This solar circuit is what I am going to use for my thesis. It charges ultra capacitors. While the voltage coming in to the capacitors is larger than the voltage they are holding, they absorb more electricity. If the voltage in the solar cells drops below the voltage already stored in the caps, the diode prevents it from flowing backward. The circuit uses a transistor to act as a voltage trigger. Based on the value of the resistor at its base, the voltage threshold can be adjusted. When the voltage coming in through the solar panels falls below the threshold, the circuit stops charging, and instead discharges from the caps through the LED.