Solar. - And here is Circuit and Project SCR Based SSS Solar Charge Control using S2800A and TL431. A force-commutated SCR makes a novel Solid State Switch in a solar charge regulator control. Prior art includes relay and transistor switches, but an SCR switch in this type of DC application may be new to the world (normally SCRs are applied in line-commutated AC applications). Advantages include robustness and the requirement of only one conductive device that performs (3) functions: switch, latch and reverse polarity diode (reverse blocking thyristor). The main SCR conducts the charging current from the solar panel to the battery while a 2nd commutation SCR performs the function of commutating (turning off) the main SCR current at the end of the conduction period. All circuitry consists of readily available discrete components. Note that this is old technology and may not be the best or cheapest solar charge control solution, but it makes a great, mind-expanding tech school lab experiment.
A TL431 is strapped to regulate at its minimum voltage of 2.5V. R19 biases it at about 1mA. It is referenced to the positive battery terminal. This is far superior to a low voltage zener because it has very low dynamic resistance and low thermal temperature coefficient.
The Original Article : "SCR Based SSS Solar Charge Control"