Battery Basics for the Backwoods

Of Amps and Ounces: How to choose your backpacking battery

The sun is dropping fast and we’re deep in the heart of an Adirondack wilderness area, a good 15 miles from the nearest wall socket. It’s the last night of a four-day trek, and for the duration of the trip, two power-hungry iPhone 4s have been tracking our course, charting our elevation gains and stitching together panoramic photos. Using an external battery, we have recharged the phones a total of four times. Now we're almost out of the woods with juice to spare, and we're feeling saucy. It’s time to splurge. It’s time for movie night.

A panoramic photo made possible by portable power

With an iPhone duct-taped to the ceiling of the tent, we settle in, crank up the volume, and press play on The Shawshank Redemption. Way out here in the middle of nowhere, the four-inch screen seems like an IMAX. For 142 minutes, we are transported out of the deep woods. I'm no longer aware of every rustling leaf and splitting twig. I even forget about my acute stealthrabidninjabearaphobiaall thanks to having chosen the right portable power source.

At the movie's turning point, watching those convicts drink beer on the roof of their prison, the sweet baritone of Morgan Freeman’s character, Red, reflects, “Me, I think he did it just to feel normal again...if only for a short while.

I pause, smile, and make a note to myself to bring popcorn next time.

Battery Basics: Keeping a charged smartphone throughout a multi-day trek is not an essential element of an outdoor experience, but it sure is nice. Skip to the bottom of the page for details on the battery that powered my Adirondack trip, but first, this primer on battery technology will help you make your own buying decisions. Here’s what you need to know:

Volts (V) represent the eagerness of electricity to move from one place to another. You want the electricity to move into the battery of your portable device, and in order to accomplish this task, a certain minimum voltage must be achieved. Many portable electronic devices require 5V as an input, because this is the voltage delivered by the ubiquitous USB (Universal Serial Bus) connection.

Amps (A) represent the quantity of electricity moved during a specific time period. If you imagine pouring out a bucket of water, amps would be the volume of water that hits the ground each second and volts would correspond to the distance the water fell (and, thus, the force with which it was impacting the ground).

Capacity. Battery capacity is measured in Amp hours (Ah) or milliamp hours (mAh) where one Amp hour is equivalent to 1,000 milliamp hours. As a point of reference, an iPhone 4's internal battery holds 1,420 mAh. If you plan on recharging your phone four times, and you lived in a perfect world free of inefficiencies, an external battery rated for 5,680 mAh would do the trick.

Batteries work through reversible chemical reactions. But here in the real world, it’s highly inefficient to transfer chemical energy from an external battery into electrical energy via a USB cable, and then back to chemical energy in the phone's battery. The result is that it might take 1,600 mAh to fully top-off an iPhone’s 1,420 mAh battery.

Construction. Batteries are composed of three main elements: a case that holds the other two elements; some electronics to regulate charging and discharging; and multiple electro-chemical cells that store energy in the form of a reversible chemical reaction. The weight of the case and the electronics are negligible, and they have almost no bearing on battery capacity. The cells are the main factor in determining both the battery's weight and capacity.

Now here's the kicker. There is no magic arrangement of cells that will yield more capacity to store energy. The performance of the cells and, in turn, the performance of the whole battery, is based entirely on the chemicals used to create that reversible chemical reaction. Each recipe of battery chemicals creates a paste that can store a certain amount of energy per unit of mass. As long as you are using the same recipe (ie: alkaline, nickel metal hydride, lead acid, or lithium polymer) the only way to add more capacity is to add more paste.

Okay, school is out. Here’s the hardware.

The battery that I settled on was the IMP1000 from New Trent.

This battery uses lithium polymer cells, which are the current state-of-the-art in rechargeable consumer battery chemistry. For about $70, this 10.2-ounce battery delivers 11,000 mAh. It gives me about five full charges on my iPhone 4. It’s not waterproof and it doesn’t strike me as particularly durable in construction, but then again neither is my phone, so I’m already committed to keeping things safe and dry when backpacking. New Trent sells a 12,000mAh version with dual USB ports for a few bucks more, but I found this model just fine for my needs.

Here are the unit’s vital specs:

  • Battery Capacity 11000mAh
  • Recharge Time (with A/C wall adapter): 6-7 hrs
  • Life recharge Cycle: 500+ Times
  • Output: DC 5V /1A
  • Charging time for iPhone 4: 1.3 -1.5 hrs

And now for some simple buying advice: Buy a lithium polymer rechargeable battery.

Try to ignore marketing and let price be the determining factor in your decision. Remember, these batteries are more or less the same on the inside. The New Trent model I bought uses lithium polymer cells—and any lithium polymer battery will have basically the same power-to-weight ratio. When choosing what battery to get, evaluate your phone or GPS device’s battery capacity, and how many times you want to recharge between visits to a wall socket. Last, consider how much extra weight you want to carry.

On a final note, I would like to say that it would be in exceptionally poor taste to refer to any external power source as “ultralight.” Ultralight isn't about making due with your GPS device or iPhone’s internal battery, it’s about leaving the electronics behind altogether. That said, each time I settle in for a wilderness movie night, I am reminded that many of the best things in life are well worth the weight.