diff --git a/app_config.h b/app_config.h
index 25e4612..d319a74 100644
--- a/app_config.h
+++ b/app_config.h
@@ -123,19 +123,21 @@ extern "C"
 #define MAJOR_VERSION_NUMBER 1
 #define MINOR_VERSION_NUMBER 8
 #elif (DEF_ELITE_MODEL == DEF_ELITE_EIS_10)
-#define ELITE_DEVICE_NAME    "EIS"
-#define MAJOR_PRODUCT_NUMBER 0
-#define MINOR_PRODUCT_NUMBER 4
-#define MAJOR_VERSION_NUMBER 1
-#define MINOR_VERSION_NUMBER 0
-#define BLE_EIS_ENABLED      1
+#define ELITE_DEVICE_NAME     "EIS"
+#define MAJOR_PRODUCT_NUMBER  0
+#define MINOR_PRODUCT_NUMBER  4
+#define MAJOR_VERSION_NUMBER  1
+#define MINOR_VERSION_NUMBER  0
+#define BLE_EIS_ENABLED       1
+#define EIS_BLE_OBSERVER_PRIO 3
 #elif (DEF_ELITE_MODEL == DEF_ELITE_EIS_11)
-#define ELITE_DEVICE_NAME    "EIS"
-#define MAJOR_PRODUCT_NUMBER 0
-#define MINOR_PRODUCT_NUMBER 4
-#define MAJOR_VERSION_NUMBER 1
-#define MINOR_VERSION_NUMBER 1
-#define BLE_EIS_ENABLED      1
+#define ELITE_DEVICE_NAME     "EIS"
+#define MAJOR_PRODUCT_NUMBER  0
+#define MINOR_PRODUCT_NUMBER  4
+#define MAJOR_VERSION_NUMBER  1
+#define MINOR_VERSION_NUMBER  1
+#define BLE_EIS_ENABLED       1
+#define EIS_BLE_OBSERVER_PRIO 3
 #elif (DEF_ELITE_MODEL == DEF_ELITE_EIS_MINI_10)
 #define ELITE_DEVICE_NAME    "EIS"
 #define MAJOR_PRODUCT_NUMBER 0
diff --git a/le_eis_srv.c b/le_eis_srv.c
index 3b2a603..13f1c11 100644
--- a/le_eis_srv.c
+++ b/le_eis_srv.c
@@ -6,6 +6,10 @@
 #include "ble_srv_common.h"
 #include "nrf_sdh_ble.h"
 
+#include "nrf_log.h"
+#include "nrf_log_ctrl.h"
+#include "nrf_log_default_backends.h"
+
 #include <stdlib.h>
 #include <string.h>
 
@@ -24,6 +28,8 @@ typedef struct
     uint16_t conn_handle;
     uint16_t service_handle;
     ble_gatts_char_handles_t bat_volt_handles;
+    bool bat_volt_notify_en;
+    uint16_t bat_volt;
 } ble_eis_t;
 
 typedef struct
@@ -33,6 +39,69 @@ typedef struct
 
 static ble_eis_t ble_eis;
 
+static void on_connect(ble_eis_t *p_eis, ble_evt_t const *p_ble_evt)
+{
+    // TODO...
+}
+
+static void on_disconnect(ble_eis_t *p_eis, ble_evt_t const *p_ble_evt)
+{
+    // TODO...
+}
+
+static void eis_ccc_update(ble_eis_t *p_eis, ble_evt_t const *p_ble_evt)
+{
+    ble_gatts_evt_write_t const *p_evt_write = &p_ble_evt->evt.gatts_evt.params.write;
+    if (p_evt_write->handle == p_eis->bat_volt_handles.cccd_handle)
+    {
+        p_eis->bat_volt_notify_en = ble_srv_is_notification_enabled(p_evt_write->data);
+        NRF_LOG_INFO("bat_volt_notify:%s", p_eis->bat_volt_notify_en ? "enable" : "disable");
+        return;
+    }
+}
+
+static void on_write(ble_eis_t *p_eis, ble_evt_t const *p_ble_evt)
+{
+    ble_uuid_t uuid = p_ble_evt->evt.gatts_evt.params.write.uuid;
+    switch (uuid.uuid)
+    {
+        case BLE_UUID_DESCRIPTOR_CLIENT_CHAR_CONFIG:
+            eis_ccc_update(p_eis, p_ble_evt);
+            break;
+        default:
+            break;
+    }
+}
+
+static void on_hvx_tx_complete(ble_eis_t *p_eis, ble_evt_t const *p_ble_evt)
+{
+    // TODO...
+}
+
+static void ble_eis_evt_handler(ble_evt_t const *p_ble_evt, void *p_context)
+{
+    ble_eis_t *p_eis = (ble_eis_t *)p_context;
+
+    switch (p_ble_evt->header.evt_id)
+    {
+        case BLE_GAP_EVT_CONNECTED:
+            on_connect(p_eis, p_ble_evt);
+            break;
+        case BLE_GAP_EVT_DISCONNECTED:
+            on_disconnect(p_eis, p_ble_evt);
+            break;
+        case BLE_GATTS_EVT_WRITE:
+            on_write(p_eis, p_ble_evt);
+            break;
+        case BLE_GATTS_EVT_HVN_TX_COMPLETE:
+            on_hvx_tx_complete(p_eis, p_ble_evt);
+            break;
+        default:
+            // No implementation needed.
+            break;
+    }
+}
+
 static uint32_t ble_eis_init(ble_eis_init_t const *p_eis_init)
 {
     uint32_t err_code;
@@ -42,6 +111,8 @@ static uint32_t ble_eis_init(ble_eis_init_t const *p_eis_init)
             .uuid = BLE_UUID_EIS_SERVICE,
     };
 
+    ble_eis.bat_volt = p_eis_init->battery_volt;
+
     // Adding vendor specific UUID to the SoftDevice
     err_code = sd_ble_uuid_vs_add(&base_uuid, &ble_uuid.type);
     APP_ERROR_CHECK(err_code);
@@ -57,15 +128,19 @@ static uint32_t ble_eis_init(ble_eis_init_t const *p_eis_init)
     add_char_params.uuid_type         = ble_uuid.type;
     add_char_params.max_len           = sizeof(p_eis_init->battery_volt);
     add_char_params.init_len          = sizeof(p_eis_init->battery_volt);
-    add_char_params.p_init_value      = (void *)&p_eis_init->battery_volt;
+    add_char_params.p_init_value      = (void *)&ble_eis.bat_volt;
     add_char_params.is_value_user     = false;
     add_char_params.is_var_len        = false;
     add_char_params.char_props.read   = 1;
     add_char_params.read_access       = SEC_OPEN;
-
-    err_code = characteristic_add(ble_eis.service_handle, &add_char_params, &ble_eis.bat_volt_handles);
+    add_char_params.char_props.notify = 1;
+    add_char_params.cccd_write_access = SEC_OPEN;
+    err_code                          = characteristic_add(ble_eis.service_handle, &add_char_params, &ble_eis.bat_volt_handles);
     APP_ERROR_CHECK(err_code);
 
+    // Register a handler for BLE events.
+    NRF_SDH_BLE_OBSERVER(m_eis_observer, EIS_BLE_OBSERVER_PRIO, ble_eis_evt_handler, &ble_eis);
+
     return NRF_SUCCESS;
 }