Power Characteristics of Solar Modules
The current and power output of photovoltaic modules are approximately proportional to sunlight intensity. At a given intensity, a module's output current and operating voltage are determined by the characteristics of the load. If that load is a battery, the battery's internal resistance will dictate the module's operating voltage.
A module which is rated at 17 volts will put out less than its rated power when used in a battery system. This is because the working voltage will be between 12 and 15 volts. As wattage (power) is the product of volts times amps, the module output will be reduced.
For example: a 50 watt module working at 13.0 volts will produce 39.0 watts (13.0 volts x 3.0 amps = 39.0 watts).
This is important to remember when sizing a PV system.
An I-V curve as illustrated to the right is simply all of a module's possible operating points (voltage/ current combinations) at a given cell temperature and light intensity. Increases in cell temperature increase current but decrease voltage.
Maximum power is derived at the knee of the curve. Check the amperage generated at your batteries operating voltages to better illustrate the actual power developed at your voltages and temperatures.
Mixing Sizes and Brands of Modules
In most cases mixing dissimilar modules in the same array is not a problem. When paralleling units of different amperage ratings, the output of the array will simply be the sum of the combined amperages. When paralleling units of different voltages, the lower voltage units will simply begin to taper off sooner as high battery voltage is reached. If used for array direct power, the array voltage will be the approximate average module voltage.
When series connecting strings of dissimilar modules, however, the amperage will be approximately that of the weakest module in the string. It pays then, to pay attention to matching the modules connected in series.
Reverse Current Protection
PV modules will leak power back from your batteries during no sun periods if not protected. This leakage is very small but over long, no-sun periods, this loss can accumulate. To prevent this we install a diode or protecting circuitry in the controller.
All controllers that we sell have reverse leakage protection. The circuit opens over periods of no sun, allowing the charging circuit to stop any reverse flow. A diode can also be used. This unit acts as a one way check valve-letting power flow in one direction to the batteries but not back to the PV module.
PV modules are very sensitive to shading. Unlike a solar thermal panel which can tolerate some shading, many brands of PV modules cannot even be shaded by the branch of a leafless tree. Once a solar cellar a portion of a cell is shaded it becomes a load and draws power instead of producing it. Watch the amp meter of your system when a hand is passed over a module and you will see a substantial drop in output. Some solar modules offer protection from partial shading. The advanced design of these modules include a diode between every cell, reducing partial shading problems. Ask your solar professionals for more information if shade protection is needed. Another rule of thumb - make sure no shading occurs between 9:00 a.m. and 3:00 p.m. around solar noon. Shading early or late is not much of a problem because these are low power producing hours anyway.
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