Many individual silicon solar cells tend to have an open-circuit voltage of approximately 0.5 volts and a short-circuit output current limited to approximately 3 amps, therefore it is necessary to combine these individual solar cells together in either series and parallel combinations to obtain higher voltages and currents. Commercially available solar panels are constructed using between 32 and 48 individual solar cells in series to give a panel capable of charging a 12V DC battery. But how many solar cells are in a solar panel? and how many solar cells do I need?. Well, as usual, it depends on your specific application.
The electrical power generated by a photovoltaic cell, ( PV ) has two components: Voltage ( V ) and Current ( I ). The output power generated by the PV cell is measured in Watts, ( P ) that the cell produces is the product of the cell's output current times its output voltage. In other words, P = V x I. The voltage output of the photovoltaic cell remains fairly constant over a wide range of input light intensities because of the cells photovoltaic effect, just as long as there is some light. The output current, however, varies in direct proportion to the amount of sunlight entering the PV cell. The more light entering the cell, the more current it produces up to its maximum. The solar cell's output voltage remains fairly stable from low to bright sunlight.
Typical 36 (6x6) Cell Solar Panel
For the purposes of this tutorial here, we will consider a standard 4" by 4" (100mm X 100mm) poly-crystalline silicon photovoltaic cell. Mono-crystalline or amorphous silicon cells are available. The absolute value of the voltage information will differ slightly, but their general performance tends to remain the same for all types of silicon PV cells for the amount of sunshine it receives on a sunny day. So how does a solar cell work.
A poly-crystalline silicon solar cell has an open circuit voltage of about 0.57 Volts at 25°C. Open circuit voltage means that the cell is not connected to any electrical load and is therefore not generating any current. When connected to a load, for example a battery, the output voltage of the individual cell will drop to about 0.46 Volts at 25°C as the generated current flows. It will remain around this 0.46 V level regardless of the sun's intensity or the amount of current the cell produces.
This decrease in output voltage is caused by internal resistance losses within the cell's structure as well as voltage drops across the metallic conductors deposited on the cell's surface to collect the current. Ambient temperature also has an affect on the PV's cell's voltage. The higher the temperature is, the lower the cell's output voltage becomes as it heats up, which is strange seeing that they spend all day sat in the sun.
While the voltage produced by a silicon photovoltaic cell is fairly constant, its output current on the other hand varies considerably. The amount of usable output current that a cell generates depends on how intense the sunlight is shinning onto the cell's surface, and also the voltage difference between the cell and the load.
Under normal operating conditions a poly-crystalline cell is rated at about 2.87 Amperes of current. This value can increase considerably on a very cold, very clear, very bright and very snowy winter's afternoon. Also altitude is another factor that affects the PV cell's output current. The higher you are, the less atmospheric conditions there is above and the more sunlight the cell will receive, assuming no clouds or snow. So expect to see current gains if used well above sea level.
When individual photovoltaic cells are assembled together into modules or panels they are generally wired in series. That is the positive connection or pole of one PV cell is connected to the negative connection or pole of the next cell, and so on until all the cells in the panel are connected together in what is called a series string. This series wiring is done to raise the voltage of the panel. We said earlier that a single cell has a voltage potential of about 0.46 Volts. This is not enough voltage to do any usable work in a 12 Volt system. But if we add the voltages together of say 36 cells by series wiring them, then we have a working voltage 16.7 Volts, and that's more than enough to charge a 12 Volt battery.
The operational voltage of a typical 12 Volt lead acid battery ranges from between 10.5 volts to 14 volts. The battery's exact voltage depends on its state of charge, ambient temperature, and whether the battery is being charged or discharged at the time. It is this battery voltage curve that the PV panels are designed to fit and so MUST provide a greater voltage than the battery possesses. If the PV panel cannot do this, then it cannot transfer electrons to the battery and therefore it cannot recharge the battery.
The output current generated by a solar panel of 36 cells in total remains the same as the current produced by one single cell, about 3 Amperes. The series wiring technique causes the voltages to be added together, but the current remains the same. We could parallel connect all the 36 cells but this would add their currents together rather than their voltages. The result of this would be a solar panel that produces 108 Amperes of electric current, (36 x 3) but at only 0.46 Volts, too low.
Most photovoltaic (PV) panel manufacturers make 12 Volt solar panels for battery charging with 32, 36, or 48 cells in the series string. They are all rated at about the same current, being composed of the same basic cell. The difference between these panels is one of voltage. The question for us to answer here is how their output voltages relate to the voltages we require for our 12V charging system.
This size of photovoltaic panel has the lowest voltage rating of only 14.7 Volts (0.46 Volts times 32 cells). This is because it has the fewest number of PV cells in its series string. This panel design closely matches the charging curve of a standard 12 Volt lead acid battery. As the battery charges-up, its terminal voltage rises. When this battery is almost full its voltage is about the same as the PV cell's at around 14.7 volts. The 32 cell module simply hasn't enough voltage to continue charging the battery when its full so cannot overcharge the average, small, lead acid battery.
The applications suitable for these small 32 cell solar panels are in RV's, boats, garden lighting and summer cabins. These applications are characterized by their intermittent use and relatively small battery charging capacity. In these these types of low power applications, a 32 cell panel can be used with or without a charge current regulator as the batteries will not become overcharged if left connect to the panel during long periods of non-use.
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This size of photovoltaic panel has an output voltage of about 16.7 Volts (0.46 times 36 cells). This is enough output voltage to be able to continue to charge a lead acid battery even though it may be already fully recharged. The 36 cell panel is suitable for a home based 12 Volt alternative energy system with high battery capacities as it has the higher output voltage necessary to recharge deep cycle lead acid batteries.
However, a 36 cell solar panel will require some form of charge regulation to prevent overcharging the battery during periods of high solar intensities or when battery usage is at its lowest.
A 36 cell solar panel tends to be more cost effective in a typical home power application because it can produce a good amount of current or high voltages at elevated temperatures. The higher voltage produced by the 36 series wired cells will more effectively recharges a large deep cycle lead acid batteries.
High ambient temperatures will cause the voltage of any PV panel to reduce slightly, but the 36 cell panel has more than enough voltage surplus to still be an effective battery charger even at high ambient temperatures.
A 48 cell panel is the big daddy of the PV industry. 48 individual photovoltaic cells connected in series produces an output voltage of about 22 volts. These large PV panels have sufficient output current capacity to charge a 12 Volt system, regardless of the battery's voltage or high temperature. However, these large panels do require some form of charge regulation in just about every application. They have the sufficient voltage necessary to raise a solar system's voltage, while charging full batteries, to well over 16 volts. This over voltage is high enough to ruin any electronic equipment rated at 12 VDC so some form of protection is needed.
Generally, a 48 cell solar module has very specific applications where high power and currents are required such as in pumping water or were long cable runs will have appreciable voltage losses even if large diameter cables are used. Another disadvantage of this PV panel is its size and additional cost compared to 32 and 36 PV cell panels. On the plus side, a 48 cell panel will perform better in very hot areas and areas with very low levels of sunlight throughout the year.
To learn more about "Photovoltaic Cells", solar panels and solar power, or to learn how to build your own DIY solar panel from individual solar cells to make Solar Power in your home a reality so you can save money on your utility bills, and to help you on your way considerand getting one of the solar books from Amazon about home made solar panel construction ensuring so that you have all the necessary information to get your solar power installation working efficiently and effectively the first time.