This is a simple charge controller that includes a dump load to dissipate excess power from the PV panel. D1 is the reference voltage that feed the 393 comparators, and its forward voltage changes with temperature. It would be better to use a low power LM317 to give a stable 1.25 volts.
Instead of a power MOSFET connecting the battery to the load, I would use a high current relay that isolates the load from the battery. Loads can generate inductive spike voltages that could damage the MOSFET.
http://chemelec.com/Projects/Charger-Dump/Charger-Dump-1.png