Sep 14, 2010

Solar Cell Backpacking

Recently, I've come to enjoy having my cell phone running while I'm backpacking for a variety of reasons. The problem is that my cell phone doesn't really last much more than a day or two on standby, let alone tracking GPS. If it's a 2 day trip, I can just take an extra battery, but in August I spent 7 days backpacking Kilimanjaro. Before the trip I did some research on various options to keep my cell phone running.

What I really wanted to do was set my self up with a solar panel. If it worked, I could use it for infinite-length backpacking. It isn't as if bringing a phone fits in the definition of ultra-light anyway.

On a recommendation from the forums at, I ordered the Powerfilm USB Charger from REI. It seemed to have the features I was looking for:

  • lightweight (<5oz)
  • fairly small
  • rugged
  • easy to attach to my pack
It also charges up 2 AA rechargable batteries which you can then use to charge the phone via a USB port. This seemed convenient instead of having to leave my phone hooked up all the time.

Before the big trip, Cristin and I did a 2-day side trip to Lassen. I took my Powerfilm for the journey. Unfortunately, while it has every feature you would want, it fails on the main one - it doesn't charge enough to keep my phone from eventually dying. It certainly helps, but not enough. I returned it. Sadly, I'm not sure that there is something on the market that will do the job.

The punchline is that I didn't have a solution for Kilimanjaro. Matt Cutts picked up a whole bunch of extra batteries, including some oversized nexus one batteries. That seemed to work well, but didn't satisfy the geek in me - it seemed like cheating.

So, this got me wondering - how theoretically possible is it to power a cell phone from solar power while backpacking? If my math is wrong, please correct me.

The amount of solar energy hitting a square meter of earth in a day is approximately 4.8 kWH. The Nexus One's battery is 1400 mAh, 3.7V and I find that I need to charge it about once per day backpacking. I don't want a solar panel much larger than the one mentioned above which is ~7x4 in = 28 sq in.= 0.018 sq meters.

1400mAh x 3.7 Volts = 5.18 WH = 0.00518 kWH.

Now we have the same units. My little panel is much smaller than a square meter, so:
4.8kWH / sq meter * 0.018 sq meters = 0.0864 kWH

So, the amount of solar energy hitting my little solar panel in a day is 0.0864 kWH and the amount of energy I need for my phone in a day is 0.00518 kWH. That's 16x more energy than I need! If I can convert just 1/16th of that power (6.25%) into battery juice, I'm set.

Unfortunately, I think the best solar panels available are only around 40% efficient. I suspect that within a backpacking product I probably won't get better than 10% efficiency. Also, efficiency decreases with heat, and the panels are close to my head / in the sun. Also, I'm unlikely to have my panels perfectly pointing at the sun at any given time - and never have any shade from clouds, trees, or my big head. So, I'd be lucky if I actually get half the amount of energy hitting my panels that I could if I were carefully aligning it. Then of course there is more efficiency lost in transferring that energy into my phone's battery. Basically, had I done the math to begin with, I don't think things look very promising. Roughly speaking: 10% / 2 / 2 = 2.5%

The good news is that we aren't that far off. With time perhaps my phone will become less power hungry. Solar panels are also getting cheaper and more efficient. Perhaps one day we'll be able to get cheap backpacking panels that are as much as 60% efficient. Maybe they'll be cheap, flexible, rugged, and light enough enough that I can just make the top of my backpack out of the material instead of having a small square hanging off the back, giving me a much larger area to absorb energy from.

I can imagine the day when the higher end backpacks come with a USB cable installed, just like a hydration system or an emergency whistle. You just plug in your digital camera, cell phone, and flashlight and your pack takes care of keeping everything charged. Would this be against the spirit of backpacking?

Update (16 September 2010):
There have been a number of great comments on the Buzz thread that got created off of this post. Questions about battery charging efficiency, piezo-electric chip alternatives, calories vs. Calories, 5000 farad capacitors, an estimate of the efficiency of my solar panels, etc. An Electrical Engineer buddy of mine, Andy Neff, weighed in and put my little bit of math to shame. If the above post was interesting, you should definitely check out the Buzz thread.

1 comment:

Bev Fox said...

I am trying to track down a solar-powered AA/AAA battery-recharger to fit on top of a backpack.
The lid of the pack is 12" x 6" (300mm x 150mm), and I am hoping to be able to power 8 batteries at a time.
Please advise?