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12 100w panels charging a 48v LiFePO4 battery - Question about Victron 100/20 MPPT controller

MilkMan

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I have 12 100w panels for charging my SOK 48V battery. I'm debating 4s3p vs 3s4p. I purchased a Victron 100/20 but I'm thinking it won't work.

Panel: Voc 24.6v, Vmp 20.6v, Imp 4.85a, Isc 5.28a
  • My original plan was 4s3p bringing total Voc to 98.4v and Isc to 15.84a. But this leaves no room for cold temperature Voc increases. If temperature drops to 0F Voc could reach 112.4v.
  • In 3s4p the Voc is 73.8v and Isc is 21.12a. The Isc is now over the 20a rating of the Victron. This also has a Vmp of 61.8 which does not leave much room for line voltage drop. (57.6v charging voltage) And don't MPPT controllers prefer much higher voltages to work efficiently?
  1. Is my only choice here to buy the Victron 150/35?
  2. Does mounting angle affect Isc? I'm mounting them flat. 6 panels per side in my attached picture.
Victron datasheet: https://www.victronenergy.com/uploa...-MPPT-75-10,-75-15,-100-15,-100-20_48V-EN.pdf

EDIT: Oh, and my trailer is pre-wired with 10 awg wire which is why I was trying to do 4s3p first. (Less voltage drop on the line) You can see the MC4 connector in the bottom right of the picture.
 

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The 20A rating of the Victron 100/20 is the battery charge current. It has nothing to do with the incoming current from the panels.

You have 1200W on a 48V system. Figuring a typical voltage close to 55V that is 1200W / 55V = 22A. Since your panels will be flat mounted you will rarely, if ever, see 1200W. So a 20A controller is just fine.

With the 12 panels in 3S4P you would have no issues using the 100/20 on a 48V setup. It will be about 21A of current from the panels to the SCC. Given the high voltage from the 3S, 10AWG will be just fine carrying that current.

But you are right that the Vmp is a bit low for a 48V system. Given that, you might be better off using the 150/35 (there is no 150/20). The 150V will allow for a 4S3P arrangement. And the 35A of battery charge current would allow you to expand to even more panels if ever needed.

One last thought - mounting 12 panels is a big PITA. Consider 4 300W panels instead or even 6 200W panels.
 
The 20A rating of the Victron 100/20 is the battery charge current. It has nothing to do with the incoming current from the panels.

You have 1200W on a 48V system. Figuring a typical voltage close to 55V that is 1200W / 55V = 22A. Since your panels will be flat mounted you will rarely, if ever, see 1200W. So a 20A controller is just fine.

With the 12 panels in 3S4P you would have no issues using the 100/20 on a 48V setup. It will be about 21A of current from the panels to the SCC. Given the high voltage from the 3S, 10AWG will be just fine carrying that current.

But you are right that the Vmp is a bit low for a 48V system. Given that, you might be better off using the 150/35 (there is no 150/20). The 150V will allow for a 4S3P arrangement. And the 35A of battery charge current would allow you to expand to even more panels if ever needed.

One last thought - mounting 12 panels is a big PITA. Consider 4 300W panels instead or even 6 200W panels.
I saw that the Victron's datasheet also lists "Max. PV short circuit current" of 20a which is why I was thinking it related to Isc. The low Vmp might be the deciding factor then. I could fit a 3 more of these panels but they would be in awkward spots that deal with shade.

The 12 panels is a PITA. I had 9 from my last rig already. I considered switching to some larger panels but the dimensions were a pain point. I couldn't get to 1200 watts and still fit in the same space. Thankfully, I've already done the hard part of mounting the rails. (See my picture) I have 4 strut channels attached with both VHB tape and lag screws. I reduced it from 96 holes to 16 holes. The right side even hit the aluminum studs but not the left. Now I just need to attach them to the rails with some strut nuts and clamps.
 
I saw that the Victron's datasheet also lists "Max. PV short circuit current" of 20a which is why I was thinking it related to Isc.
You are correct. That is an additional constraint I overlooked. So that rules out having the panels in 3S4P with the 100/20.

So I think you have no choice but to go with 4S3P on a 150/35 (which has a max Isc of 40A which you wouldn't get close to at 3P).

You could do 3S3P with the 100/20 if you want to save about $170 compared to the 150/35 and you can get by with 900W instead of 1200W.
 
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