Adding panels - the plan

Surfpath · October 2013

A wee update:

Right now we use about 2.8 - 3.5 Kwh/d and seem to comfortably meet that with ~2,000w of panels.
Our 370amp/hr bank typically goes from 95/90% SOC to 70/65% SOC each day.
My FM80 usually goes from bulk to absorb by 10/11am.
I don't experience any "winter" short light conditions. My temps range from 72F coldest to 92F warmest (80% of the time in the mid-to-high 80's).

I'm in the process of finishing a small 1BR guestroom/kitchenette & see the need to add a little more PV to handle:
A. LED lighting for that space (~100Wh/d when in use), plus
B. a converted chest fridge (I believe 0.60 KWh/d is reasonable to expect for this item).

Currently my 9 panels are combined in 3 series of 3 panels each using an MNPV-6 box. The 4th fuse in this pre-wired combiner box is unused, so adding another series would be plug and play.

My panel specs:
Model number SPI-M220-60US Polycrystallines
Max power: 220 Watts
VOC: 36.29volts
IMP: 7.65 amps

Sadly SPI went out of business last year. Their panels are very solidly made. So I'd have to get different panels

Other practical info: The maximum DC voltage into my controller ranges from about 103 to 108v. The maximum amps I have seen at the controller was 47 amps, but typically there's between 26 and 36 amps, depending on how sunny it is outside.

I installed #6 AWG ANCOR marine cable (+ and -) from the combiner box to the CC - Basically two #6AWG conductors through 33mm electrical conduit. Here's the ANCOR allowable amperage chart for my wires (help, it's a little challenging for me to decipher).

SO my questions:

Do you think 3 more panels might meet my new needs?
Will my existing wiring handle the additional amps?
If I am within 5-10% IMP on the new panels will this be OK?
Is there anything else I should be thinking about?

Cheers and thanks for checking this out,
SP

Surfpath · October 2013

Re: Adding panels - the plan

Hey...almost 100 views & no thoughts? Man, was it the garlic I ate for lunch?8)

OK, some more info. I am interested in 3 of these Sonali panels? The 230watt version. I presume they are polycrystalline.

Electrical Specifications
Nominal Maximum Power (Pmax) 230W
Optimum Operating Voltage (Vmp) 30
Optimum Operating Current (Imp) 7.67
Open Circuit Voltage (Voc) 36
Short Ciruit Current (Isc) 8.1

The panel is about the same size as my current ones (except thickness 1.57" vs 1.97" for the SPI's).

Whadya tink? Will they work for the system I have described in the OP?

Surfpath · October 2013

Re: Adding panels - the plan

Interestingly, there is a recent NAWS thread on a chap in Haiti who is currently installing some of these same panels. May be useful to preview.

As a side note I am interested in what YehoshuaAgapao had to say in his thread.....to quote:

You got 72-cell panels. Not a very good fit for 150V MPPT controllers. 3 to a string will overvoltage in cold weather (particularly at dawn), 2 to a string will frequently undervoltage on a 48-volt battery bank. Midnite solar MPPT controllers are more flexible than the rest, so you are probably better of getting one of those to use with 72-cell panels.

60-cell polycrystalline panels are perfect in strings of 3. 96-cell for strings of 2. 48-cell strings of 4, 36-cell strings of 5. 54-cell, 72-cell, 80-cell are not very good fits. This is for polycrystalline panels. Monocrystalline is different - more volts per cell.

Niel goes on to say that "most likely by extrapolation the vmp is 30.6v and the voc is 36.6v. this changes things as these are 60 celled pvs and not 72 celled pvs". He is right. I see that the Sonali spec sheet also lists 60 cells. My SPI's are also 60 cells. Either way, I am interested in learning a little more about the whole "# of cells is best for # of strings" concept.

BB. · October 2013

Re: Adding panels - the plan

OK--Lots of information--But a few questions.

First, you can either match the new panels by Imp or Vmp... Within 5-10% maximum difference between panels is fine.

It sounded like you wanted to put the one each new panel in series with the existing panels--which would bring you to Vmp-array~120 VDC. That is probably too high for a 140 VDC maximum input controller (you asked about Imp variation--that would only apply to adding panels in series).

If you add a 4th string in parallel, then you would worry about Vmp-string for the new panels--And that seems to be fine. If all panels are 60 cell Vmp~30 volts per panel, should be perfect for parallel string addition (current flow per string does not matter--other than for selecting the fuse/breaker for the combiner box and new parallel string connection).

Next question--What is the installed wire size (gauge) and run length (oh, I see 6 awg)? Is the combiner box located at the array (one pair of heavy wires from array+combiner to charge controller; or is the combiner next to the FM80 charge controller)?

