Small RV array of two 100 watt panels. I have read that I can add two more 100 watt panels in a 2s2s type array and stay within the limits of my 20 amp charge controller. Struggling to understand it an hope someone could direct me to an image to help me understand it. Any kicks the correct direction on this would be greatly appreciated.
2s2p array image
- Thread starter GlennDale
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rodrick
Free energy enthusiast
If it’s a 12 volt battery system you can use 240 watts without over paneling you need to see if your scc can be over paneled most can be over paneled between 25 and 50 percent this allows for low pv days but your scc will only use 240 wattt clipping off the extra pv wattage you may what to look at a 3p or 3s depending on panel voc and scc max voltage if it’s a 24 volt system you can double the wattage making it 480 without over paneling
Rednecktek
Solar Wizard
What kind of charge controller? PWM you'll be looking at a 4p array whereas MPPT you can use a 2s2p effectively.
I stole this image and added my values using the values of the panels and the charge controller off their respective information sheets. So I have two, 2 series tied panel strings. Series tied panels the voltage add and the amperage stays the same. So each two panel string produces 42.8 volts / 5.5 amps. These two series strings are then connected in parallel. Parallel tied panels amperage adds and voltage stays the same. So this 2s2p array will produce 42.8 volts / 11 amps. If my math is incorrect please kick me the correct way. 
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yodamota
Solar Enthusiast
math looks correct.
sunshine_eggo
Happy Breffast!
You're over-paneled, but you are within typical allowed parameters. Some MPPT recommend no more than 50% over-paneling, and you're under that as well. In most cases the only issue is that too much PV may override any reverse polarity protection the unit may offer, i.e., don't accidentally wire your panels backwards.
Your MPPT will not output more than 20A * 14.5V = 290W; however, it will output that 290W for the same length of day a 400W array would put out 290W.
Your MPPT will not output more than 20A * 14.5V = 290W; however, it will output that 290W for the same length of day a 400W array would put out 290W.
sunshine_eggo
Happy Breffast!
What does that have to do with anything?
I wasn't saying your setup wouldn't work. I was pointing out the over paneling and the limitation. A 20A controller is 20A OUTPUT, not input.
While you can safely put 400W on the MPPT as proposed, you will not get more than 20A out of the MPPT or about 290W assuming 14.5V on a 12V system.
Yep!
In the real world, nine times out of ten those 4x100w panels won't make more than 300wh between them anyways
I'm not 100% sure but I think you might be confusing the PV amps and battery charging amps.
Amps are relative to/dependant on volts. Watts are the best unit to work by
1 amp x 100 volts = 100w
100amps x 1volt = 100w
MichaelK
Solar Wizard
A few additional comments I could make. There are the pen&paper calculations, and then there are the real-world numbers. Looks like you are handling the pen&paper ones just fine. Now the wrinkles.
First, you are doing the math with the Voc, which is for checking max voltage the controller is exposed to. For power production, you want to be using the Vmp, which is likely to be more like 18V. So, what the controller is actually seeing as the battery charges is more like 36Vmp, not the 42Voc.
Second, the panels on the RV are mounted flat on the roof, or are they angled towards the sun? When the panels are not directly oriented towards the sun, their output has to be de-rated. 60% re-rating is a good start. The value that gets de-rated is the amperage, not the voltage. Voltage stays about the same, no matter what the sun intensity is. But, the amperage goes down as the sun makes a more and more acute angle with the sun. Same for cloudy days.
So, looking at the real-world output of four panels, what you are most likely to see on a daily basis is (400W/12.5Vcharging) X 0.6 (60%) de-rating = 19.2A. So, your controller will handle that. Suppose you park the RV on a South-facing hill in June, when the sun is at it's highest possible point. The output might exceed 20A for a short while, but your controller will just clip off the extra power. Too high volts will fry the controller. Too high amps just gets clipped off and is unused.
First, you are doing the math with the Voc, which is for checking max voltage the controller is exposed to. For power production, you want to be using the Vmp, which is likely to be more like 18V. So, what the controller is actually seeing as the battery charges is more like 36Vmp, not the 42Voc.
Second, the panels on the RV are mounted flat on the roof, or are they angled towards the sun? When the panels are not directly oriented towards the sun, their output has to be de-rated. 60% re-rating is a good start. The value that gets de-rated is the amperage, not the voltage. Voltage stays about the same, no matter what the sun intensity is. But, the amperage goes down as the sun makes a more and more acute angle with the sun. Same for cloudy days.
So, looking at the real-world output of four panels, what you are most likely to see on a daily basis is (400W/12.5Vcharging) X 0.6 (60%) de-rating = 19.2A. So, your controller will handle that. Suppose you park the RV on a South-facing hill in June, when the sun is at it's highest possible point. The output might exceed 20A for a short while, but your controller will just clip off the extra power. Too high volts will fry the controller. Too high amps just gets clipped off and is unused.
I appreciate the help. This RV lives in the PNW, 47 degrees north, plus 4 months of thin overcast and a shit load of of trees. The panels will never see maximum output. Just in case I go to Joshua Tree should I put a some sort of over voltage protection between the panels and the MPPT?
Rednecktek
Solar Wizard
Howdy neighbor!
No, your array VoC is still well under the limit of the SCC even in perfect sun so don't worry about it.
MichaelK
Solar Wizard
Please go back and re-read paragraph 3, because you don't really seem to comprehend what I wrote. Voltage does NOT go up with more intense summer sun. Voltage actually goes down. It is the amperage that goes up with additional solar intensity.
sunshine_eggo
Happy Breffast!
Supplemental to the above made very basic:
PV voltage and current:
1) light gives voltage
2) intensity of light gives amperage
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