There are several ways of approaching this. The solar cells are going to have to generate at least enough voltage to drive your control electronics and then you're going to have to use a switch mode converter to adjust the final output voltage to the charging voltage. 1.5 to 13 volts is a pretty wide supply range. It could be as simple as using enough solar cells to generate the charging voltage and making sure they can supply enough current to change the batteries, no circuitry needed, but that's not going to be universal. The amount of current you need to supply depends on the number and type of cells in the battery and the amp hour capacity of the pack. Lipoly's need a relatively intelligent charger so you should think about starting with a 5-12 volt solar array that can handle the current requirements. Micro controllers are going to be the easiest solution, as the circuitry required to intelligently charge a battery is a bit difficult to do purely from analog parts. Suggestions would be easier if you could supply more information like the normal use for the charger, their capacity and your experience.