Your link is to a Marine maximum current ratings of ~102 amps (inside conduit in engine room)... The NEC is more conservative and would list maximum current for 6 awg as 55-75 amps (depending in insulation type, and less for higher air temperatures). Your proposed 2,730 Watt array (if I followed your question correctly) would be:

2730 watts * 1/90 Volt Array-Vmp * 1.25 NEC derating * 1.25 NEC solar derating = 47.4 amps

So, assuming you do not have too much voltage drop (very long wiring from Array to Charge controller), you should be OK.

For Controller to battery cabling:

2,730 watts * 1/42 volts minimum battery voltage * 1.25 NEC derating * 1.25 NEC Solar Derating = 102 amps

Which, is pretty much the maximum current rating of the FM 80:

80 amps * 1.25 NEC derating = 100 amps wiring+fuse/breaker ratings

So, if I was a conservative engineer--I would be looking at a minimum AWG of ~2 AWG from Controller to Battery bank. Although, if exposed cabling, you could probably get away with 2-4 AWG cable.

So, what is the fuse/breaker/wire gauge from the charge controller to the battery bank. With a FM80 controller, fusing/wiring should be sized to handle 1.25x80 amps or 100 amps minimum (if designed for full controller rating).

Just to compare my rules of thumb against your actual measurements.

9*220 Watts * 0.77 panel+controller deratings * 1/59 volts battery charging = 26 amps

You typically see 26-36 amps--So that appears to still be a valid rule of thumb for conservative solar array derating.

The maximum "cost effective array" for the FM 80 would be:

80 amps * 59 volts charging * 1/0.77 panel+controller derating = 6,130 Watts maximum array...

So, you are not controller limited yet.

Your 370 AH @ 48 volt battery bank 5% to 13% to 25% rates of charge:

370 amps * 59 volts * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,418 Watt array minimum
370 amps * 59 volts * 1/0.77 panel+controller derating * 0.10 rate of charge = 2,835 Watt array nominal
370 amps * 59 volts * 1/0.77 panel+controller derating * 0.13 rate of charge = 3,686 Watt array max "cost effective"
370 amps * 59 volts * 1/0.77 panel+controller derating * 0.20 rate of charge = 5,670 Watt array max cost effective battery charging
370 amps * 59 volts * 1/0.77 panel+controller derating * 0.25 rate of charge = 7,088 Watt array probably best not to exceed

You are looking at:

9*220 Watt + 3*250 Watt = 2,730 Watt Array

Which is pretty close to our "optimum" 10% rate of charge for a typical off grid system...

Sounds like you have a nicely balanced system, but you could pump up the array even higher -- Especially if you have heavier day time loads if you ever need it.

-Bill

Surfpath · October 2013

Re: Adding panels - the plan

Hi Bill,
Thanks for the reply. FMI on my panel set up, including a general diagram see here (especially post # eight).

I think my panels are wired in 3 panel series (right?). So, instead of adding another 3 panel series, you suggest that I add a 4th string of panels in parallel? I may be mixing up what you recommend. Either way...just to recap - I live in a warm/hot country. My temps range from 72F coldest to 92F warmest (80% of the time in the low-to-high 80's).

There is a 55 foot wire run from the combiner on the roof to the controller in the solar room. The 33mm or 1.25" conduit for this wire contains two (+ & -) #6 AWG wires, plus one #8 ground. The ANCOR wire I used seems to be really good multi-strand flexible wire, expensive stuff. Perhaps that is why the amp ratings are higher?

My controller to battery cables are 6 feet of 2/0 gauge.

My Flexpower 1 panel has an "80 amp charge controller breaker".

Post eight on this thread here lists the DC breakers and suggested cabling for my system. I also installed a 100amp Blue Sea battery terminal fuse on my first + battery cable.

Hope that added info helps.

Given the panels I am looking at it would then be a 9*220 Watt + 3*230 Watt = 2,670 Watt Array. So I'd be closer to the 10% nominal charging rate.

Surfpath · October 2013

Re: Adding panels - the plan

Ok Bill, I think I get what you mean by
"It sounded like you wanted to put the one each new panel in series with the existing panels--which would bring you to Vmp-array~120 VDC. "

What I want to do is create a relatively similar 4th panel series using of 3 of these new panels - I don't want to add a 4th panel to each of the 3 existing series. That way my VOC should stay the same. The real issue, as you point out is can my #6 handle the increase in amps.

BB. · October 2013

Re: Adding panels - the plan

It is close from a US NEC point of view--But you are not in the US... From a functional point of view, it will probably work OK--But if your runs are long, you may lose more than 3% of the solar energy in cable losses.

