With smart phones becoming more useful as backcountry navigation tools (try our favorite, Gaia GPS), it’s also vital to have a means of charging them. While always putting your phone in airplane mode is good practice, having some back up power is fortunately inexpensive and lightweight.
Anker makes great auxiliary batteries in a variety of sizes. I have one of their smallest ones, it’s about as big as my thumb, and charges my phone about 1.5 times. (For you engineers who have to know, it's 3350 mAh.)
In addition, you can get a tiny 4 inch long charging cable for your iPhone. I'm sure there's one for Android folks as well.
The auxiliary battery and the charging cable together cost about $20, and weigh 3.5 ounces.
Personally, I consider a battery and charging cable the 11th essential, and carry this tiny additional weight with me on most every trip.
Here’s the setup I like, and it comes with a nice little padded mesh carrying case. (Sharpie pen shown for scale):