Recently I did a factory reset on my Android phone and I noticed an app, that I had taken for granted, was missing.  Every time I stepped into a hot-spot my WiFi would turn on and when I left it would turn off.  This app had worked so well that I though it was feature of Android. I guess not. I had installed the app Y5- Batter Saver.  This app simply did what was mentioned above. Turn on your WiFi when you had saved a hotspot to the phone and when it was out of range it would turn off WiFi and turn on your wireless carries data.  Simple as that! Also it worth mentioning that your phone wastes less battery on WiFi than on 3G/4G connection.