Charge Controllers regulate the current coming from the solar, wind, or other generating source to keep the batteries from becoming overcharged. The three types of charged controllers are shunt/relay, pulse width modulated (PWM), and Maximum Power Point Tracking (MPPT).
Shunt controllers are the most basic and oldest charge controllers, which simply disconnect the source from the batteries at a certain voltage. These can be seen on some older systems, but nowadays have been replaced with PWM or MPPT controllers.
Pulse Width Modulated, or PWM controllers, are industry common and can adjust the charge rate depending on the battery’s capacity. PWM controllers do this by flickering the power on and off to allow for a very slow charge that the batteries need when they are approaching full capacity.
Maximum Power Point Tracking controllers, or MPPT controllers, are the most electrically sophisticated controllers on the market, and have the ability to determine the voltage for optimal power production and then step that voltage down to the system voltage. With MPPT controllers, sometimes as much as 20% more power can be obtained from an array, giving you a higher output from your solar panels. Furthermore, by being able to step down voltage from the array to the system voltage, you are able to run a much smaller wire from your panels to your controller, saving money that otherwise would have gone towards much thicker wire.