Geo
New Member
Posts: 2
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Post by Geo on Jun 3, 2017 13:34:34 GMT -8
OK foks, here's the question!
(Assuming the battery & charge controller are properly matched) - is it safe to have a larger solar array than your controller supports? And, does this depend on if it's MPPT or PWM?
Example 1: I have a Rover 20A MPPT controller - it specifies that it will take a max of 260 watts of panels in 12V mode. I'm looking at buying a 300 watt panel locally (the price is right!) But, that's too large! Is that going to cause problems like fire / burning stuff out? Or is it just going to not be able to make use of the remainder?
Example 2: My friend has a lot of roof space, and a 35A PWM controller and 12v 100Ah AGM battery bank. It seems that if he were able to pile on an excess of panels, he'd be in better shape on cloudy days, right? But, that 35A controller reasonably only handles - what, 450 watts of panels? Will he be OK if he sets up two of those 300 watt panels instead? (Actually, in this case, he couldn't because they're higher voltage panels, but for the sake of argument let's imagine they aren't).
Basically - does power at the panel higher than the charge controller's rating just go to waste, or does it risk burning stuff out? Is this the same if it's PWM or MPPT?
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Post by tattoo on Jun 3, 2017 15:00:59 GMT -8
I say no but rabird will know for sure.....
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Post by rabird on Jun 3, 2017 16:52:37 GMT -8
Geo, I read the manual PV Overcurrent The controller will limit the battery charging current to the maximum battery current rating. Therefore, an over-sized solar array will not operate at peak power. page 19 www.renogy.com/template/files/Manuals/RNG-CTRL-RVR2040_V1.0.pdfIn rare circumstance a 300 watt panel will 'make' more that 300 watts (extreme cold pointed directly at bright sun). At very low batt voltage that may exceed 20A, 300/12 = 25A 300/14= 21A Most of the time a 300 watt panel will make far less than 300 watt! I believe that answers both questions. some controls shut off at over current, others clip excess power, looks like the rover clips.
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Geo
New Member
Posts: 2
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Post by Geo on Jun 4, 2017 8:33:49 GMT -8
Rabird - thanks!
I note that on Page 1 (Page 2 of the PDF) under "Charge Controller Safety" it says "Do not exceeed 20A (ROV-20) ... The short circuit (Isc) of the solar array should be less than 20A (ROV-20) ..."
These panels are less than 9A Isc (due to being higher voltage, presumably). So that looks good!
But what *is* the "maximum battery current rating?" What defines that? Is that just "20A" for the Rover 20A? Is it someting I can set in the Rover? (Or, is it calculated somehow?)
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Post by rabird on Jun 4, 2017 12:42:26 GMT -8
that's why it is a 20a controller cuz it designed to be up to a 20A charger. They use different components in the 30A, 40A etc.
Note on page 22 specs that the max PV input short circuit is 25A, these specs never make sense! "The short circuit (Isc) of the solar array should be less than 20A (ROV-20) ..."
I assume it is when the controller scans the panel from zero volts (Isc) to Voc (note spec <100vDC PV input) to determine max power pt. PWM does not do this, PWM operates at battery voltage, so buddies PWM and 300w would only be 9A @ battery v, ~100w. see images on page 5 and assume mppt is 33v or so!
so the controller scans the panel's power curve and decides to operate @ 33.33v, 9A, 300watts. The internal bucking gadget converts that to batt voltage after losses. So a dead battery starting off in full sun, 300w/12v is 25A but the controller is designed to limit to 20A and so it should not sure how that works but the controller could force the the panel to operate at 26v x 9A = 234w 234/12=19.5A
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