Hello BPL! I've been secretly bouncing around the boards for several months, but this is my first post.
My wife and I are hoping to complete a PCT thru hike in 2013 (but our backup plan is the JMT)
For this reason (and general interest in "preparedness" and DIY projects), I became interested in a solar panel system that would fit the following requirements
-- charges 1+ smartphones
-- charges AA/AAA batteries
-- internal backup battery
-- minimal weight
-- waterproof (at least resistant)
-- modular (fewer panels for light-weight hiking, more panels for more intensive electronics use or low light scenarios... whatever inspires)
-- easily attached to a backpack
I've experimented with a few commercial systems, but nothing made me confident enough to take it on a long brutal adventure (either due to high weight, low power, or awkward form factor). Those reviews are appropriate for another thread, but the point is I decided to make something myself. The challenge and pleasure of DIY alone would make the project worthwhile!
If you haven't looked into it yourself, the state of the market for solar smartphone charging seems to be that solar panels are not efficient enough to actively provide amperage to keep a smartphone running and charging; most systems reply upon an attached battery pack that receives a low current from the solar panels, and then provide a high current for the smartphone when charged in.
The guts of the system are the electronics package from bootstrapsolar.com. His components are completely open source and based on the Arduino development system. In theory I could have designed them myself and saved lots of cash, but the energy barrier to learning electronics (not my field) was too great at this point in my life. This system is awesome since it actively regulates the power coming in (solar panel, battery, wall outlet) into the power sinks (2 USB outlets, battery) keeping everything regulated and under control, with LEDs to tell you the charging state. You can even convert the dual USB charger circuit into a mono 1.5A for certain devices. The battery is capacious, and adds a bit of weight, but is small.
Small and light USB-powered 2xAA / 2xAAA charger.
Instead of using the pretty Bootstrap Solar bamboo case, I decided to use a hard-shell waterproof Pelican 1010 Micro Case (the OtterBox 1000 was too small). This adds 6.5 ounces of non-functional weight, and some bulk, but I consider it worth the penalty for the protection of the electronics in severe environments. I had to trim some of the cables, rubber case lining, and USB connector housings to get everything to fit, but once done everything fits snugly with very little shaking. I drilled holes to fit the DC and USB cables.
I also skipped the Bootstrap Solar panel; even though it has much higher amperage, crystalline panels are fragile and hard to attach to a backpack. The only other product out there that was DIY friendly is the Powerfilm Weatherpro 7.2V 200mA panels. These come in lighter non-Weatherpro versions, but the extra plastic coating makes them much more durable, and a service rep told me that the non-Weatherpro versions only last a few months in constant exposure to the elements. I hooked up these up in parallel to increase the peak amperage to 400mA. I hope to not have to upgrade to 4 panels; must test first.
To keep everything absolutely waterproof I used these connectors on my cables. They connect with normal non-waterproof fittings as well.
I did not find any small, low-weight solutions for USB connections, but these are not necessarily going to be exposed to the elements.
I made a ~4ft waterproof connector cable to extend from the solar panels to the electronics case.
DAP Marine Silicon sealant for the cable ports in the Pelican case
Liquid electrical tape for waterproofing wire splices
Heat-shrink tubing for cleanliness and durability at wire splices
Hot glue for attaching components to the Pelican case
Size 00 grommets to provide linking and mounting ports for the solar panels
Case+electronics, no batteries: 11 5/8
AA/AAA charger: 1 4/8
Dual solar panel (no extension cable): 4 oz
Extension cable: 1 1/8 oz
TOTAL: 18 1/4 oz
This weight total is much higher than I had envisioned at the start. But most of the weight is due to the large battery (greater charging security during extended bad weather) and the waterproof case (greater shock and water security for the electronics), so I've rationalized the weight cost to myself.
I have not had an opportunity to test the kit yet, but will update this when I do. (No current estimates for the next opportunity)
Photos will come tomorrow