Ok maybe this isn’t a real solar light as it comes from LED, but the idea is laudable. Idea is pretty simple and not new – harvest solar energy during day and store it in to battery and deliver light to plant during night. In fact there are some difficulties in doing that. Author have used a Luxeon K2 LED that requires over 2,79V, while two NiCad batteries can give around 2V.
After some experiments he changed battery to Li-Mh with 3.7V and built an ATTiny15 microcontroller based circuit where LED is controlled by PWM depending on how much energy is left on battery. Battery voltage is measured via MCU ADC channel and depending on voltage level PWM signal is adjusted. This is more fun project to work with as practically LED light cannot replace sunlight.