Question About Outback FM-80 Charge Controllers

[TheFerretman]

Hello Everybody:

I'm getting ready to upgrade my existing Conergy panels (230W) to some newer ~400W panels. I'm slightly confused by some of the numbers I'm seeing.

The FM-80 can handle 80 amps, which (typically) I downrate to 72 amps to give me some space. The new panels though are all putting out ~11 amps/panel according to the specs.

From the back-of-the-envelope math that means I can only put 6 or maybe 7 panels per charge controller, is that right? This means in order to handle the input I'll need to expand out the system to perhaps 7 charge controllers? That seems like a lot....

Any help here would be appreciated.....thank you!

Steven from Colorado

 

[MichaelK]

Hello Everybody:

I'm getting ready to upgrade my existing Conergy panels (230W) to some newer ~400W panels. I'm slightly confused by some of the numbers I'm seeing.

The FM-80 can handle 80 amps, which (typically) I downrate to 72 amps to give me some space. The new panels though are all putting out ~11 amps/panel according to the specs.

From the back-of-the-envelope math that means I can only put 6 or maybe 7 panels per charge controller, is that right? This means in order to handle the input I'll need to expand out the system to perhaps 7 charge controllers?

The important number is the the OUTPUT amps, because with a MPPT that is almost always higher than the input amps.

What is your system voltage? Covering all the bases, for each voltage we would see.....

(80A X 13V charging)/90% de-rating (conservative) = 1155W: 2-3 400W panels
(80A X 25V charging)/90% de-rating (conservative) = 2222W: 5-6 400W panels
(80A X 50V charging)/90% de-rating (conservative) = 4444W: 10-11 400W panels

Because the FM-80 is a 150V controller, it's more critical to match up the string voltage first before getting to the string amperage. What is the Voc of those panels, and how cold do your winter lows get? That will decide how many panels you can put in a string. Once that is determined, we can determine how many strings you can have.

 

[TheFerretman]

This is a 48V system, ground mounted, fixed slope, roughly 21 degrees per row.

We get awesomely cold sometimes....double digit negatives a couple of months ago.

A typical candidate panel I'm looking at is the 430W Solaria panel:

Solaria 430W Solar Panel PowerXT-430R-PL - A1 Solar Store

Solaria 430W Solar Panel PowerXT-430R-PL solar panel is the highest power solar module in residential US market. Look into detailed descriptions, ratings, reviews, pictures, and more. Look into detailed descriptions at A1 Solar Store

a1solarstore.com

This shows a Voc of 45.7 V, maximum string voltage of 1000V, and an Isc of 11.87 A.


Steven in Colorado

 

[MichaelK]

OK, tossing your numbers into Midnight's string calculator, this is what I'm seeing....


Because the FM-80 has a V limit of 150V, three panels would exceed that
With your winter lows in mind, it looks like three in series would reach 161+V. Keep in mind that the calculator is geared towards optimizing Midnight products, but what we can see is the specs for the FM-80 more or less parallel the Midnight150


So, it looks like 5 parallel strings of 2 panels in series (2S5) is max for you. BTW, those panels are more than 1$/W, so I think you might find better deals elsewhere. Remember, when you buy retail the shipping is inbedded in the price. Looking on Craigslist, I see better deals right away. https://cosprings.craigslist.org/fod/d/salida-new-and-used-solar-panels/7567465742.html

 

[TheFerretman]

Interesting. I've been using the Outback spreadsheet, though it seems very similar.

It is unclear to me how the existing panels are wired in with the strings. I'll have to ask my solar guy about this.

Looking at those numbers, I see five strings of two panels each per controller total of ten? Doesn't that top out at 118.70 A, which exceeds the FM?

I reckon I'm confused here, definitely. Appreciate your help though; the guy who wired them had to have run strings but's not clear to me what he did.


Steven in Colorado

 

[MichaelK]

It is unclear to me how the existing panels are wired in with the strings. I'll have to ask my solar guy about this.

Looking at those numbers, I see five strings of two panels each per controller total of ten? Doesn't that top out at 118.70 A, which exceeds the FM?

