Background Research: In order to determine if this was possible, I needed to do some research. First, I tried to find out if anyone had attempted this before. A quick web search told me that a few people had powered their cell phones while camping using some portable solar panels, but nobody had apparently attempted to do what I was hoping to do.
My next step was to determine what solar panels (or solar cells) were available. Very quickly I determined that there are two types of solar panels: (1) those with an integrated Lithium battery pack which the solar cells charge; and (2) those without a battery pack which only charge your phone when both your phone and the solar cells were in the sun. I chose the solar panels with the battery pack since this meant I could leave the solar cells in my car all day while I worked, and charge my cell phone at night.
A quick bit of research found the following solar cells, all of which were suitable for my purpose:
I initially chose the SolarMio 31 since it offered better charging in low light (cloudy) conditions, but it did not have as large of a battery pack as the Solio Mag. The SolarMio was also less expensive than the Solio Mag. The Brunton SolarRolls did not have an integrated battery, so those were out of the question. However, when I arrived at my local REI store, I found they were offering a Brunton Freedom charger. This charger used a Lithium battery and was half the price of the SolarMio! The battery is a 2,200 mAh battery, which is more than enough to charge my iPhone, and well over double what is offered with the SolarMio (900 mAh). The photo to the left is the Brunton Freedom.
Hypothesis: If I try to use only a portable solar cell or panel, I should be able to power my iPhone 3G without the need for any supplemental power from an outlet or car charger.
Experiment: I plan to hang my Brunton Freedom in my car window every day while I am at work (roughly 9 a.m. to 5 p.m.) facing South. I will then use the Freedom to charge my iPhone. I will not use any other supplemental power.
Data Collection: I placed the cell phone in my car window while I was at work, starting on Monday, February 22, 2010. I left it there until I left for home each day.
Analysis: The Brunton Freedom was able to fully charge my cell phone on Monday night. This was likely due to the full battery in the Brunton device, since I had pre-charged it before Monday using a wall charger.
On Tuesday evening, the Brunton solar panel battery was only able to charge my device to 80%. This was probably due to the fact that the device only had 50-75% of the battery charge remaining, which was not sufficient to fully recharge my depleted cell phone battery. Unfortunately, it was mostly cloudy or overcast on Tuesday, so the Brunton device was not able to fully recharge.
After work on Wednesday, the third day of the experiment, I had to stop the test. My cell phone battery was at 25%, and the solar cell battery did not have enough of a charge to charge my cell phone at all. What was especially disappointing was that the solar cell on the Brunton Freedom was insufficient to even charge the internal battery over 25% after the device was left in the full, bright sun for eight hours. I would have expected it to be at least half full, but it wasn’t.
In case anyone is wondering, I have included the graphs showing the solar intensity each day. Except for a few tiny dips, the sun was shining clearly the entire day on Wednesday.
I have subsequently found out that Brunton offers the same model solar panel with two 100 mA solar cells, which would presumably charge the internal battery twice as fast. Even with this second panel, I have my doubts that it would have charged the internal battery sufficiently to fully charge my cell phone.
Conclusion: It is not possible to charge an iPhone (3G) with a small, portable solar panel as the only source of power for the phone. Supplemental power will be necessary or the phone use must be extremely limited.
Finally, if you plan to buy a Brunton Freedom, make sure you understand it’s limitations. If I had a week to leave it in the sun, this might have been a different story.
May 20, 2012 Update: I recently tried this experiment again with a Joos Orange solar charger. This time, I was able to make it an entire week before fully depleting the solar charger. The technology is getting closer, but is not quite there yet!