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Lee Crosson avatar image
Lee Crosson asked

MPPT 100/50 Listed Input Capacity vs Calculated

Using the formula (Panel Wattage x Number of Panels / Battery Voltage x 1.25) for MPPT sizing that I have seen listed at Alt E Store and several other sites is giving me numbers that just don't jive with what I would expect vs the datasheets for the actual MPPT. Could someone check my math or help to clarify?

Using this formula for my pair of Seraphim 320W panels gives me 66.66A, which is clearly beyond the capacity of the 100/50.

However, wiring the two panels in parallel should, using numbers from Seraphim's datasheets, yield a Voc of 45.5V (45.5V x 1) and a combined Isc of 17.92A (8.96A x 2), which would be well within the capacity of the 100/50.

Additionally, the 100/50 also lists a max nominal PV of 700W, which further makes me think that it would be a good fit for these panels, even at the theoretical max of 640W. After 30% of realistic losses the charge controller should only be seeing something like 450W, yes?

Any help would be *greatly* appreciated. I'm new to this so please excuse any idiocy on my part!

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Matthias Lange - DE avatar image
Matthias Lange - DE answered ·

Try to use the online calculator or the excel sheet (better) and enter all panel specs you can find.

https://www.victronenergy.com/mppt-calculator

https://www.victronenergy.com/upload/software/VE-MPPT-Calc-3_7.xlsm

The 100/50 should be good for two panels in parallel.

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Lee Crosson avatar image Lee Crosson commented ·

Thank you, Matthias. I did run the specs through Victron's calculator after posting this. It's recommending the 150/35. I'm fine with bumping up from the 100/50 if need be, I'd just like to have a better understanding if I do so.

What am I missing? If the '100' or the '50' aren't actually the hard limits, then why use them? If they are, where am I going astray with my calculations? Should I not simply be using the sum of the Voc or the Isc, respective of wiring the panels in series or parallel?

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Vance Mitchell avatar image Vance Mitchell Lee Crosson commented ·

The 100 is a hard voltage input limit and you need to allow for the panels outputting a higher voltage when cold. Your panels might output over 50v if the temperature is low enough.

The 50 refers to the maximum charge amps, on a 12v system this allows for around 740w (14.8V x 50A). Adding more solar over this limit would be wasted but a small amount over (say 10-20%) can help when the panels are not producing their maximum.

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