As you jet off on a winter vacation or a holiday-season trip to see family, chances are that you’ll face a 21st-century problem: mobile device battery angst. It’s a never-ending itch at the back of your thoughts. Though we all love to use our smartphones to stave off boredom or to navigate a new city, every moment of use eats precious battery time. And, particularly during a journey, you never know when you will find power next. Thanks to my job, I experience more battery angst than most people, so I have tried some battery life apps. You may, too. But unlike with other types of apps, you need to be wary.