A charge controller has an important role in a renewable energy electrical system with batteries. It functions primarily as a device that regulates the voltage to prevent the battery banks from over-discharging or under-discharging, depending on model.

Simple low-powered controller units are installed between the battery bank and renewable energy source to allow energy to flow to the batteries until they reach a set charge. When that is achieved, the battery is disconnected from the source by a semiconductor transistor or mechanical relay "switch". It is reconnected only once the battery bank has discharged enough for a recharging.

Advanced models use the one-step charge control that comes with advanced modulation techniques using pulse width and a more accurate charging algorithm for a 3-stage battery.

Large PV systems need advanced battery charging controls with automatic equalization and multi-step programs or power point tracking functions to give maximum results.

Almost all wind or hydro turbines need a consistent electric load to avoid spinning at excessively high speeds that can cause destruction or damage. The turbine should be hooked up to the battery bank with the charge controller configured to function as a diversion regulator which will shunt and divert extra energy from the turbine to somewhere else, such as to a water heater or space heater. Any extra energy will be consumed.

On grid systems without batteries, you don't need a charge controller because all available electrical energy will be channeled to the grid. But almost all inverters connected to the grid have an "energy boosting algorithm" or maximum power point tracking.

Series Controller

The series controller is connected between the battery bank and PV array in series, allowing the array's output to flow to the battery through the controller, which checks on battery voltage. When the battery is fully charged, the charge controller will disconnect the battery from the PV array to stop excess charging.

Since the battery voltage depends on the home's electrical load, it drops when energy flows out of the cells. When the recharge set-point is reached, the controller automatically reconnects the PV array to restart the charging process. The series controller functions like an automatic switch which controls current flow to the batteries depending on their charge state.

At night, the only task performed by the charge controller is to disconnect the battery bank and the idle PV array to prevent a backflow of energy into the array from the batteries. This happens even during daytime if the battery voltage level is higher than that of the PV array. If backflow were allowed, the PV array's large surface area would absorb energy flowing out of the battery and dissipate it as heat: this energy loss can be quite significant and a real concern during long winter nights.

Thus, almost all series controllers have current flow locks that work in both directions to protect against the backflow of electricity.