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randallemarshall avatar image
randallemarshall asked

Are there losses by maxing out an MPPT 100/20? Should I upgrade to a 150/35?

I have 12 panels that are each 100 watts for charging my 48V battery. They will be wired as 3 parallel strings of 4 series connected panels. They parallel together in a fused combiner box. Each panel's Voc is 24.6V and the Isc is 5.28A. As a result this will be a Voc of 98.4V and an Isc of 15.84A. With most electronics I hesitate to max out the rated device specifications because they're overrated but Victron is generally more reliable.

And actual numbers when connected to my battery/load should be lower. Vmp is 20.6V and Imp is 4.85A. So only 82.4V and 14.55A total. Then to top it off the panels will be mounted flat so less current will be expected.

Self-consumption is 19ma for the 100/20 vs only 10ma for the 150/35 but that is so little anyway. I'm not exactly sure what temperature compensation is referring to but it's the same on both units.

  • Both the 100/20 and 150/35 are rated for peak efficiency of 98%. Both units are rated full output up to 40°C. Will the 100/20 get hotter than the 150/35 with these solar specs? This is for a mobile application so I won't always have them in a comfy 20°C room.
  • Is there any other way the 98% efficiency number may be affected by how maxed out the unit is?
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2 Answers
wkirby avatar image
wkirby answered ·

98.4V is a bit too close to the 100V limit. That array could easily exceed 100V in colder ambient temperatures. The specifications are normally based on 25ºC.
You'd probably be better off with the 150/35 in that case.

The temperature compensation is for lead batteries. Raising the battery charge Voltages when cooler and lowering them when hotter.

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randallemarshall avatar image randallemarshall commented ·

Thank you for teaching me this. That about solves my dilemma then.

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

With 98,4V the Voc is way to close to the max of 100V.
The Voc will go up at temperatures below 25°C.

If you want to go for a 4s3p setup you should use a 150/35.
With the 100/20 you should do a 3s4p setup.

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randallemarshall avatar image randallemarshall commented ·

Thanks, both of you taught me a lesson about temperatures on Voc.


3s4p may be possible. My combiner box is meant for 4 parallel strings so I have space. But I previously ruled it out because 4p brings the short circuit current to 21.12A which is over the 20 amp rating of the 100/20. Is there something there I'm missing?

EDIT: I'm purchasing the 150/35. Thank you for the help.

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