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Old 16-08-2012, 12:24   #1
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Solar Panel output voltage?

I want to buy 4 240W panels to keep my 12v 840AH batteries topped up but I do not know what sort of panels to look for.

The output voltage seems to be high for the larger panels (36v ish).

I understand that I have to use a MPPT controller, but if the output voltage is far above the charging voltage for my batteries will I be "wasteing" some of the power of the solar cells? Or will the MPPT controller deal with this variation in voltage?

Lastly do certain panels work better with certain controllers?

Thanks
John
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Old 16-08-2012, 12:26   #2
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Re: Solar Panel output voltage?

The MPPT controller, with a few exceptions, will take the higher voltage, reduce it, and increase the charging current.

The panels and controllers don't care or know what brand eachother are.
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Old 16-08-2012, 12:27   #3
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Re: Solar Panel output voltage?

Quote:
Originally Posted by John Duckworth View Post
I want to buy 4 240W panels to keep my 12v 840AH batteries topped up but I do not know what sort of panels to look for.

The output voltage seems to be high for the larger panels (36v ish).

I understand that I have to use a MPPT controller, but if the output voltage is far above the charging voltage for my batteries will I be "wasteing" some of the power of the solar cells? Or will the MPPT controller deal with this variation in voltage?

Lastly do certain panels work better with certain controllers?

Thanks
John
Short answer, as long as the panel output is within the voltage range specs of the MPPT controller it will make maximum use of the panel output. So, nothing is wasted using a higher voltage panel.
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Old 16-08-2012, 12:35   #4
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Re: Solar Panel output voltage?

John:

Make sure that your MPPT controller has a maximum input voltage rating higher than the Voc (open circuit voltage) of your panels. Also 960 watts of panels will put out 960/12 = 80 amps of current if they are all wired in parallel. The maximum output current specification of your controller needs to be 100 amps because NEC requires a 25% safety factor. That will mean an expensive controller.

Or you could wire two sets of two panels to one controller. Then you will need two 50 amp controllers.

David
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Old 16-08-2012, 13:39   #5
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Re: Solar Panel output voltage?

I faced the same question last year (except my panel are 230W only). I talked directly to technicians at several MPPT manufactures. Some manufacturers will void warranty if nominal voltage of solar panels does not match (or is not close enough to) nominal voltage of their MPPT controller. Outback FlexMax are designed to work with high voltage panels (up to 150Voc). So, be careful with your choice.
BTW, the difference between MPPT and ordinary controller is that, unlike ordinary controller, MPPT will not waste available power, it will use all available power and bring voltage down to nominal charging voltage while converting excessive voltage into more current.
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Old 16-08-2012, 14:05   #6
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Re: Solar Panel output voltage?

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Originally Posted by BambooSailor View Post
I faced the same question last year (except my panel are 230W only). I talked directly to technicians at several MPPT manufactures. Some manufacturers will void warranty if nominal voltage of solar panels does not match (or is not close enough to) nominal voltage of their MPPT controller.
The concern is the maximum voltage of the panel. Most controlers are damaged by even brief periods of exceding their maximum voltage. Therefore most manufacturers recommend staying well below even the Voc for the panel ( or panels if they are wired in series)
The panels should also not be capable of exceeding the maximum current, at the battery voltage, although often this will not damage the controler, as usually they self limit.
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Old 16-08-2012, 14:37   #7
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Re: Solar Panel output voltage?

Yea looking at your amount of panels 4x240=960w / 12v =80a you may to go with 2 Tristar 45 about $400 or could even go with 1 Midnite classic 150 $610 or a Lite for $500 which is a 96a cc at 12v.
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Old 16-08-2012, 15:39   #8
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Re: Solar Panel output voltage?

Just doing some research on MPPT controllers. Interesting case to be made, particularly for larger installations, to have different panels controlled by different controllers. This way when some panels are shaded by clouds etc for example, and some not, then the controllers for those different groups of panels will optimize differently -- resulting in more total output than a single controller for all the panels. Read that some manufacturers/installations are starting to consider building MPPT controllers into the panel itself so that each panel can be optimized individually.

Also reading about different MPPT algorithms. Such technical things are not usually humorous, but the common name of one algorithm "Perturb and Observe" I found funny -- sounds like a great bar game to me!
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