구글링을 하면서 찾아보던중

import android.app.Activity;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.os.Bundle;
import android.widget.TextView;


public class Main extends Activity {
private TextView contentTxt;
private BroadcastReceiver mBatInfoReceiver = new BroadcastReceiver(){
@Override
public void onReceive(Context arg0, Intent intent) {
// TODO Auto-generated method stub
int level = intent.getIntExtra("level", 0);
contentTxt.setText(String.valueOf(level) + "%");
}
};

@Override
public void onCreate(Bundle icicle) {
    super.onCreate(icicle);
    setContentView(R.layout.main);
    contentTxt = (TextView) this.findViewById(R.id.monospaceTxt);
    this.registerReceiver(this.mBatInfoReceiver, new IntentFilter(Intent.ACTION_BATTERY_CHANGED));
}
}

다음과같은 소스를 구해서 사용해보았는데요, 배터리 잔량은 표시가 잘되는데 usb를 꼽거나 빼면 강제종료됩니다.

무엇을 추가해 주어야 하는지요.