Solar controllers are needed in most PV systems to protect the batteries from overcharge and excessive discharge. The minimum function of the controller is to automatically assure the battery is fully charged and to keep the battery fully charged without damage to the battery.
The basic criteria for selecting a solar controller includes nominal operating solar PV voltage and the solar PV panels rated current. A solar controller should be able to handle the rated current of all the panels combined.
For example, A 85W solar panel is rated at 5.02 amps. The controller selected should handle 5.02 amps. Two 85W panels in parallel 5.02 amps + 5.02 amps = 10.04 amps.
The next criteria is selection of the desired features. Is an indicator required to indicate the solar panel and charge controller is working? Is an LED charging indicator sufficient or would you rather have a meter that displays battery voltage and charging current? Is conformal coating an important feature? Is Gel, AGM, or lead-acid battery selection important?
Solar boost controllers maximize the power transfer from the solar panels to the battery through Maximum Power Point Tracking (MPPT). The charge controller with this feature constantly seeks the maximum power point on your solar panel to transfer its energy. Typically this converts, for example, the 17 V from the solar PV panel down to the 13 V needed to charge your battery. During the conversion, the current increases from the solar PV panels up to a possible 25%.
Diversion Load Controllers are controllers that monitor your battery voltage and limits its maximum value by diverting some of the energy into loads. When used in combination with solar PV panels, a diode is inserted between the solar panel and the battery bank eliminating discharge power in the evening. Most Diversion Load Controllers use a resistor to dissipate the excess energy in the form of heat or in some cases the energy is placed into the onboard hot-water tank.
Photovoltaics, the process of making electricity from sunlight, is growing in popularity among alternative-energy enthusiasts and for good reasons. In operation, PV panels are absolutely pollution-free (the same can't be said for their manufacture, of course) and require very little maintenance. What's more, solar cells are steadily dropping in price and are now competitive with other energy alternatives in many situations.
As is the case with so many of these independent power-generation systems, however, a photovoltaic setup requires some means of energy storage and the most popular medium now is the lead-acid battery. During the day, when sunlight is plentiful, the electricity generated by the PV panel produces chemical changes in the battery cells. Then at night—and during other nonproductive hours—that chemical process can be reversed to retrieve the stored power from the battery.
But charging a lead-acid battery isn't a simple chore. These sensitive electrical instruments require specialized care: There must be a harmonious relationship between the photovoltaic generator and the storage battery if the system is to perform efficiently and provide the years of service that it's capable of.
Sunlight, like wind, isn't a constant force. Fortunately, though, it's much more predictable than wind! Seasonal changes and weather notwithstanding, we receive about six hours of productive sunlight each day. Of those hours, the period between 10:00 AM and 2:00 PM offers peak solar radiation and the bulk of the photovoltaic-accessible energy.
Because charging occurs, at most, for only a quarter of the day, we should stuff as much power as possible into the cells during that period. On the other hand, we also must respect the requirements of the battery in order to insure that it gets fully charged and isn't damaged.
A dead lead-acid battery will accept a very heavy initial charge with little trouble . . . but only at first. As the battery progresses through its topping cycle and its chemical makeup changes, it takes on a completely different set of charging characteristics. When 70 to 80% of the total capacity has been placed in the cells, the electricity being forced in will begin to decompose the water inside the battery breaking it down into its elemental components of hydrogen and oxygen.
You may have noticed this effect without being aware of what was actually going on. The situation is often called "boiling", a misnomer that refers to the percolating appearance of the rising gas bubbles. The process is more properly called gassing and if allowed to continue, it can permanently damage the cells. To prevent this from happening, current is normally reduced just as gassing begins. At the lower rate (often referred to as a trickle charge), the battery can be lifted to 100% capacity without danger.
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