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thepilot avatar image
thepilot asked

MPPT 250/100 max input current

Hi

Just looking to clarify if my assumptions are correct.

I'm looking to install a 4kw solar array, two series arrays of 5 x 400w panels. The max current for each array is 11.7A and voltage will be 226.6V (These are assumed max with temperature corrections for coldest temp). Combined these will produce 226.6V and 23.4A which should be fine for the MPPT 250/100. Stepping down in the MPPT to a 48V battery system (so approx 56V max for charging) would mean a current output of approx 92A.

If I were to add 3 more solar panel arrays as above and combine them to 226.6V and 93.6A in total into the MPPT it would be an array size of 16kw in total (Low temp corrected and its nearly 21kw max). So the max input current at 226.6V is 93.6A and the max output from the MPPT would be 100A (battery bank would be 48V so max 56V for charging). Steping down the voltage would mean the available current would be approx 378A but the MPPT would limit this to 100A output.

Firstly are my assumptions correct?

Secondly what happens to the excess current in the MPPT....does this get dissipated into heat?

Lastly would it be more efficient to limit the input to the MPPT so that the max current output is as close to the 100A limit and therefore add more MPPT's rather than send all the panels to one MPPT?


Many thanks


MPPT Controllers
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5 Answers
Alexandra avatar image
Alexandra answered ·

@thepilot

The max short circuit current accepted in is 70A. The reason for this is that sometimes the pv input is short circuited to prevent charge.

See the tech spec.

General calculations are batt voltage x charging amps + 30% for watts connected in.

48x 100 =4800 x 30% = 6240W ÷ 400W panel size = 15.6 or 16 panels max.

Run your scenarios through the mppt calc sheet or the Victron online calc.

And yes things will get very hot when run at max like that leading to derating. It is better to add more mppts rather than waste roof space and panels.

And the point of an mppt is to not have to have the current in close to the current out, that is more of a pwm charge controller setup.

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thepilot avatar image thepilot commented ·

Many thanks Alexandra. I'll spec my system in relation to that.

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roedolfdel avatar image
roedolfdel answered ·

I have a similar question...

I am trying to "max out" my 250/100 MPPT. (It says 250/100 so I want it to do just that... thats why I bought it). If I run 2 strings of 5 460W panels (Voc 42V and 14A) the MPPT at max capacity and ask it to charge at 100A like as it says on the box... will this be a problem or bad practice? Im struggling to find a series/parallel solar panel combination that will satisfy its capacity. I mean sure you can go and use low wattage solar panels but then you "waste" roof space.

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jetlag avatar image
jetlag answered ·

Here again the hint to check the online MPPT calculator from Victron.

For your setup I would asume, that the Voc voltage in cold conditions might be over 50V per panel, and so a string out of 5 panels in series would go over the allowed 250V (I have also some 460Wp panels installed).

The power itself with ~4,6kWp should be no problem.

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roedolfdel avatar image
roedolfdel answered ·

Thank you Jetlag for your answer. So I've been doing allot of reading... I just need to charge my planned 40kwh battery bank withing 3,5hours in Summer and 7 hours in winter. (Winter is half of our Summer array in South Africa). I can go up to 600W panels, do 2 strings of 9 (Voc 41V and 18A). That will take it to 369Volts and 18Amps = 10800Wp

That should be enough right? And it should comply with a Victron 450/100 MPPT (If I sell my 250/100 ofcourse). Max charge amps for the 450/100 is 100A or 5760W. Sooooo that brings me to my next question: If my 40kwh battery bank is 800Ah and the max charge rating of the MPPT is 100A then it will take 8 hours to charge? I only have 7 hours of useful sunlight in Summer and 3,5 hours in the winter so that mean I will never be able to fully charge my battery with my system?

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jetlag avatar image
jetlag answered ·

Well, a 40kWh battery is really a big one. And yes, I would agree with your assumtion. If you have 7 hours of pure sunshine delivering the full 100A into the battery, there will be some time before and after this "peak". So normally in such a sunny day it should be possible to gather the missing 100Ah. But nevertheless, the PV array is "too small" for that huge battery. Another question is, - do you consume the whole battery energy every day? Or is this only a kind of buffer for not-sunny days?

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