ATtiny85 Solar Charge Controller
PWM85 solar charge controller consumes less than a single mA when running.
Pulling power literally out of thin air via solar panels is quite an amazing thing, but if you’re charging batteries you can’t (or shouldn't) hook your cells up directly. Instead you need a charge controller, like this PWM85 by Adam Welch. As its name suggests, it uses PWM power regulation to supply just enough power to keep battery cells topped off.
As also implied by the name, it's based on an ATtiny85 microcontroller, along with a MOSFET and other associated electronics to accomplish its PWM functions. Welch’s efficient circuit design meant the device sipped just a single mA of power when running, and with a bit of programming assistance – thanks to someone named Mark – he was able to get this down to 0.74mA. Once the battery is fully charged at 13.5V, a blue LED lights up to indicate this condition, with a brighter light showing a shorter duty cycle for the charging circuit.
The PWM85 will work with 36 cell panels typically sold as 12V devices, which can be rated at up to 100 watts. It includes an ISP header so that you can adjust its behavior if you so desire.
Code and circuit/board info for this PWM85, as well as a 5V version, are available on GitHub. If you’d rather not build it yourself, you can buy the board on Tindie.