old_contents: var BATTERY_UUID                    = '180f';
  var BATTERY_LEVEL_UUID              = '2a19';
  
  function BatteryService() {
  }
  
  BatteryService.prototype.readBatteryLevel = function(callback) {
    this.readUInt8Characteristic(BATTERY_UUID, BATTERY_LEVEL_UUID, callback);
  };
  
  module.exports = BatteryService;
  
new_contents: var BATTERY_UUID                    = '180f';
  var BATTERY_LEVEL_UUID              = '2a19';
  
  function BatteryService() {
  }
  
  BatteryService.prototype.readBatteryLevel = function(callback) {
    this.readUInt8Characteristic(BATTERY_UUID, BATTERY_LEVEL_UUID, callback);
  };
  
  BatteryService.prototype.onBatteryLevelChange = function (data) {
    this.emit('batteryLevelChange', data.readUInt8(0));
  };
  
  BatteryService.prototype.notifyBatteryLevel = function (callback) {
    this.onBatteryLevelChangeBinded       = this.onBatteryLevelChange.bind(this);
    this.notifyCharacteristic(BATTERY_UUID, BATTERY_LEVEL_UUID, true, this.onBatteryLevelChangeBinded, callback);
  };
  
  BatteryService.prototype.unnotifyBatteryLevel = function (callback) {
    this.notifyCharacteristic(BATTERY_UUID, BATTERY_LEVEL_UUID, false, this.onBatteryLevelChangeBinded, callback);
  };
  
  module.exports = BatteryService;
  
subject: Add battery level notification