I finished my solar powered guitar amplifier and I want to brag about it.
I put a full write up on my website with a ton more photos and a video demonstrating how it sounds: http://www.crazybutable.com/projects/solar-stella
I used the medium 2 watt solar panel, the 2700 mAh lipo battery, the USB/solar charging circuit, and the thermistor (which came in handy when I accidentally locked it in my car all day, unintentionally baking it and charging it at the same time). It sounds awesome and looks even better. The Adafruit components were top notch and worked perfectly the first time I set everything up.
I designed the amplifier circuit itself almost two years ago. One of the things I wanted to do was to make it sound really good at low supply voltages (unlike the usual battery powered guitar amp chip, the LM-386), and, in fact, from day one I wanted this amp to work well with solar power.
For one reason or another I wasn't able to pick up everything I needed to get this amp to work on solar power until fairly recently, and let me tell you I let out a huge sigh of relief when I powered it up and everything worked right the first time! The battery is probably too big, I don't think I've ever come close to exhausting it.
I do have a few questions though. Do I have to drain the battery every now and then like I do with my laptop battery? Also, I notice that in full sun, after I charge it to the point the green done light turns on, if I briefly shade the solar panel, the done light goes out and it looks like it starts charging the battery again. Is this safe? Will it get overcharged?
What if I have it in the windowsill, and it gets enough power for the charge light to blink at me constantly, but never gets enough charge to get in the "done" state. Will the battery fully charge if I leave it there long enough? Will it overcharge?
Obligitory disclaimer: I sell the Stella amp as a kit on my website (without the solar stuff), but it is available as 100 percent open hardware.