Last summer I spent 5 of the longest days of the year on the Tahoe Rim Trail. My plan was to charge my GPS (2xAAA) and my 30 gig video Ipod from the sun. To accomplish this I had a Solio charger and another homebrew charger I constructed from a couple of thin film solar panels, a 2xAAA series battery holder, a blocking diode, a little wire and of course some duct tape <img src="/forums/images/graemlins/grin.gif" alt="" />. It was similar to this. In direct sun it was pumping 100 milliamps @ about 3 volts into the cells and in theory could fully charge two 1.2 volt 900 milliamp/hour AAA cells in less than 9 hours. It sounded pretty on paper.

The panel I made was approx. 12"x4". In theory it would produce about as much power as my GPS consumed in a day. And the Solio would do the same for my Ipod which included listening to tunes for a couple of hours on the trail and hour or so of movie viewing each night.

In reality, they didn't even come close. I'm glad I brought extra cells just in case.

If I had been able to simply lay the chargers out in direct sun all day they would have had a chance. And that probably would have been possible had I been base camping somewhere.

But I wasn't and therein was the problem. It was impossible to keep the chargers in direct sun and therefore maximum output while I was on the move. Most of the miles were in forest cover. The panels were mounted on my pack and sometimes my own head shaded them. Deep canyons were too shady. I had to constantly stop and adjust the orientation of the panels when I changed direction. They blocked access to my pack. The whole experiment was a PITA for very little power harvested. After spending all day in the charger the two AAA's moved my GPS's power meter up only one bar of four. I would get a good hour or so of sun on them in the evening when I made camp but only by constantly adjusting as the sun set.

I could build an array twice as big and harvest more power but that would border on impractical because it would cover so much real estate on my pack.

My conclusion: Until more efficient technology arrives, it's simpler, lighter, and cheaper to just carry extra cells. Unless you are out for a very long time, or just staying in one sunny place.