First, you need to remember the electrical mantra; In series volts add while amperage stays the same. In parallel, amps add while volts stay the same. So, if you wire two panels in series, the Vmp will be 37.9V + 37.9V at 11.35A = 11.35A at 75.8V. For five in series, it's (11.35A at 75.8V) + (11.35A at 75.8V) +(11.35A at 75.8V) + (11.35A at 75.8V) + (11.35A at 75.8V) = 56.75A at 75.8V.

BTW, 75.8 is barely over the minimal practical MPPT voltage of 72V, which is what is appropriate for a bank that might reach 60V, so that really is a better reason to look at other panels with a better voltage range. Going back to that Craigslist seller I linked, I see this item...

Jinko 265 watt 60 cell Poly Solar Panels for $125.00 each (13) More than double the W/$!

With 60-cell panels, the Voc is likely ~38V, so three in series that's 114V, a much better voltage range for a 48V battery bank. That's about the voltage I myself am running. Even at -40F, the Voc is likely to be only ~142V, lower than your limit.

Alternatively, a different choice is to switch controllers and go with a higher voltage model. Outback makes their 300V model, and Midnight makes 200V and 250V models. Those choices will allow you to have a higher solar voltage that will help cut down on copper wire sizes from the panels to the controller. My Midnight200 controller could handle three of the panels in series, with a Voc of 171V at -40F.

 

[TheFerretman]

Thank you; to be fair I just tossed out a random panel on the list of the ones I'm looking at.

Looking more closely I'm seeing as you say a different between the 60-cell panels and the 72-cell panels, which seems to be leading to the higher voltage numbers you're talking about. Am I correct from what you're saying that this seems to better size my system voltage wise for the 60-cell jobs?

I'm kinda not a fan of swapping out the controllers, if only because everything else is Outback. I have pondered it; if the FM-100s weren't SO much pricier than the FM-80s I might have.

I've pinged my solar guy about how he set up the strings; at a GUESS it's been too long that he doesn't recall. In a way that's positive because when we put in the new system we can label/color things more better, I guess (trying to look at the bright side here).

Thank you....much to ponder.


Steven in Colorado

First, you need to remember the electrical mantra; In series volts add while amperage stays the same. In parallel, amps add while volts stay the same. So, if you wire two panels in series, the Vmp will be 37.9V + 37.9V at 11.35A = 11.35A at 75.8V. For five in series, it's (11.35A at 75.8V) + (11.35A at 75.8V) +(11.35A at 75.8V) + (11.35A at 75.8V) + (11.35A at 75.8V) = 56.75A at 75.8V.

BTW, 75.8 is barely over the minimal practical MPPT voltage of 72V, which is what is appropriate for a bank that might reach 60V, so that really is a better reason to look at other panels with a better voltage range. Going back to that Craigslist seller I linked, I see this item...

Jinko 265 watt 60 cell Poly Solar Panels for $125.00 each (13) More than double the W/$!

With 60-cell panels, the Voc is likely ~38V, so three in series that's 114V, a much better voltage range for a 48V battery bank. That's about the voltage I myself am running. Even at -40F, the Voc is likely to be only ~142V, lower than your limit.

Alternatively, a different choice is to switch controllers and go with a higher voltage model. Outback makes their 300V model, and Midnight makes 200V and 250V models. Those choices will allow you to have a higher solar voltage that will help cut down on copper wire sizes from the panels to the controller. My Midnight200 controller could handle three of the panels in series, with a Voc of 171V at -40F.

 

[MichaelK]

Looking more closely I'm seeing as you say a different between the 60-cell panels and the 72-cell panels, which seems to be leading to the higher voltage numbers you're talking about. Am I correct from what you're saying that this seems to better size my system voltage wise for the 60-cell jobs?

I'm kinda not a fan of swapping out the controllers, if only because everything else is Outback. I have pondered it; if the FM-100s weren't SO much pricier than the FM-80s I might have.

I myself am using some 72-cell panels for some of my strings. What works out nicely is that the Vmp of three of the 72-cells in series is almost exactly the same as the Vmp of four 60-cells panels in series. So, my arrays are a combination of either 3 panel strings, or 4 panel strings. Pretty much all the panels in both my systems are mix and match.

That random panel you selected appears to have a higher than average Voc, so I'd say discount it from continued selections. My 72-cell panels have a Voc of 37.5V, and three in series at -40F would reach 140V, which would be safe for your controller. Use the string calculator to make your final decision.

Yes, the FM-100 price is scary!