A Sun Powered USB Charger
After trolling the internet this summer for an inexpensive, well made, solar USB charging device, I came away with the sense that I could make one for considerably less than current prices. Solar is in vogue, and it is amazing what kind of poorly made overpriced gadgets are being marketed to the general public. One search for solar battery charger on Amazon, and you'll see what I mean.
After working through the numbers with my solar panda project, I put together a list of parts with some beefier specs than what you can find commercially available. While not inexpensive, this unit can charge faster and store more than anything I've seen for the same price. It consists of these items from Adafruit:
The panel can put out slightly over 500mA of current in optimal weather conditions. In theory, with a load attached to the Minty, there should be enough leftover current to charge the battery at the same time. The nice feature of this charger is its ability to manage the incoming and outgoing current, so the battery is not constantly discharging and charging. This significantly increases the useful lifetime of the battery.
These components are wrapped up in a laser-cut fluorescent blue acrylic enclosure. The design of which you can see below. The blue lines are what will be cut, with the red and green for engraving. I placed standoffs where you see the small round holes, and am using a modified 'tab in slot' design to enclose the inner square that houses the components. The standoffs will make the whole structure that much more structurally sound. I also added the "outboard" standoff mounts so the unit can be placed vertically on a surface without the main body coming in contact with that surface (a bit of ding prevention). The outer square of slots is for a second, bigger bottom plate, which will let me later add other components (charge meter, temperature, etc).
You'll notice that I left space in each corner of the enclosure. This gives the unit some amount of ambient airflow to keep temperatures below critical levels. Temperature can be an issue with charging rates of over 500 mA. I did discover that when plugged into a USB power source, the chip on the charger gets warm to the touch. Since I replaced the resistor on the charger to allow for up to a 1000 mA charge rate, temperature monitoring is essential. You can't really tell from the pictures, but I added a thermistor as suggested on Lada Ada's tutorial for the battery charger. This will proportionally limit the charging rate as the board gets hotter, and completely cease charging at a safe upper temperature limit.
The enclosure design file is available at my showcase on Ponoko, and looks very similar to this:
Here are a couple of pictures of the assembled prototype:
And here's a closeup of the components:
I would really like to combine the charger and MintyBoost into one custom PCB, add a charge meter, and use headers on the board for the USB input and output power. This would let me place the USB jacks using parts that are ready made for panel mounting. You'll notice that I had to do some hand cutting of the acrylic to place the Minty in a suitable spot, so the inner compartment needs to be resized, as well.
Over time, UV exposure can make acrylic brittle and prone to cracking/breaking. There are UV resistant materials available, and I need to do some homework on an option that also looks as good as this version. In the meantime several microcontrollers, a couple of Droid phones, and various other gadgets will all have a chance to sip on sun power!