Most models of lights use a single solar cell which produces a low voltage, and a varying amount of current depending on the size of the cell and the amount of light striking the surface.
The solar cells are wired directly to a battery through a diode, which prevents the battery's current from flowing back through the solar cell at night. During the day, the battery charges, then at night, the solar cells stop producing power and a photoresistor turns on the LED light that is your solar garden light.
