A charge controller simply regulates the electricity going to your battery bank and prevents it from being over-charged. It monitors the levels of the batteries, and if they are full, it interrupts the flow from the solar panels. Controllers are ranked on how much amperage they can handle. A charge controller is highly recommended when battery backup is used, because it protects the batteries and lengthens their lifespan. Most controllers have maximum power point tracking (MPPT) included, as to regulate and optimize the flow of electricity coming in from the solar panels, and increase the amount of energy the system can produce. Charge controllers are only used when battery banks are used as part of the system.