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What is a solar charge controller and how does it work?

Photovoltaic panels are the consumer’s choice. With the price of carbon-based fuels on the rise, the power companies that rely on fuels such as coal and oil to generate the bulk of their electricity have been forced to raise their rates; subsequently, homeowners are now opting for solar power to satisfy their electricity needs. Photovoltaic systems are popular with homeowners because the energy is free and recent technological advances have reduced the initial cost of these systems.

Batteries require a steady charge.

Due to the intermittent nature of solar energy, it is not practical to connect appliances directly to a photovoltaic panel. Instead, the power absorbed by the panels during hours of sunshine charges batteries, where it is stored for use as needed. The output of photovoltaic panels depends on certain variables such as the location of the solar panels, the time of day, and the season. These factors all affect the angle of incidence of the sunlight striking the photovoltaic cells and consequently, the temperature of the cells. These dynamic conditions that cause fluctuations in the output power of the panels are not conducive to good battery health. As a general rule, batteries like to be charged at a current rate that is about 20% of their Amp-hour rating. For instance, a 10 Ampere charge would be ideal for a 50 Amp-hour battery. Since overcharging batteries can cause them to quickly fail, a solar charge controller is necessary in order to insure the proper charge current is maintained.

What is a solar charge controller and how does it work?

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 the 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 discussed earlier, the actual values are dynamic and thus the need for a charge controller. There are some very sophisticated solar charge controllers available today, which use maximum power point tracking, or MPPT, allowing 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.

The proper charge controller is necessary.

As you can see, a solar charge controller is an absolutely essential component of any photovoltaic system. Although it is true that a simple functional charge circuit can be built with a few parts obtained from your local electronics part store, this option is not recommended for the average person. With so many variables associated with solar power, the use of a solar charge controller that has the ability to track all the pertinent parameters and provide the optimum charge for your batteries is recommended instead. The use of a quality solar charge controller will ensure that you enjoy the maximum life from your batteries and the most from your photovoltaic system.

Author – Charles LaRose

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