With smartphones ever more common as vital backcountry navigation tools (try our favorite, Gaia GPS), it’s also crucial to have a means of charging them. While always putting your phone in airplane mode at the trailhead to save battery is good practice, having some backup power is inexpensive and lightweight.

Some people only will take an auxiliary battery on a multi-day trip. That works great provided you remember to charge your phone fully in your car driving to the trailhead, but that is a little task that's easy to forget. For me, it's more reliable to just carry a fully charged extra battery and cable as standard practice.

Anker makes great auxiliary batteries in a variety of sizes. With the larger batteries of modern phones, probably 5,000 mAh is the smallest do you want to go, which should give you one full charge. (Geek note: “mAh” means milliamp hours, a way to describe battery capacity.)

If you're going on a longer trip, sharing your battery with partner(s) or don't mind carrying a little more weight, go with a 10,000 mAh battery. That should easily last one person a week or so.

I like having both - this small one for short term emergency backup, and a 10,000 for longer trips.

In addition, you can get a tiny 4 inch long charging cable for your iPhone; no need to bring the long cord you use at home. I'm sure there's one for Android folks as well.

Tip: cheap charging cables can get damaged getting banged around in your pack and fail when you most need them. Try to buy cables that are a little more durable, and consider bringing two of them on longer trips, they are inexpensive and extremely lightweight.

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. 

Note - If you search online, you'll find loads of different battery options, many from off-brand companies. I’ve tried a few of these trying to save a few bucks, and been disappointed. I recommend you pay a tiny bit more and get a name brand like Anker.

The photo below is a few years old; now I'm using a 5000 mAh battery. but you get the idea.

(Sharpie pen shown for scale)