Seeking clarity on max. array power

Discussion about the FM100, FM80, and FM60 Charge Controllers

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bruapple99
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Seeking clarity on max. array power

Post by bruapple99 » Fri Nov 22, 2019 10:48 pm

Hi,
A small matter for start - as another forum user mentioned recently, I find that OB's string sizing tool does not allow system voltage selection (I'm using MS Office 2013) - for any of the four SCCs selected, system voltage is set to 48. I wonder if this is intentional, or a software problem - any ideas?

I'm seeking clarity on the max. array power for an FM100 on a 24V system. From different sources I get different figures:
  • FM100 user manual March 2018, p. 65 - 3000W (STC conditions)
  • FM100 datasheet April 2019 - 3400W (nameplate STC)
  • (Neither of the above say anything about derating from the figures given.)
  • OB's string sizing tool (downloaded this week) - states Max. power 6kW on 48V, so presumably it would be 3kW on 24V.
    As system voltage in the tool is fixed at 48V, I enter double the intended # of strings, on the assumption that will give the same output current as the actual # of strings on 24V. Halving the power figures under String Sizing Options again - for 24V - gives 'good' for 2800W and 'caution' for 3360W.
  • My equipment dealer - 24V (nominal U-batt) x 100A = 2400W
  • OB Knowledge Base article 'How to size a PV array for the FLEXmax charge controller', May 2014 - gives data for FM60 & FM80 only, but states "For best practice the array should be 15% less than the maximum rating listed below [given as STC]. Multiply the maximum rating by .85 to determine best array size for your battery bank/controller setup. Downsizing the array from the maximum rating allows for longer controller life, greater efficiency, and reduces the chance of nuisance breaker tripping."
    If this is applied to the figure from the April 2019 datasheet, it gives 3.4kW x 0.85 = 2.9kW.
So, four different figures, ranging from 2400W to 3400W, which is a big difference.
I'd be grateful for a definitive answer from Outback, but if that is not forthcoming, perhaps someone else out there can help ...?
Thanks in advance!

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Kurt Lundquist
Alpha Energy Engineering
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My RE system: VFX3524, Mate3, 200Ah 24V AGM Battery Bank
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Re: Seeking clarity on max. array power

Post by Kurt Lundquist » Mon Nov 25, 2019 8:02 am

I can't speak for OutBack on this but personally I would shoot for ~3kW or less depending on your location and goals. You are looking at 8-10 standard 72 or 60 cell type modules in 2 series strings. 9 modules is possible in 3 strings of 3.

-Kurt
Kurt Lundquist
Renewable Energy Project Engineer
http://alphaenergy.us/

Heppner
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Re: Seeking clarity on max. array power

Post by Heppner » Tue Nov 26, 2019 4:28 am

What happens if you go for more than 3kW? Does it damage the device?

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Kurt Lundquist
Alpha Energy Engineering
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Joined: Tue May 31, 2011 7:23 am
My RE system: VFX3524, Mate3, 200Ah 24V AGM Battery Bank
Location: Arlington, WA

Re: Seeking clarity on max. array power

Post by Kurt Lundquist » Tue Nov 26, 2019 7:40 am

I suggest sticking with 3,500W or less with a 24V battery bank.

-Kurt
Kurt Lundquist
Renewable Energy Project Engineer
http://alphaenergy.us/

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IanMcCluskey
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Re: Seeking clarity on max. array power

Post by IanMcCluskey » Tue Nov 26, 2019 11:27 am

Kurt is correct, and I am partially responsible for the various numbers.

First, our new manuals coming out with the FM100 AFCI will all say max. 3500 W of connected PV.

We started including power output in watts for folks who look for that spec, but the 6,000W power rating is really 100 A at any reasonable battery voltage of 24 V or above.

In summary, 3500 W is the maximum warrantied PV rating on an FM100 at 24 V. The product does an excellent job limiting output, so even at that rating, the highest output you will ever see is ~101 amps for less than a few seconds.
Ian McCluskey
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http://www.alpha.com

jnh
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My RE system: GVFX-3524, FM-80+FM-60, ~4.9kW PV in mixed panels on movable platforms, 415 Ah AGMs @ 24V, 350Ah @ 12V, Hub4, Mate, FNDC, Brultech GEM 32-ch AC datalogger, Midnite E-Panel, homebrew DC-DC crossfeed to 12V system, Honda EU-1000i, Iota DLS-27-25
Location: St. Augustine, FL, US

Re: Seeking clarity on max. array power

Post by jnh » Tue Nov 26, 2019 11:40 am

Heppner wrote:
Tue Nov 26, 2019 4:28 am
What happens if you go for more than 3kW? Does it damage the device?
I haven't used an FM100, but on the FM80 and FM60 (and other brands of charge controller's I've worked with), hitting the output amperage limit causes them to move their input voltage away from the optimum MPPT point (letting it drift higher), basically leaving excess power at the panels. This is easy to do with a PV input, as compared to something like a wind turbine where you normally must accept all generated power, then send excess to a dump load to avoid over-speed.

There will be brief excursions over the limit during very dynamic sun conditions (clouds rolling through)... I think I've seen 83A out of my FM-80 for a few seconds before its starts throttling back, but it never lasts long enough to trip the 80A DC-out breaker.

Anyway, nothing should be damaged, at least not in the short term. Operating at 100% of capacity for many hours every day does make the CC's run hotter, of course, potentially accelerating wear and tear and reducing the lifespan of certain internal components like electrolytic capacitors, especially if the controller's installed in a very warm environment.

My PV arrays are slightly above the recommended maximum for their controllers, although due to various higher-than-ideal losses (voltage drop over very long feed-in cable, not all panels at optimum angles, slightly mismatched panels in some strings, partial shading at certain times) they never approach 100% of STC even during edge-of-cloud events, so that mitigates it somewhat.

I also made a slight mod to my FM-80's fan circuit (search "Quieting a noisy Flexmax-80") mostly for noise abatement, but which also helps it run cooler - its thermostat can still kick the fan up to Dustbuster speed when necessary, but hardly ever does, so that should help extend component life.

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