Lukas Fässler from Soldernerd has designed a Solar Charger using PIC18 seried microcontroller. In this article that is also posted in his website, he decribes the design flow as well as provides the design files.
While I like the Arduino platform I had to admit that it’s probably not ideal for a low-power design. Yes there are some things you can do to reduce the 50mA or the Arduino draws but it will never be truely low power. So I designed a stand alone version with plenty of extra features that I hope to draw only a small fraction of the current. I particularly care about the idle current, i.e. the current it pulls when the solar panel is not producing any energy. In winter, the panel might be covered by snow for weeks and you don’t want your charger to drain the battery during that time. With this new design I’m aiming for an idle current in the tens of microamps. Even if it ends up being 100 microamps that is still 500 times less than an Arduino uses just by itself. And that means it draws less than 1Ah (ampere-hour) per year so you will never drain the battery no matter how little sunshine there is.