When you have a solar power system with batteries to store the energy you capture, you need to have a charge controller. A charge controller is a regulator for all the power that you collect with your solar panels, to prevent them from overcharging your batteries and shortening their life. Its functioning is very similar to a regulator in a car.
Solar panels are usually rated to put out about 17 volts or so, and your batteries are rated at 12 volts. You can charge your batteries with about 14 or 15 volts, but the panels have to be rated higher, as often they will not be working in the optimum conditions of bright sunlight directly overhead. If they didn’t have the higher rating, then a lot of the time you would not have enough power to charge your batteries effectively. The charge controller takes care of all this matching of output and input for you.
Charge controllers are available in a couple of types, the difference being the cost and the efficiency – how much of the solar generated power ends up in the batteries. The more basic controller has several stages of control and pulse width modulation control, which means the effect of the control is to send out a series of short electrical pulses whenever it is working. If the batteries are low on charge, the pulses are relatively long, with short breaks in-between; as the batteries charge up, the pulses become shorter with longer off periods. Incidentally, this can cause some interference or buzzing if the installation isn’t behaving properly.
The more expensive and better controllers are maximum power point tracking controllers, which give very close to 100% efficiency. They do this by providing a continuously variable voltage output, selected to give the maximum power that the batteries can take in at any particular state of charge. This involves some high frequencies for conversion of the voltage to the optimum level. These controller are particularly effective at using the output from your panels in non-optimum conditions, i.e. when it’s cloudy, and they do provide a worthwhile increase in usable energy.
Whatever type of charge controller you get, it is very worthwhile to have the gauges to show you how well your system is working. Sometimes these are optional extras. Basic units may have only a couple of lights, which show you have a voltage and that you’re getting a charge. Better are the types that show you actual voltage and current, and these may be regular meters or electronic displays.