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wikel avatar image
wikel asked

Advice on MPPT spreadsheet calculator "Accepted" status

Looking for a bit of advice. When using the spreadsheet it listed my config as "Accepted" but I'm concerned with the values on the higher temp voltage reading.

This is for a smaller RV setup so I'm limited to using 6 12v Grape Solar 180w panels connected to a 48v Lifepo4 bank. The spreadsheet is showing 50.4v as "Min PV voltage @ max. temp" but I'm not seeing that be acceptable to charge a nominal 48v bank consisting of 16 3.2v max charged cells (51.2v when max charged), let alone pushing a 56v-58v bulk absorption voltage.

Am I missing something or how is 50.4 "accepted"? Thanks for any assistance!


MPPT Controllers
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4 Answers
Pat Davitt avatar image
Pat Davitt answered ·

The problem is not with the Charge Controller. It's with the voltage output of the panels. Three of those Grape solar panels in series only output a Vmpp of 59.06 volts. When compensated for high panel temperature you get the 50.4 volts. I have a 48V system, and I just checked, my panel array shows a voltage input of 87 and a Voc of 94.5. These are numbers you need for a 48V system.

A 24V battery system would be much more suited to your solar array.

Pat

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John Rushworth avatar image
John Rushworth answered ·

Maybe try using series only to raise the voltage. That way the controller will start charging earlier and finish later. The sheet currently accepts your configuration based on a nominal 48V lead acid bank. The MPPT needs +5V to start working so the sheet is saying the controller is acceptable as it won't fail on too high voltage or start self limiting on amps. It doesn't mean it is the ideal controller. As you are using lithiums with higher nominal voltages I suggest trying series to get the voltage up. Basically get that voltage up at the extremes above the dotted line ,into a more comfortable zone.

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Cory Wikel avatar image Cory Wikel commented ·

Yeah 6 would be too high on the cold temp side and exceed the current 150/85 MPPT. I could do 5 panels in series but I was trying to minimize shading issues. As someone mentioned I could go to a smaller nominal voltage bank as well (say 36v vs 48v).

I had tried to find a similar sized 24v panel but have unable to find anything close enough in size of the 12v units.

I was thinking it looked strange with the hot output being well below the desired charging profile for a 48v bank, so I'm glad I asked.

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John Rushworth avatar image John Rushworth ♦♦ Cory Wikel commented ·

I see, that explains why you went for 36 cell panels as opposed to 72 cell and is why you now find yourself having to series even more for a 48V bank than is usual.

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Pat Davitt avatar image Pat Davitt commented ·

You could wire all 6 panels in series. That would require one of the 150V charge controllers. Your 48V batteries would then be happy.

Pat

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John Rushworth avatar image John Rushworth ♦♦ Pat Davitt commented ·

Would 6 not be around 160V in cold temperatures? If so then he’d need to go to a 250V controller.

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Pat Davitt avatar image Pat Davitt John Rushworth ♦♦ commented ·

John,,

You are correct, my bad. I still think the OP's best bet is to go with a 24V battery system. That way he can keep his existing series/parallel arrangement and not get into the "shading" issues inherent in an all series setup. As others have said a 36V battery system is a odd duck.

Pat

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Mark avatar image
Mark answered ·

Why have you currently selected a 85 Amp output charge controller when your max current for a 48v bank is only ~24A?

That's way too big for that setup...

If you really want to keep 48v I would go all PV in series with the smallest 250v charge controller (I think 250V/60A is the smallest).

Or go a 24v system with 2 strings of 3 panels & a 100V/50A MPPT.

If you go 36v I think it's a bit odd & I don't think there are many 36v inverters out there.

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John Rushworth avatar image
John Rushworth answered ·

Cory. I spoke to a more knowledgeable person than myself. He said he is concerned about the temperature readings. And:

This specific solar module (configuration) reaches the min. voltage at high temperatures quite early because of its temperature coeff. It will work under most conditions, but everything should be setup correctly.

My question would be is he 100% sure about the real temperature of the solar cell at maximum irradiation (he states 65°C)? Of course, if this would rise further than 65°C things get worse.

If on top of that wiring in reality could give more voltage drop (inserting connections and/ or switches) than in theory calculated, the regulator might stop charging too early missing some yield throughout the day.

My answer can only be very simple as the setup made now is too close to what is possible: I would always suggest taking one more panel in series in this case.

I also think the current rating of this MPPT is chosen quite high for this setup; input voltage as well as max. current.

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