There is an old well-tried solar charge regulator called Micro M+. The circuit is here modified in a number of ways to both consume less energy and fit on an Arduino for collection of data.

The original Micro M+ circuit is well described in an article (see files).

This project will be in a number of steps to modify and extend the regulator. Step 1 to make it work with comparator instead of OP-amp and FET instead of ordinary transistors. Future steps will make it an Arduino shield for reading a number of analog & digital points into a memory for future analysis.