Using the diagram below can be made a simple solar panel battery connection switch electronic device that allows charging a battery using a solar panel.
When voltage is too low, the relay is not actuated, so that the battery is not connected to the solar panel.
When voltage is sufficient for activate relay and light dependent resistance (LDR) R2 receives enough light for the opening of T1, the relay is activated and the battery will be charged.
Due to its inherent hysteresis, the relay remains activated and when panel voltage drop a certain level. A charged battery connected can not operate the relay when light falls because R2 will lock transistor T1. With P1 will adjust the lighting when the relay actuation occurs.
Because consumption is determined primarily by the relay, it is important that it be miniature type, with high resistance of the coil, but still be able to switch up to 10A.