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Mr Solar's Monthly Newsletter #74
February 2002
www.MrSolar.com
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Question of the month.
Hello, I am interested in buying some new solar modules to add to my system. I currently have Siemens SP75, 75 watt solar modules and would like to add the BPSX75, 75 watt modules to my system. Is this possible?
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Modules of different sizes, types and\\or different manufacturers can be mixed in the same solar array. However, for the system to work properly this must be done with caution.
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Background Wiring Terms
There are two major terms used in wiring: series and parallel.
Parallel wiring keeps the voltages the same, while increasing the current. Parallel can be described as positive to positive, negative to negative.
Series wiring increases the voltage in a system, while the current rating remains the same. Think of a flashlight, as you add batteries a positive to negative connection is made.
Lets say you have two 60 watt, 12 volt solar modules that produce 3.5 amps each. If they are wired in parallel, the array will produce 7.0 amps at 12 volts. If they are wired in series, they will
produce 3.5 amps at 24 volts.
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Explanation of Answer
Using the above terms allow me to explain what happens when you mismatch modules using the following examples.
Here are the modules I will use in my examples:
SX40, 40 watts, 2.37 amps @ 16.8 volts
SX60, 60 watts, 3.56 amps @ 16.8 volts
SP75, 75 watts, 4.4 amps @ 17.0 volts
SX75, 75 watts, 4.54 @ 16.5 volts
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Series Wiring
If you have a 24 or 48 volt system you should try to avoid mixing modules sizes when wiring in series.
If you have a 40 watt module and a 60 watt module and try to wire these for a 24 volt system you are wasting 20 watts of power.
When you make the series connection the modules voltages will add together to 33.6 volts. Since the current remains the same in a series connection, the total current produced drops to the lowest output module, 2.37 amps (SX-40).
When wiring in series you should always use modules that have similar output ratings.
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Parallel Wiring
If you decide to mix modules, you want to do this in parallel wiring.
If you are mixing the SP75 and the BPSX75 their currents will add up to 8.94 amps. Since the voltage remains the same in the parallel connection the total voltage drops to the lowest of the two modules, 16.5 volts (BPSX75).
As you can see you still want to be careful not to add too low of a voltage module to the system. For example the SM46 by Siemens, 3.15 amps @ 14.6 volts is a self regulating module. This module was made with a low voltage for RV applications so a charge controller was not needed. The problem is as a module heats up the voltage drops. So on hot summer days the battery will never be totally recharged. If you want to add a module like this to your parallel wiring you must include a blocking diode to keep this module from pulling the array
voltage down. A blocking diode is like a one way valve letting the module add its current to the total until its voltage is too low.
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Mixing Technologies
If you have a thin-film or amorphous module and want to parallel wire it with a crystalline module, a blocking diode must be used. These modules perform differently under the same environmental conditions and the blocking diode will only allow the highest voltage panels to add current to the total. In a situation like this you could still be wasting some power. A better way to wire these module types to the same system would be to
purchase a second charge controller. This will allow you to isolate the two arrays and allow each to
charge the batteries independently.
(Around Oz - we use this system and
find it the best bet as you usually cant fit two or more of the same panels
on an A'van plus Jeff has used several different brands over the years)
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Sale Items
http://www.mrsolar.com/sale/
120 watt solar modules, $535
75 watt solar module, $335
10 watt Solar module, $99
http://www.mrsolar.com/sale/
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Have you purchased something from us in the past?
Tell us how we did at:
http://www.mrsolar.com/survey/
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http://www.mrsolar.com/classifieds/
Don't forget about our classified section.
If you are looking to sell or buy used equipment, this is the place for you.
Here are some of the items listed.
10 watt solar panel $50
6 watt solar panels $40
3 watt 6 volt solar module $20
C-40, 40 amp controller $75
http://www.mrsolar.com/classifieds/
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