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peternielsen avatar image
peternielsen asked

Choosing the right SmartSolar chargers

Hi,

I have 3x4 and 3x3 constellation of the attached panels. I have tried to use the calculator and i am wondering why it's suggesting 150/60 for my 3x4 side when the datasheet states Nominal PV power, 48V = 3440W and i have Total PV Power : 3720W and then the same on the 3x3 side where i have total PV power 2790W and the datasheet states 2600W, i am sure i am misunderstanding something but what is it ?

Amerisolar-AS-6M30-PERC-Black-Module-1640x992x40mm-Eng.pdf

The next question is about MC4 vs without . As far as i can see all MC4 connectors are rated at 30A. My 3x4 side will be 40A since each series is of 10A and the 3x3 side will be 30A. Does that mean that if i buy a 150/60 MC4 i can't use it with 40A with the MC4 ? If so why then make it at all ?

I should mention that i will run a 48V system (16S of LiFePo4 Batteries)

Best regards,

Peter

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2 Answers
wkirby avatar image
wkirby answered ·

3x4 array. This configuration is good for a 150V controller.
3720W @ 48V = 77.5A battery current at maximum power. So you could use a 150/70. You'd be limited to 3360W when the battery is at 48V and the full power of the array will be available when battery Voltage is higher than 53V. The limits are minimal and a PV array only makes full power in ideal conditions. The 70 limit is automatically imposed by the MPPT and an oversized array will not damage it. If you don't want the array to be limited at all then a 150/85 is the answer.

3x3 array, this configuration is also good for a 150V controller.
2790W @ 48V = 58.2A
A 150/60 would be perfect for this array. Its maximum charge current is just above the capability of this array at 48V battery Voltage. As the battery Voltage increases it will be in its comfort zone.
Only if the array produces over it's rated capacity for those rare cloud edge effects or really cold sunny days then it will limit it's self to the 60A.

I don't know why that solar calculator is acting like it is.

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peternielsen avatar image
peternielsen answered ·

@WKirby Sorry i didn't see your answer, Thank you very much. Not sure i understand all of it but one thing i was wondering. The 3x4 array could be converted to 2x6 array in order to increase the voltage so the voltage drop over 6mm2 cable 15meters is very low compared to how much the voltage is dropped if use 120V. What's your take on that? Am i running to close to the MPPT charger of 250V ? The array would run on 240V 20A. If i use the calculator it states i can't probably because i am close to max ? I don't know if i can trust the calculator. Maybe @Guy Stewart (Victron Community Manager) knows if there are issues with the calculator ?

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klim8skeptic avatar image klim8skeptic ♦ commented ·

I work out a 6 series string to 285.24v @ -40c.

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Alexandra avatar image Alexandra ♦ commented ·

@peternielsen

Panles outputs are rated at certain temperatures. Usually about 25°C. So colder they can produce more than their rating under ideal solar irradiation conditions. Hotter they produce less.

If you scroll a bit further down you will see a graph that you set for your area and it will help you see outputs over the year for the array setup you have chosen.

If your panels are 310w and you are installing 12 then your potential is 3720 watts. If you choose a lower amp output mppt then you will loose so to speak the full peak potential of your array as it will limit to that amps multiplied by battery voltage.

Running at lower string voltages is mostly noticed under lower light conditions. And 15meter runs in my experience don't affect output as much unless you are trying to combine all the strings into one closer to the panels. Then with the higher combined amperage you will need larger cable and possible use the TR rather than the Mc4. If you use the Mc4 then it is best to spread all the strings very all the terminals. Heat is an issue in the cables.

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peternielsen avatar image peternielsen Alexandra ♦ commented ·

@Alexandra So i have two options, combine all the strings from each side of the roof in a combiner box close to the panels then run two sets of cables that will carry 120V 30A and 120V 40A or i can pull all the cables closer to the MPPT charge controller and combine them there. I believe it's best to combine them close to the panels. No ?

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Alexandra avatar image Alexandra ♦ peternielsen commented ·

@peternielsen

The big mistake most people make is running one cable down that is too small and have hot cables. So this does depend on cable size and whether you are using the tr or the mc4. If it it the mc4 split your array over all the mc4 connections available on the model you buy, if TR get biggest cables recommended.

You do not mention what you are using but I can say from troubleshooting problems on systems that the cable most people use after combining at the panels is the same size as before and gets hot (and this is obviously a hazard) plus it means energy is being lost as heat. That why on the lower amperage mppts there are 2 mc4 pairs and on the larger mppts there are more.