-Bill

Surfpath · October 2013

Re: Adding panels - the plan

Using Outback's String Sizing tool, I added another series of three panels to see what it had to say. I used my original panel specs, which are pretty close to the proposed Sonali's.

Though, It said "max power current of array 30." Are these the MAX amps that this expanded array will produce?

Currently I typically see low 30's when bulking on my 1,980 watts. Occasionally it has gone higher (45 amps once), but I suspect these are those "edge of cloud" events that y'all been talking bout.
Attachment not found.

Surfpath · January 2015

Re: Adding panels - the plan

Happy New Year to all!

I am finally going to add another string as I proposed in this thread from a year and a half ago. I thought it may be a good time to quickly revisit the addition before pulling the trigger.....

Recap: My current panel specs are:

220 Watts
VOC: 36.29volts
IMP: 7.65 amps
VMP: 28.77
(9 panels in 3 strings of 3 panels in each string = 1980 watts).

I am still thinking about buying 3 of these Sonali panels (they still seem to be available)

Based on the previous discussion in this thread, if I am to add a new 4th string (of 3 panels), then I just need to make sure VMP is within 10% of the existing panels. Since the existing panel is 28.77vmp I could go up to 31.64 and be within 10%.

The Sonali 240 Watts's are:
VMP 30.60
IMP 7.84
VOC 36.6
ISC 8.3

Sonali 250's are:
VMP 31.32
IMP 7.98
VOC 37.2
ISC 8.45

Splitting hairs whether I get the 240w or the 250? They cost the same. Thoughts appreciated.

706jim · January 2015

Re: Adding panels - the plan

I'd go with the 250's. VMP closer to what you have now than the 240's.

BTW, I will shortly have the same size battery bank as you (8 L16's at 24 volts) but will be making do with a LOT less PV, at least for another year.

Photowhit · January 2015

Re: Adding panels - the plan

Surfpath wrote: »

Though, It said "max power current of array 30." Are these the MAX amps that this expanded array will produce?

I'm only a year and change late to point this out, but perhaps you've figured this out already, This is the out put of the array going into the charge controllers, not the current that is going into the battery. This represents the panel rated numbers, likely less in normal situations.

Surfpath wrote: »

Currently I typically see low 30's when bulking on my 1,980 watts. Occasionally it has gone higher (45 amps once), but

This it the current coming from the charge controllers going into the battery bank, the MPPT type controller tracks the correct voltage to charge the battery bank and converts the incoming power into the optimal voltage and current to charge the bank. So typically you will see more current coming out of the controller at lower voltage.

Photowhit · January 2015

Re: Adding panels - the plan

Surfpath wrote: »

There is a 55 foot wire run from the combiner on the roof to the controller in the solar room. The 33mm or 1.25" conduit for this wire contains two (+ & -) #6 AWG wires, plus one #8 ground.

This is a pretty long run for 30 amps on 6 gauge wire. I don't have time to check, but you might see what voltage drop you will have. I think I have a similar run from my future(NO I'm not done yet) front porch array to my charge controller, and I think I needed #4 minimum.

Surfpath · January 2015

Re: Adding panels - the plan

Photowhit wrote: »

This is a pretty long run for 30 amps on 6 gauge wire. I don't have time to check, but you might see what voltage drop you will have. I think I have a similar run from my future(NO I'm not done yet) front porch array to my charge controller, and I think I needed #4 minimum.

According to the NAWS Voltage drop calculator:

"1 conductor(s) per phase utilizing a 8 AWG Copper conductor will limit the voltage drop to 2.23% or less when supplying 30 amps for 55 feet on a 102 volt system".

So 6 AWG should be fine for my current system. I remember checking this before installation. With the proposed, 4th, string this transmission should also be more efficient (ie more amps), not so?

I used the calculator again with 55 amps as an estimate of the max current I expect to see from the 4 strings, and I got...

"1 conductors per phase utilizing a #6 Copper conductor will limit the voltage drop to 2.15% or less when supplying 45.0 amps for 55 feet on a 102 volt system".

But, Photowhit, you said something interesting: "So typically you will see more current coming out of the controller at lower voltage". In other words the current that is actually transmitted on the combiner box to controller run may be lower than what the FM80 records because BULK is more about Amps than volts. Am I on the right track?

On a related point, even though I am pushing the stated NEC amp limit for typical #6 AWG (and the NEC does not apply to me), I have to restate that I installed 6AWG ANCOR boat cable wire on this combiner to controller run. It is way more stranded (and flexible), and slightly thicker in diameter than what you see in typical house electrical wire. The ANCOR wire chart lists a working limit of 102 amps for their 6AWG conductor (at 105 degrees C).

