A charge controller regulates battery voltage and current to control the charging rate, or the state of charge, for batteries and/or loads. Solar (photovoltaic) panels output varying voltage. Under full sun, the voltage from a solar panel may be quite higher than the nominal voltage of the system. The need for a charge controller arises because many electronic devices are sensitive to voltage fluctuation, and overvoltage can destroy them. In addition, batteries can quickly be destroyed by overvoltage.
Charge controllers are available in many voltages, currents, and configurations, such as: no controller, voltage regulator, basic charge controller with low voltage disconnect (LVD), pulse width modulation (PWM) controller, and maximum power point tracking (MPPT) controller.
A quick note on charge controllers: Most charge controllers have a prescribed order of connection, and disconnection, that must be followed. That order of connection is often Battery first, Panels second, Loads third. That order of disconnection is usually exactly opposite, i.e., Loads first, Panels second, Battery last.