If you Google
"(MPPT) maximum power point track design" you will get numerous hits. Typically new designs use a microprocessor to track the solar panel and battery voltages and adjust the converter voltage to achieve maximum charging efficiency. This is not a trivial design.
For example
here's a design using a Fuzzy Logic controller. The advantage of Fuzzy Logic is that it's generally easier to optimize the control for a particular application as compared to a PID controller, especially for a non-linear system or incompletely defined requirements.