A charge controller is an essential part of all battery based power systems, whether the power source is PV, wind or hydro-electric. Its primary purpose is to keep your batteries from becoming overcharged. Most batteries must be protected from overcharge and excessive discharge, which can cause electrolyte loss and even damage or ruin the battery plates.  Most charge controllers also have a mechanism that prevents current from flowing from the battery back into the array at night.  In addition, advanced charge controllers offer maximum power point tracking (MPPT), which can extract more power out of your solar array.