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PV Solar Panels Construction, Considerations Part Three, Connections
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Calculating the Solar Cell Requirements
The first thing to do to build a solar panel is to define the required power capability and know the solar cell dimensions and their electrical specifications. For the purpose of this tutorial 3 by 6 inches commercially available solar cells will be used for the design of a solar panel.
The solar cell electrical specifications are:
Max voltage: 0.5 Volts at open circuit voltage, 4 at max current and max sun light*
*This is a solar cell manufacturer data, actual figures can be different.
Max current: 3.6 Amps at max sun light
Watts per cell: = 0.5 Amps X 3.6 Volts = 1.8 watts
The solar panel must deliver:
Open circuit voltage = 12 Volts
Current = 6 Amps
Voltage calculation in an open circuit voltage condition
Number of cell to be connected in series to achieve the desired voltage = 12/0.5 = 24
Voltage Calculation at max current
At max current voltage will be 0.4V x 24 = 9.6 volts, sufficient for a 6 volts battery charging system
Current calculations for 3.6 amps
Number of cell to be connected in parallel = 6/3.6 = 1.6, since the cells can not be broken, 2 cell columns in parallel will be used.
To achieve the required goals the panel will be composed of two cells connections in series formed by the cell columns A + B in the picture which is actually one series connection, and by the cell columns formed by C + D, each of these columns provide the required nominal 12 volts because they connect 24 cells to make the 12 Volts. Please note that each separated 24 cell column provides each only 3.6 amps, therefore the parallel connection of cells columns A+B and C+D is necessary to obtain the specified 6 amps, although because each cell can generate 3.6 amps, the actual generated current will be in the order of 7.2 amps.
A panel can be designed to meet a combination of current and voltage requirement, toward this end the proper selection of cells power specifications and arrangement of series and parallel connection must be established. This particular example might not represent most practical solar panels since the most desired voltage levels are above 24 volts, to make it compatible with most batteries, regulators and ac to dc converters, a smaller current capability cells perhaps 1.8 amps could accommodate twice as much cells and render 24 Volts on open circuit condition for the same panel area, although the current will be a half, the total power delivery by the panel will be the same because the power relationship:
P=V X I, Power = voltage multiplied by current
A B C D
+

+

+
+


Series connection
A + B
Series connection
C + D
+
_
9.6 Volts, 7.2 Amps
Expected Power =
9.6 X 7.2 = 69.2.4 Watts
Buss
Wires
Rect Diode
The rectifier diode is necessary to prevent current feed back from the batteries or from the regulator to the cells, the diode current capability must exceed by 50% to 100% the maximum current generated by the panel.
Note:
Do not attempt to use two rectifier diodes in parallel in the same wire to increase the current rating without the proper resistor network, any or both could be burned out.
If you can not find a rectifier with the required current rating is better to insert a single diode in each one of the positive buss wire branches 1 and 2, for this example two 6 amps diodes will be OK.
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Solar Panel, Cells Series and Parallel Connections