Solar Charge Controllers for Photovoltaic Systems

The output of your photovoltaic panels varies greatly depending on certain variables such as the location, the angle of the sun, and the temperature of the panels. These fluctuations are not good for your batteries. Since undercharging or overcharging batteries can cause them to quickly fail, a solar charge controller is necessary in order to insure the proper charge current is maintained.



A solar charge controller is an electronic circuit that adjusts the output voltage and current of solar panels to match the needs of the batteries being charged. In order to charge a battery, your charger--or in this case--your solar panel must be at a higher potential than the battery being charged. In other words, the voltage of the panel must be greater than the opposing voltage of the battery under charge, in order to produce a positive current flow into the battery. But how much more should it be? The current flow can be calculated using Ohm's law: E=I*R; where E is the voltage in expressed in volts, I is the current in Amperes, and R is equal to the resistance measured in ohm's. The effective voltage in this circuit is the output voltage of the solar panel minus the opposing voltage of the battery. We can determine this difference in voltage necessary to cause our desired current flow by plugging our known values into the equation. Assuming a 12 volt battery with a 50 Amp-hour capacity, our target charge rate should be about 10 Amps. There is no resistor in this circuit except for the internal impedance of the battery, which is a fraction of an ohm. In this case we will approximate it to be 0.2 ohms. Using Ohm's law we multiply 10 Amps by 0.2 ohms and we get 2 volts as a result. This means that our solar panel must be 2 volts higher than our battery voltage to maintain our target charge rate. This example is meant to show that the rated output voltage of your solar panel must be greater than the battery being charged for your solar charge controller to work. As I mentioned earlier, the actual values are dynamic and thus the need for a charge controller. Go Green Solar has some excellent solar charge controllers available at a reasonable price, which allow them to adapt to different battery and panel combinations such as 24v, 36v, 48v, etc. These controllers use DC-DC converters to match the voltage and use digital circuitry to measure actual parameters many times a second to adjust the output current accordingly.