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 time. (For you engineers who have to know, it's 3350 mAh. mAh means “milliamp hours”, a way to describe battery capacity.)
This is about the smallest, both in size and capacity, auxiliary battery I've been able to find. If you're going on a longer trip, sharing your battery with partner(s) or don't mind caring a little more weight, you probably want to choose a battery that has a higher mAh capacity, such as 5,000 or 10,000.
According to the manufacturer, a rough rule of thumb is that about 3000 mAh can charge an iPhone 8 about one time. So, If you think you might need to recharge your phone three times on a trip, go for the 10,000. (Personally, I only carry this 3350 mAh battery for short term emergency back up for climbing, and have a much larger one for longer trips off the grid.)
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 every trip.
Here’s the setup I like, and it comes with a nice little padded mesh carrying case. (Sharpie pen shown for scale):