[Alex] decided to build a solar charger for his car battery. He had an 18V solar panel able to provide up to 83mA. You cannot connect panel directly to the battery because charging voltage cannot exceed allowed safe limit and also solar panel may become a load for battery in dark time, and this way discharge it.
So he ended up with simple circuit utilizing LM317 and couple resistors setting voltage so that battery would be charged at recommended 13.2V. To prevent back supply, a Schottky was used. Of course, it adds some voltage drop (0.7V worst case). This was taken into account while calculating voltage adjusts resistor divider. As a test [Alex] left solar charger for three days connected to his battery, and it charged up to 12.35V which is about 75% of capacity. Not bad at all.