My wife tell me that the more strands in the cable, the larger the surface area to carry the current. Sounds good to me.

Surfpath · January 2015

Re: Adding panels - the plan

Maybe a silly question but...before I pull the trigger....is it OK to add a string of polycrystalline panels when my other 3 strings are monos (the new Sonali's are poly, my existing SPI's are mono).

Just to be clear I am not mixing poly's and monos on the same string, I am just adding a 4th string and it will likely be a set of poly panels.
-SP

ps. The VMP of the 250 watt Sonali's is 8.8% higher than my current SPI's.

Photowhit · January 2015

Re: Adding panels - the plan

Surfpath wrote: »

But, Photowhit, you said something interesting: "So typically you will see more current coming out of the controller at lower voltage". In other words the current that is actually transmitted on the combiner box to controller run may be lower than what the FM80 records because BULK is more about Amps than volts. Am I on the right track?

I think,... sort of...

You have 3 strings of these panels;

220 Watts
VOC: 36.29volts
IMP: 7.65 amps
VMP: 28.77

The input current to the charge controller, is doubtful to ever be over 3 x 7.65 amps, and more likely 3 x 7.65 x about .75(or about 18amps), since panels typically only put out 75% of their rated value.

So I'm assuming what you are seeing is what the charge controller is putting out to the battery bank. MPPT CC's put out voltage a bit higher than the batteries and it keeps track of where your battery voltage is (MPPT = Multi Power Point Tracking) it converts the incoming wattage say 1500 watts (18 amps at @85 volts) to 1500 watts at charging voltage, which when early in the morning a depleted battery may be 1500 watts as 31 amps at 48v and later in the day as the battery nears absorb 1500watts as 25amps at 58volts.

I'll check the wire runs tomorrow, I just popped in. I don't use NAWS calc until the tell me if they want one way runs or total runs. 55 or 110 feet for you...

Photowhit · January 2015

Re: Adding panels - the plan

Fine to mix poly's.

westbranch · January 2015

Re: Adding panels - the plan

The amps are added when you connect strings together... what are the amps for each of those strings of 2 different types of panels, they need to be close...

Surfpath · January 2015

Re: Adding panels - the plan

westbranch wrote: »

The amps are added when you connect strings together... what are the amps for each of those strings of 2 different types of panels, they need to be close...

Hi Westbranch,
The amps (IMP) for my current panel (the SPI's) is 7.65, and for 7.98 for the proposed Sonali's.

Actually here are the two specs:

SPI 220 Watts (current panels: 3 strings of three panels per string)
VMP: 28.77
IMP: 7.65
VOC: 36.29
ISC: 8.25

SONALI 250 Watts (new string of 3 panels)
VMP: 31.32
IMP: 7.98
VOC: 37.2
ISC: 8.45

Howdy Photowhit.
So I see, unless I have some sort of clamp on meter I will unlikely know how much actual current is coming from the combiner box to the charge controller. However, it is very unlikely to be higher than what the MPPT controller sends to the batteries. That means that my combiner to controller wire rating (102 Amps max according to ANCOR Marine - #6 Gauge) is likely appropriate, conservative even.

But using your little 'formula' an approximation of the current on this run may be: 4 (strings) x 7.98 (using the Sonali IMP) x .75 = 23.94 amps (may be a somewhat higher, understand).

Hey, perhaps I should put in another string! The FM80 can handle it.

That way I'd have ~3400-3500 watts.

As BB said higher up on this thread, it'd put me a bit below the 13% rate of charge - good for the rainy days:

BB: Your 370 AH @ 48 volt battery bank 5% to 13% to 25% rates of charge:

370 amps * 59 volts * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,418 Watt array minimum
370 amps * 59 volts * 1/0.77 panel+controller derating * 0.10 rate of charge = 2,835 Watt array nominal
370 amps * 59 volts * 1/0.77 panel+controller derating * 0.13 rate of charge = 3,686 Watt array max "cost effective"
370 amps * 59 volts * 1/0.77 panel+controller derating * 0.20 rate of charge = 5,670 Watt array max cost effective battery charging
370 amps * 59 volts * 1/0.77 panel+controller derating * 0.25 rate of charge = 7,088 Watt array probably best not to exceed

gww1 · January 2015

Re: Adding panels - the plan

surf
You cc will show the amps that it is recieving as amps in and it will show the convertion on its screen as amps out. It will record your highest unloaded panel voltage. It will show you what the voltage has droped to during the convertion. My panels come in as high as 108 volts and when loaded buy the battery will drop to about 80 volts when charging about 60 volts to the battery. I won't tell you how perfect the cc is recording but you can check it with a clamp meter if you are only charging and don't have loads running. The seperate cc will have to be added in you head as I find the mate to be as high as 300 watts off at times. When you put all your monitor info together it is pretty easy to see what is going on.

