String Sizing Tool for MX and FM Charge Controllers

Discussion about the FM100, FM80, and FM60 Charge Controllers

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blurr95
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Re: String Sizing Tool for MX and FM Charge Controllers

Post by blurr95 »

I would like to know how to change the fm8o to 24v as well.
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Re: String Sizing Tool for MX and FM Charge Controllers

Post by Mike Curran »

Blurr- check out this thread: viewtopic.php?f=29&t=13283&p=69722&hili ... age#p69722

It refers to changing to 48 volts but the same steps apply for a change to 24 volts.
https://ei.tigoenergy.com/p/pZXn7SZQyO45
https://enlighten.enphaseenergy.com/public/systems/Hctc107221
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Re: String Sizing Tool for MX and FM Charge Controllers

Post by blurr95 »

Thanks for the link, it will come in handy when I get things hooked up. But I was talking about changing the setting in the string sizing tool from 48v to 24v for the FM80. I am trying to figure out how to configure the panels.

Jason
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Re: String Sizing Tool for MX and FM Charge Controllers

Post by fcwlp »

blurr95 wrote: Sat Mar 21, 2020 1:04 pm hanks for the link, it will come in handy when I get things hooked up. But I was talking about changing the setting in the string sizing tool from 48v to 24v for the FM80. I am trying to figure out how to configure the panels.
HI Jason, on the string sizing tool, there is not an option to change the system voltage. Use the string tool to get your string Voltages based on temp correct but then divide the max watts shown by 2. Keeping the FM80 24V watts to 2,000.

Since you are currently designing your system, there are major advantages of going to 48V, likely 2,000 more watts of PV for same inverter cost and CC cost. If you were planning on two strings of batteries at 24V, you can use the same batteries to make 1 string at 48V with the same capacity and fewer issues.
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Re: String Sizing Tool for MX and FM Charge Controllers

Post by blurr95 »

I'm not designing the whole system, just the array. I already have a 24v inverter, I just need to figure out the panels for the FM80.
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Re: String Sizing Tool for MX and FM Charge Controllers

Post by blurr95 »

I don't understand what it is that I need to divide by 2, I guess I just need to know if 9 Canadian 255 watt panels will be good for my FM80, 3 strings of 3. Will this work? Sorry about not understanding but solar panels are new to me, and it's a lot to learn.

Thanks, Jason
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Re: String Sizing Tool for MX and FM Charge Controllers

Post by fcwlp »

Jason, attached is the string sizing tool run for 9 CS 255W panels module model # C6P-255. You should confirm that this is your actual module model number.

I put your min temp at -40C to be on the safe side and the string voltage is 137V which is good. If you look at the STC line you see that your array watts are 2,295W and for a 48V system caution is indicated at 4,590W. This would indicate that for a 24V system, caution is indicated for 4,590W/2 = 2,295 which also happens to be your STC PV array watts.
OutBack String Sizing Tool for blurr95.pdf
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Re: String Sizing Tool for MX and FM Charge Controllers

Post by blurr95 »

Thanks so much for doing that for, I really appreciate it. Seeing it done, It makes sense now what you said before. I just couldn't get my head around it. Do you think It would be okay to use these panels, or should I scrap this idea and find different panels. Thanks again for your help.

Jason
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Re: String Sizing Tool for MX and FM Charge Controllers

Post by fcwlp »

Jason, your o'k to use the nine CS C6P-255 panels with the 24V FM-80.
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Re: String Sizing Tool for MX and FM Charge Controllers

Post by blurr95 »

Ok, thanks again for doing this for me. You have helped me tremendously.

Jason
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I install and maintain grid-tied and off-grid systems in my area and consult on solar system design/operation.
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Re: String Sizing Tool for MX and FM Charge Controllers

Post by fcwlp »

Your welcome
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Re: String Sizing Tool for MX and FM Charge Controllers

Post by Mendez »

Mike Curran wrote: Sat Mar 21, 2020 7:48 am Blurr- check out this thread: http://outbackpower.com/forum/viewtopic ... age#p69722

It refers to changing to 48 volts but the same steps apply for a change to 24 volts.
Greetings, this thread link takes us to the main page of the forum. Do we know what happened to the original link? How do we get to this thread? Thank you.
http://outbackpower.com/forum/viewtopic ... age#p69722
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Re: String Sizing Tool for MX and FM Charge Controllers

Post by Mendez »

fred2258 wrote: Sun Jul 20, 2014 1:33 pm Clipboard-1.jpg

How come the string sizing tool recommends a max watts of 2000 at 24V and the manual recommends a max of 2500 watts?
Does some have a copy of this original tool? Thank you.
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Mike Curran
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My RE system: Outback - Garage roof (2007/2017):
- 8 (2x4) Evergreen 180's into one FM80 (2007/2020 - replaced MX60)
- 6 (2×3) ALEO S79-300's into one FM80 (2017)
- 2 grid-tied VFXR3524A-01's series stacked, replaced GVFX's (2020)
- 12 Surrette/Rolls 2V x 1766Ah (2007)
- Hub10.3, Mate3s, FNDC, RTS. Tigo ES maximizers on each PV module.

Westinghouse Solar - Barn roof: (2012)
- 30 (2x15) 235W panels with Enphase M215 microinverters, grid-tied

Outback Skybox - Barn roof: (2019)
- 14 Talesun 275W in series (DC array input to SB inverter/charger)
- 3 SimpliPhi 3.8 batteries, 48V, 225Ah total
- AC coupled input from 14 Talesun 275W, Enphase M215 microinverters

Battery systems operate in grid-tied, net metering w/backup mode
All self-designed and self-installed
Location: Chagrin Falls, Ohio

Re: String Sizing Tool for MX and FM Charge Controllers

Post by Mike Curran »

https://ei.tigoenergy.com/p/pZXn7SZQyO45
https://enlighten.enphaseenergy.com/public/systems/Hctc107221
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Re: String Sizing Tool for MX and FM Charge Controllers

Post by redsierra »

Hello.

