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libor avatar image
libor asked

What are true solar array limitations of Multi RS?

Hello Victron & the community,

After reading hundreds of posts and lot of documentation, I still feel I need to verify this, to be sure I'm not doing something advanterous and avoid invalidating warranty.

The Multi RS manual states these conditions:

  • maximum rated voltage of the solar charger is 450 V
  • rule of thumb: Keep an additional 10% safety margin
  • each tracker is 13 A (reversed polarity), maximum short circuit current of 16 A for each tracker
  • open circuit voltage of the PV array must be less than 8 times the minimum battery voltage when at float (--> 51.7V * 8 = 413.4V)


So for example, maximum per-tracker power capacity for above mentioned Voc x 8 would be 413.4V * 16A = 6608 W

Yet the product is branded as inverter with MPPT trackers 3 kW each.

Looking into various posts, people don't discuss any other limits than those from manual chapter 4.3. Specifically @Alex_Pescaru (but found similar statements in other posts) advices:

  • "Over-paneling on a MPPT (4kWp on a 3kWp tracker) it's not necessary a problem as long as you respect the maximum input voltage of the tracker and the maximum short circuit current of that tracker."

In my example with 8 x 455Wp panels on 1 tracker of Multi RS:

  • 8 * 455 Wp = 3640 Wp (BTW in reality never exeeds 89% --> 3240 Wp)
  • 8 * 49.5V (VOC) = 396V (well bellow 413.4V and 12% bellow 450V)
  • 455W / 41.7V (voltage at max. power) = 10.9A (well bellow 16A)


Thus voltage and current seems to be well bellow rated values, including 10% adviced margin.

I tried to use Victron MPPT caluclator, but not sure if I can trust it, it behaved funky. When I entered 9 panels, nothing appeared, then after reducing some standalone trackers appeared. I have to reduce panel quantity all the way to 2 panels (Longi Solar LR4-72HPH-455M) for Multi RS + Easy Solar II + Multiplus II to appear. Then I increasing count, Multi RS stayed there up to 7 panels, which makes little sense, since it's not respecting the 3 kW claim in marketing materials, but is bellow my calculations (should go up to 8).

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3 Answers
Alexandra avatar image
Alexandra answered ·

@Libor

The important factor is battery float. VOC is 8x float up to the max of 450v at min temps. For many batteries the 450v is just on 8x float (56v). But for pylontec and other 15s or less it is not.

Not breaking the other input rules for the mppt aside.

The choice really is between clipping and not and your install conditions (bad irradiance etc).


So you can 'over panel' but the max it will produce it 3kW per tracker in optimal conditions. If you can live with some losses then put what you want there without violating the input rules. (The 3kw is not a rule just a limitation)


In most the install conditions where I am, 70% of total installed wattage is what we can rely on actually happening most days. So usually we 'oversize' to make the full production happen.

In other words if i want 3kw then I would aim to install 4300w or about there depending on panels availability.

Volts x amps ÷ panel size.... Then check VOC of panels total in string. Make sure it is within safe limits for the colder temps and cloud edge effect and battery float. (It usually averages 6 or 7 panels in a string.)

As a side note the online mppt calculator used to tend to lean towards no clipping of production. So would want you to be under or close to the max. I haven't used it in a while.

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libor avatar image libor commented ·

Thank you for your great answer, it explains a lot.

"So you can 'over panel' but the max it will produce it 3kW per tracker in optimal conditions. If you can live with some losses then put what you want there without violating the input rules. (The 3kw is not a rule just a limitation)"

That's perfectly fine, during sunny seasons I have unused over-production and during cloudy and winter times I will never reach 3 kW with the panels. But this comment finally clarifies the reason behind 3 kW branding.

"Volts x amps ÷ panel size.... Then check VOC of panels total in string. Make sure it is within safe limits for the colder temps and cloud edge effect and battery float. (It usually averages 6 or 7 panels in a string.)"

The values in above calculations are real, taken from battery datasheet (float voltage) and "Voltage at max power" (Vmp) and "Short Circuit Voltage" (Voc) from the datasheet of the exact panel model I have. It seems people calculate it the same way (as per some posts in history of this forum). However I noticed I took lab conditions values (STC) and there are also lower nominal values (NOTC 20˚C 1m/s), but current is still well within limits (even reduces, if I take new power for the temp). I don't see any other data, i.e. for cold temperatures.

