Originally Posted by John_V

You may also want to look at the Wahoo RFLKT as well. While not as fancy or as expensive ($79 USD) as the Elemnt, it will display almost any data value that Cyclemeter tracks and the screen is so user defined that you can place that data anywhere you want it. I use one, along with HRM and speed/cadence sensors, and can ride well over 4 hours and still have about 50% of my phone's battery life left after the ride. And I'm using an iPhone 5c that's over two years old with the original battery.

Here's a web site with some tips to help extend the battery life of an iPhone. It has some stuff for an iPhone 7 but if it doesn't apply to your iPhone, like a new feature, you can skip over it.