Ultimately, the goal is to power a Pi-based computer with this charge controller. The Pi will then communicate through Wifi (or a cellular data connection) with the user and do more complex tasks, such as running a web server, etc. And since I already have an Arduino in the charge controller that monitors the solar panel and battery, I can now send the power data from the Arduino to the Pi. This can be used to let a remote user retrieve power status and for a clean shutdown of the Pi on low battery.
Initially, I used a Banana Pi (almost same as Raspberry Pi) but the power consumption seems a bit high. In the simplest setup where the Aduino and the Pi communicate through a USB cable, the Banana Pi (actually Pro) uses about 2.7 Watts.