So answering what is best really depends on your setup.

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peternielsen avatar image peternielsen Alexandra ♦ commented ·

I am using 6mm2 cable so according to the calculations made together with @WKirby the cable should be ok. There is loss but i think it's acceptable.

Also i plan to use the Tr model in order to be able to plug the cable directly to the MPPT charger since i can't use MC4 with 40A since they are not rated for it.

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peternielsen avatar image peternielsen peternielsen commented ·

@Alexandra

So as i see it have two options :

1. Combine all strings close to the panels then run one set of 10mm2 x 15m cable for the 120V 30A and one set for the 120V 40A.

2. Run all the cables 7 sets which is 14 cables using 6mm2 x 15m as close as i can get to the MPPT charger then combine and run either 6mm2 or 10mm2 from there to the MPPT charger.

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wkirby avatar image wkirby ♦♦ commented ·

240V is getting a bit too close. Remember, this is at 25°C. If you gett good sunshine and the panel temperature drops below ~7°C then the open circuit Voltage will be more than 250V. In hot climates then you'll probably be fine.
Maybe better to go with 4x3 and suffer the power loss in the cable.

Power loss in the cable. 15 metre run of 6mm² = 30 metre round trip.
6mm² cable ~3.3mΩ / metre x 30 = 0.1Ω in total.
So, at 30A, 30²x0.1 = 90W lost to heat in the cable. At 20A you lose 40W in the cable.

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peternielsen avatar image peternielsen wkirby ♦♦ commented ·

@WKirby i was always calculating using 15m but i can see i need round trip according to your calculation so in total i will lose one the 3x3 array 9W and on the 3x4 array i will lose 16W so in total 25W out of 6510W (21 panels of 310W) ? That would be 0,38%, doesn't sound right. What am i doing wrong ?

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wkirby avatar image wkirby ♦♦ peternielsen commented ·

Let's use the figures from the datasheet which you so kindly linked earlier.
One panel has 9.46A flowing through it at nominal power of 310W, power loss in your cable is 9.46²x0.1 = 9W
3 panels in series = 9.46A @ 930W, power loss in the cable is 9W.
4 panels in series = 9.46A @ 1240W, power loss in the cable is 9W.
You see the power loss in the cable is the same bacuse the current is the same even though you are transmitting more power as you add panels in series. That is why raising the Voltage makes electricity transmission more effeicient because the Voltage does not cause heat, the current causes heat. Power loss rises exponentially with current increase and gets inefficient really quick.

Now connect strings in parallel:
2x strings in parallel will cause 18.92A to flow through the cable = 35.8W cable loss.
3x strings in parallel will cause 28.38A to flow through the cable = 80.5W cable loss.
4x strings in parallel will cause 37.84A to flow through the cable = 143.2W cable loss.

So yes, for your 3 series x 4 parallel array = 143.2W and your 3x3 array is 80.5W for a total cable loss of 223.7W at full power.


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peternielsen avatar image peternielsen wkirby ♦♦ commented ·

@WKirby so i am missing something essential then. Looking at a calculator as this https://photovoltaic-software.com/solar-tools/dc-ac-drop-voltage-calculator the voltage 120V and 40A is typed in and i get a loss of 288W. What am i missing ?

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wkirby avatar image wkirby ♦♦ peternielsen commented ·

You are not doing anythging wrong I think.
I have been unable to do simple multiplication. I once wrote: 3.3mΩ x 30 = 0.01Ω. Well it doesn't, does it. It is, in fact, 0.1Ω.
This changes everything and now the calculations are a lot more realistic.
I have altered my mistakes and I'm sorry to have misinformed you. Thank you for questioning me. I don't know what came over me.
I shall edit my posts to prevent anyone else being misinformed.

In fact, I am soon to be running a 25m 6mm² cable, I'm glad the mathematics have been done here.

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peternielsen avatar image peternielsen wkirby ♦♦ commented ·

@WKirby thank you very much for getting back so this changes alot since i have a loss of 455W !! which is extremely high i would say, that's 7,5% of the overall production goes into the cable. Why is it that we need roundtrip of the cable i mean if the cable is 15m why do we use 30m to calculate ?

According to the calculator i linked to earlier it's stated :

L : simple length of the cable (distance between the source and the appliance), in meters (m).
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wkirby avatar image wkirby ♦♦ peternielsen commented ·

b : length cable factor, b=2 for single phase wiring, b=1 for three-phased wiring.

Current has to flow all the way back to the solar panel array to complete the circuit.

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