: String) : ServerMessage()
 
     /**
      *     Unregister {
@@ -49,7 +49,7 @@ sealed class ServerMessage {
      */
     @Serializable
     @SerialName("unregister")
-    class Unegister(val channelID: String, val status: UInt) : ServerMessage()
+    data class Unegister(val channelID: String, val status: UInt) : ServerMessage()
 
     /**
      *     Broadcast {
@@ -60,7 +60,7 @@ sealed class ServerMessage {
      */
     @Serializable
     @SerialName("broadcast")
-    class Broadcast() : ServerMessage()
+    data object Broadcast : ServerMessage()
 
     /**
      *     pub struct Notification {
@@ -93,6 +93,15 @@ sealed class ServerMessage {
         val headers: HashMap<String, String>?
     ) : ServerMessage()
 
+    /**
+     *    Urgency {
+     *         status: u32,
+     *     },
+     */
+    @Serializable
+    @SerialName("urgency")
+    data class Urgency(val status: UInt) : ServerMessage()
+
     /**
      *     Ping,
      */
diff --git a/app/src/main/java/org/unifiedpush/distributor/sunup/callback/BatteryCallbackFactory.kt b/app/src/main/java/org/unifiedpush/distributor/sunup/callback/BatteryCallbackFactory.kt
index 1d40057..d940f88 100644
--- a/app/src/main/java/org/unifiedpush/distributor/sunup/callback/BatteryCallbackFactory.kt
+++ b/app/src/main/java/org/unifiedpush/distributor/sunup/callback/BatteryCallbackFactory.kt
@@ -3,21 +3,31 @@ package org.unifiedpush.distributor.sunup.callback
 import android.content.Context
 import org.unifiedpush.distributor.callback.BatteryCallback
 import org.unifiedpush.distributor.callback.CallbackFactory
+import org.unifiedpush.distributor.sunup.BuildConfig
+import org.unifiedpush.distributor.sunup.api.MessageSender
+import org.unifiedpush.distributor.sunup.api.data.ClientMessage
 
 object BatteryCallbackFactory : CallbackFactory<BatteryCallbackFactory.MainBatteryCallback>() {
 
     class MainBatteryCallback : BatteryCallback() {
 
         override fun onBatteryLow(context: Context) {
-            // TODO: Subscribe to normal and high urgency messages only
+            if (BuildConfig.URGENCY) {
+                MessageSender.send(context, ClientMessage.MinUrgency(ClientMessage.Urgency.Normal))
+            }
         }
 
         override fun onBatteryOk(context: Context) {
-            // TODO: Subscribe back to all messages
+            if (BuildConfig.URGENCY) {
+                MessageSender.send(context, ClientMessage.MinUrgency(ClientMessage.Urgency.VeryLow))
+            }
         }
     }
 
     override fun new(context: Context): MainBatteryCallback {
+        if (BuildConfig.URGENCY) {
+            MessageSender.send(context, ClientMessage.MinUrgency(ClientMessage.Urgency.VeryLow))
+        }
         return MainBatteryCallback()
     }