Battery Function
Sometimes the sun doesn't shine and the wind doesn't blow. Because solar and wind power offer intermittent energy, batteries are typically used to store excess energy for times of poor energy output.
Charge Controller Function
Batteries are only designed to absorb a certain amount of charge, and overcharging them can cause damage. Solar panels and wind turbines frequently produce higher charges than their battery banks can handle. Controllers protect batteries by opening, diverting or shunting generator circuits.
Considerations
The charge controller you use must match the capacity of your battery bank. For this reason, if you are using both wind and solar power, you can't use a single charge controller for both systems. Most solar panels generate 12 to 18 volts, and require charge controllers with a maximum capacity of 20 volts. Most wind turbines can generate as many as 100 volts.