question

marky0 avatar image
marky0 asked

Is the MPPT Calculator correct when oversizing a PV Array ?

The excellent technical article by Bob Hopman details how to calculate the mppt with an oversized array. But Bob's description does not seem to be consistent with the Victron provided online excel Spreadsheet Tool or online calculator ?

The technical article states

"Determining the maximum PV short circuit current.

Get the maximum PV short circuit current from the PV Panel datasheet. Multiply by the number of panels in parallel in the array. Having more panels in series does not change the number.

The result of the calculation may not exceed the Max PV short circuit current as specified in the MPPT Datasheet."

Both the spreadsheet tool and online calculator don't seem to use the PV short circuit current Isc in any of the calculations. Take the online calculator, selecting a custom module, any value for Isc can be used and there is no impact on the MPPT recommendation !!! The same with the spreadsheet : any Isc value can be entered into column H of the "Modules" tab and no warning is given in the spreadsheet calculation. They both do however use the maximum power current Impp. It appears that it is the Impp that cannot exceed the Max PV short circuit current of the MPPT not Isc which contradicts Bob's technical article.

So my question is which one is correct ? The technical article or the spreadsheet tool/online calculator ?

Thanks

Mark

MPPT Controllers
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1 Answer
matrixrat avatar image
matrixrat answered ·

Human diversity. Different engineering minds will no doubt come up with different algorithms.
Looking after three systems here, off grid:-


1: 5kw 24v panels wired 2S 5P into MPPT 100|50, setup midsummer, now almost winter.

Predecessor blew a MPPT 150|100 on same panel array by DISCONNECTING BATTERY FROM CHARGER whilst charging. Repeated that costly mistake on a replacement so that's when I got called in.
I think two out of two is a pretty good indicator that yanking out cables is NOT how to switch it off with 65v coming down the line thumping 80A into 24v battery array as it was BOTH times.


2: (2x 250w 12v) panes in series P (2x100w 12v) panels in series P 1x 250w 24v panel into MPPT 75|15. Adding more.


3: Camper 1x 250w 24v panel > MPPT 100|30. provision for plugging additional 1Kw worth of same panel.


No smoke or excess heat, everything works.

Bottom line is charger stops when Vsolar is less than (Vbatt + roughly 5v) so using 2S 12v panels will not break the Victron MPPT charger and harvest more energy for the day so as far as I'm concerned the choice is obvious, put your panels in series.

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