It is too late for crying now because I am deep into a 24 volt system, but I do now understand the major advantages of a 48 volt system. That said, why can't Outback modify their string sizing calculator to be able to select a 24 volt system?

If I understand the info above correctly all you have to worry about is if the wattage of the string is showing either green or yellow for in the calculator for a 48 volt system then you would be good to go with half of that amount?

In the attached image, for instance, one of my arrays is 1500 watts. The calc shows that 3000 watts is good BUT the figures above are all in red. Is that a concern or if the system wattage is either green or yellow I am good to go with half of that amount as long as half that amount is either green or red for ALL the figures in the string column?

Knock wood....no issues yet!

I am changing out some other arrays so I would like to be super clear on this before I start.
Screenshot 2023-01-23 23.17.05.png
So, if this is the case, is this proposed set up OK in this string calculator?
Screenshot 2023-01-23 23.17.05.png
Thanks
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Screenshot 2023-01-23 23.08.21.png
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Re: String Sizing Tool for MX and FM Charge Controllers

Post by raysun »

I've never had much luck with the string sizing tool. Mainly because it doesn't work well with Excel on my smart phone. That, and it's easy to compute strings manually.

The 24V v.s. 48V system issue is, as you've determined, related to nominal battery voltage, and the charge controller's maximum output (charging) current capacity. For an FM80, that capacity is 80A. The Input power from the PV array is proportional to the output power to the battery. For a given maximum current, the output power into a 48V battery is 2X the output power into a 24V battery. In simple math, the maximum PV array, at 48V nominal battery voltage: 48V x 80A = 3840W. Being as the battery voltage while charging is higher than 48V, the array power can be "cheated" upwards slightly: 50V x 80A = 4000W. On the other hand, the original FlexMax design engineers would sleep better at night if the users left a 20% margin so: 50V x 80A @ 80% = 3200W. Ye pays yer money an' takes yer chance. Personally, the choice is to derate maximums. For 24V nominal calculations, the products are /2.

The second critical limit is the FlexMax maximum input voltage. It is set by the breakdown voltage of the charge controller's power Field Effect Transistors (FET). The specified maximum is 150V. Exceeding this voltage at the FM80 PV IN terminals can quickly destroy the FETs. No bueno. The maximum PV panel voltage is specified as V(oc), or the maximum measured into an open circuit. When does a PV panel typically "see" an open circuit? During times when the solar radiation is strong enough to induce a voltage in the cells, but not strong enough to induce a current. Early mornings are prime time for this, but also sudden cloud cover, and low solar angles relative to the panels. There's another factor - the negative temperature coefficient (NTC) of solar cells. The lower the temperature, the higher the V(oc). The "worst case" V(oc) is seen on extremely cold, crystal-clear mornings with intense sun at a low angle to the panels. As I chronically slept through math class, the NTC calculations intimidate me. My solution was to move to Hawaii. In practice, I look at the V(oc) @ STC published by the manufacturer, and calculate PV string size where the sum of the V(oc) stays well away from 150V. This "spitball" approach can be refined, of course, by estimating the "worst case" V(oc) by incorporating the most common factors: all time lowest temperature and the NTC of the panels in question. If that number gets above 135V, I get nervous. However, I've seen systems touch 145V and live to tell the tale. The FMs complain mightily at that limit, however.

So what's all that mean, in practical terms, to the average Joe who just wants to make stuff work and keep it working? In order for the FM to work as a charge controller, the input voltage (from the array) must always be at least 1.5V higher than the output voltage (to the battery). In order for the FM to continue working as a charge controller, the input voltage must be at most 145V. Give or take a few watts of optimum efficiency, the best choice for string voltage lies comfortably between these two extremes. For example, a PV panel with a V(oc) of 45V @ STC could form a string of 3 with a sum V(oc) = 135V. The V(oc) could rise nearly 15V and the FM would survive. A string of 2 would be a conservative choice with a sum V(oc) = 90V @ STC. An absolute minimalist configuration would be a string of 1. The panel voltage at maximum power would probably stay above the 24V battery charging voltage, however, the charge controller likely wouldn't be able to pick an optimum MPPT value in all cases. For myself, the safe middle ground is a string of 2. The string of 3 would come into consideration if the distance between PV array and charge controller was long enough to induce significant (> 5%) voltage losses in the transmission line, and I wasn't living in a location with weird temperature extremes.

In my actual arrays, the REC Alpha 440W panels have a V(oc) = 53V, so there's no question of string length for my 48V system. We leave the solution to the reader. (String Sizing Tool not necessary.)
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Re: String Sizing Tool for MX and FM Charge Controllers

Post by redsierra »

WOW…..that was GREAT info. Thanks. Unfortunately this location is in an area of weird extremes! The lows in winter can be -40! The highs in summer can be +30!

I DO have long runs between all my arrays and the charge controllers. Probably 150 feet (have to verify).

I have a place in MX where we spend the winters so this system is unattended.

I am heading to the lake now to check on some stuff related to this and will reread and digest this more when I return.

Thanks for this as it clearly took some time to write it. Especially in such an understandable format.
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Re: String Sizing Tool for MX and FM Charge Controllers

Post by raysun »

With the values cited in the SST images above, my sense is, given the V(oc), minimum temperature, and notional distance between arrays and controllers, strings of 3 would seem to strike an acceptable "risk/reward" balance.
For the current configurations, the FM/MX controllers keep the "Lifetime Maximum V(oc)" value stored in NVRAM.
Screenshot_20230124-050327.jpg
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