1. The charge controller restricts the rate at which current is added or drawn from an electric battery. With a charge controller, overcharging can be prevented and the battery can be protected against overvoltage or complete draining.
2. The primary function of a charge controller in a stand-alone photovoltaic (PV) system is to protect the battery from overcharge and over discharge. Any system that has unpredictable loads, user intervention, optimized or undersized battery storage (to minimize initial cost), or any characteristics that would allow excessive battery overcharging or over discharging requires a charge controller and/or low-voltage load disconnect. There are two methods for controlling charging – shunt and series regulation.
https://en.wikipedia.org/wiki/Charge_controller (1.); https://energypedia.info/wiki/Charge_Controllers (2.)