If you are thinking of installing a small solar panel to create a 12V system, maybe for some lighting in a shed or to service a leisure battery in a vehicle, this efficient and simple PWM charge controller might be of interest.
This opensource controller is suitable for up to 36 solar panels with an open circuit voltage up to 21V. Pre-assembled it’s ready to go out of the box configured to work with 12V lead acid batteries. With some tweaks to the supplied code and an ISP programmer you can reflash the onboard ATtiny85 to get it to play nicely with other battery flavours if required.
It’s very efficient, whilst your target battery is charging and under the charged voltage threshold, the device itself consumes less than 1mA. Excellent when you want to send as much energy from the sun to your battery as you can! Supplied with a random colour 3D printed mounting bracket we love the simplicity and efficiency of this project from AdamWelch.