A charge controller is an essential part of nearly all power systems that charge batteries, whether the power source is photovoltaic modules, wind turbines, hydro, fuel, or utility grid. Its purpose is to keep your batteries properly fed and safe for the long term.
The basic functions of a controller are quite simple. Charge controllers block reverse current and prevent battery over-charge. Other controllers also prevent battery over-discharge, protect from electrical overload, and/or display battery status and the flow of power.
The key to extending battery life in a solar system is to protect the batteries from both a complete discharge and over-charging. A charge controller is a power system component whose functions include regulation of solar charge current, and in some cases to provide low battery protection.
A charge controller, or charge regulator, prevents overcharging by limiting the current flowing into the batteries. They are analogous to a switch in the charging circuit which opens when the batteries have reached a 100% state of charge. A good regulator will also provide a float current to keep batteries topped up.