Start with this. It is self-contained without intervention from the AVR. You can use the AVR as a supervisor, to disconnect the load to protect the battery when the sun doesn't shine. We can modify the circuit later to enable the AVR to speed up re-charging.
In the meantime, start simple. This basic circuit will limit initial charging current to ~ 1/2A (R5 = Vbe/Ic). It will also limit the maximum battery voltage to 13.7V (adjustable with R6) after the current tapers.