I cannot find any specific references to calculation of the effects of cold weather or partial shading. Is the 10% margin meant to mitigate risks from this variation?

My panels are standalone, south-facing and sun-exposed in the garden and are only shaded very late in the day by my house, when the overall power is reduced, so I'd assume such effect shouldn't be noticeable in my case.

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Alex Pescaru avatar image Alex Pescaru libor commented ·

Shading will degrade performance (ie power, Voc, Isc, etc) so you are not in danger in shade... :-))

As for temperature, on every panel technical sheet, you will find something like below (this taken from your panel's sheet). Then you do the math for your target temperature.

Eg:

STC for your panels is given for 25*C, where the panels have VoC 49.5V

For a temperature of -10*C, so 35*C offset, you will apply a correction of 0.265% x 35 = 9.275%. See here that I am taking 0.265 (with + sign), because the correction is given per rise in *C and we are computing for temperature evolution in the other direction.

So the VoC at -10*C will be 9.275% more than 49.5V, meaning 54.1V.

temp.jpg


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libor avatar image libor Alex Pescaru commented ·

Thank you for clarifying @Alex Pescaru.

That's what I thought about shade.

I somehow missed the little chapter Temperature rating --> makes sense now :-).

I get 433V for -10˚C, still safe under 450V. Taking it around and still assuming linear function, the limit would be reached at around -26.5˚C.

However, I thought (based on examples) that the condition "open circuit voltage of the PV array must be less than 8 times the minimum battery voltage when at float" is calculated with nominal values, but now I'm getting unsure. In case it should be applied here too (?), 8 panels would be over limit (bellow 8.5˚C), while 7 panels perfectly fine.

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Michelle Konzack avatar image Michelle Konzack libor commented ·

I use 12 modules of the exact same type because the are 210x104cm in size and fit EXACT the roof of my FireWood Storrage with 12,7m lenght.


However, I use the SmartSolar MPPT 250/100-Tr VE.Can and use only 4 modules in series because of the calculation:


450V / ( 49,5V + ( 49,5V x 0,00265 x 50degC ) = 8 Modules


Where

  • 450V is the maximum MPPT voltage
  • 49,5V is the Voc (STD)
  • 0,00265 is the Temperature Coefficient
  • 50degC is the temperature difference from STD (+20degC) which is in my case -30degC in February.


If I have in February -30degC cold cloudy days in Estonia and IF the sky open, the Voc can make a huge jump and burn the MPPT.


However, the RS450/100 has a 450Voc, 13A input and 3kW Power limit, hence you can go with 8 panels in series without any problems up to -50degC under STANDARD conditions.


8 x ( 49,5V + ( 49,5V x 0,00265 x 50degC ) = 448,5V

and 3,640Wp solar panels


If it can go colder in your are, you have to go with 7 solar panels in series

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libor avatar image
libor answered ·

Thank you for reasurance @Michelle Konzack, very helful reasurance!

Only fun fact/context follow:

I'm in (relatively) sunny South Moravia, so I don't think we ever had or ever will have -30°C. Maybe 30 years ago I remember temporary record-braking temps around -25˚C. Also, it gets warmer before even sun goes up. Those are temperatures during the coldest day 2019-2020:

1715608125588.png

So before the solar radiance get into any meaningful values (which are lower than in sunny season already), the temperature is like -6°C.

This is just another safety margin and I won't rely on it.

Also, I have ground mounted solar array, so I can easily reduce the number of active panels to 7, if I'd think there will be a cold week with even remote change of getting under -20˚C (quite unrealistic, unless El Nino goes on a grand tour :-D). Just for staying on the safe side.


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andrii-podanenko avatar image
andrii-podanenko answered ·

I have 2*4640W installed on my MultiRS Dual Tracker.

I see it is usually at 3000 when I have plenty of solar. Spikes could be to the maximum, though. But with 1.16 firmware I see it stays around 3000 usually



Here is a screen from today/yesterday trackers

1000015766.jpg



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libor avatar image libor commented ·
Thank you @Andrii Podanenko,

that's reassuring and also an answer to my unsaid question, if it could handle two such solar arrays, since I'm in planning process of a house extension and will have more suitable roof area.

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andrii-podanenko avatar image andrii-podanenko libor commented ·
2 arrays work great in my setup ( South and West facades)
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yurii avatar image yurii commented ·

Hello, I want to ask you - what kind of invertor do you use for ~9kWt of power?

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Related Resources

Mutli RS Solar Datasheet

Mutli RS Manual


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