I think you are going to like the outback system over all because of the above.

There may be some surprises even if your panels are exactly the same. If you got the money to get to 13% charge rate I would do it while you are installing cause after it is up it is hard to make yourself repeat the process again. You would be running you cc pretty hard but when you do the math and you discount the panel rating by 77% it is going to hurt to see how much 77% really is.

Even if you lose a small bit of the panels due to the mppt averaging what is coming in, you still will make more with them then without them and when you are actually using the system you are going to be very glade that everything is done and doesn't have to be revisited.

I could use 2000 more watts of solar panels to end up where you are going but have a hard time spending more and dread having to do more racking and can't decide best layout for looks and shading. These would not be issues if I would have done it all to begin with.

Just my thoughts and I hope they help more then hurt.
gww

Photowhit · January 2015

Re: Adding panels - the plan

I like the way you're thinking, GWW has more scoop on the outback charge controllers, I've never installed one or used one. You'll have to check out the various info you can display.

I obviously would prefer over paneling, I have the potential for a 20% charge rate with 4,000 watts of array and a 800ah 24v battery bank. but it's on the silly side of unlikely. It would have to be a cold day, with a discharged battery and have the sun break out during solar noon. I actually will add to my array! So thinking about adding to get into the 13% charge rate is something I can agree with, your charge controller could easily handle most days of a 4000watt array, with no limiting. You could justify going even larger array to a point. Remember most days your battery voltage will climb with the sun during the day, by the time you reach absorb(this is the point that the charge controller/battery reduce the incoming amperage) The voltage will be around 58 volts... 4000 watt array in perfect weather (cold/clear) may produce 4000watts/58v=69amps

Surfpath · January 2015

Re: Adding panels - the plan

Photowhit,
I am pulling the trigger, 1500 watts of Sonalis heading my way for a total of 3,500watts. I also posted to the Midnite forum about adding a 4th & 5th string to my combiner box, and they said it can be done, replacing my fuses with breakers.

I am glad I used that higher amperage #6 AWG marine cable for the combiner run. I installed it because it was so flexible and the conduit run involved a few tight 90 degree angles. I am also luckily to have enough Southern facing roof to do this.

softdown · January 2015

Re: Adding panels - the plan

Midnite's string sizing tool is much better than Outbacks...for almost everybody. Especially when you don't have cold weather to deal with.

You keep thinking that stranded wire carries more amperage than solid. The opposite is true. You also said that more amps makes your wire runs more efficient. Actually....it is more volts that increases wire efficiency.

Congrats on a successful solar install. Then having the fortitude to add to a working system. Let us know how it works out!

Surfpath · October 2015

Sorry for the lateness in reply. A quick update. In July in added an additional series of panels (750w) to my existing array. Now we have 2730watts and ~ a 10% rate of charge. It was dirt easy to do because I used the same DIY panel mounting as before (unistrut with hangar bolts - cheap and awesome), and I had a free fuse available in the Midnite Combiner box to connect the series to. As per the discussion and research above the new panels are still within the limits of my wiring and Outback FM80.

The highest recorded amps I have seen on the cc since the install has been ~43. But typically they are in the 28-35 range. The latter is kinda surprising to me because I thought the new array would consistently produce 35-45amps. Of course on cloudy days when the sun peeps out, and the CC tries to maximize whatever power comes in, I get the 40's.

The benefit to the new panels is that I now go into float about 2-3 hours earlier than before (typically), and on really rainy days I dont need to turn off the fridge at night. If you may recall I have not had a generator for 2 years (after the Yamaha dealer fiasco) and I have been fine.

General lessons: either design a system at a 10% rate of charge to begin with or leave room to expand to at least that %. If you have a combiner box, leave room to expand.

So, it was a good upgrade. I got the panels fairly cheap, 250watt Sonalis at ~ 80 cents per watt.

Now all I need to do is update my signature. Hold on...where the heck is my signature???
-SP

Photowhit · October 2015

Congrats! Looks like a nice addition to your system!

Surfpath · October 2015

Thanks. It took me exactly 2 years to make this addition! I will probably come back to this thread in a year or so to see what I'll have to do to add one more string. But for now all is good.

Plus I found how to update my signature.

Adding panels - the plan

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