I had not used the stm32 series microprocessors before, so this seemed like a good opportunity to learn.  I can use floating point calculations with no noticeable performance penalty even running at only 8mhz.   I have used the stm32duino environment as working with registers directly on this processor is very complex compared to the avr I am more familiar with.   Despite this I needed to anyway for the pwm output to achieve the variable deadtime needed in driving the mosfets.   I have achieved variable deadtime in the past using timer interrupts on avr series, but this limits the overal maximum frequency to around 20khz.

The design uses a basic buck converter, however it can be reversed and act as a boost converter as well.  So there is not a solar or battery specific side, but a high and low voltage side.  The parts cost approximately $12 in small quantities.