Wiring panels series? Parallel? Both?

Ironbull

Trying to put a system together and not quit sure how to wire the panels. Here's what I have:

8 12v 100w panels
Vmp 18.9v
Voc 22.5v
Imp 5.29a
Isc 5.75a

This is for 24v system with a 450ah battery bank. Along with a Outback Flexmax 80 CC and Samlex 2000w inverter. 

bill von novak

Ironbull said:

Trying to put a system together and not quit sure how to wire the panels. Here's what I have:

8 12v 100w panels
Vmp 18.9v
Voc 22.5v
Imp 5.29a
Isc 5.75a

This is for 24v system with a 450ah battery bank. Along with a Outback Flexmax 80 CC and Samlex 2000w inverter. 

The Flexmax has a max input voltage of 150 volts.  So 2 strings of 4 each would put you at 90 volts with plenty of margin for cold mornings (Voc goes up when it's cold.)

Ironbull

Would that give me enough amps to properly charge my batteries? 

Do I need a combiner box also? If so where should it be located? At the panels or with the cc and inverter? 

BB.

In general, for a 24 volt battery bank on a good quality MPPT controller, you want the Vmp-array-std to be around 35-100 VDC. And, Vmp-array-std minimum really should be ~1.3x29v=37.7 to ~40 volts.

Specifically, you really want the Vmp-array-std to be around 2x the battery voltage (2x 29 volts charging = ~58 volts Vmp-array-std). 

Maximum voltage should be around 100 VDC Vmp-array-std... But that limit is for really cold climates (hard winters). If this was installed, on a warm tropical island, then you can get much closer to 140 VDC.

More or less, the Vmp-array-std minimum is set by your battery bank voltage and how hot it gets in your area (Vmp falls as panels get hot... Very hot days, it gets close to 0.80 * Vmp-std (standard conditions, aka "marketing numbers" on back of panel). To low of Vmp-array and your solar power harvest will suffer (Vpanel has to always be higher than Vbatt voltage for, almost all, MPPT controllers).

And Voc (voltage open circuit) is set by the coldest weather you get and the maximum rated input voltage for the controller (typically ~140 to 150 VDC)... If your array Voc-cold gets too high, then you can damage the MPPT transistors. And many controllers will log an over voltage error that will invalidate your warranty.

Because of the complexity, many MPPT controller manufacturers have a string calculator program to help address the issue. Here is Outbacks:

http://www.outbackpower.com/downloads/string_sizing_tool/string_tool_instruc.pdf

For your system, I would suggest 4 panels in series x 2 parallel strings (Vmp-array=75.6 volts; Imp-array=10.58 amps).

3x panels in series would be very nice too--However, since you have 8 panels, you cannot use all of them (6 or 9 panels / 3 ). You could go 5x easily, but again, 8 panels on hand.

Also, you may want to install more panels/larger array later... Mixing 100 Watt and 300 Watt panels typically requires a 2nd charge controller... Or you get >200 watt panels and sell the "12 volt" panels to others (typically >200 Watt panels are about 1/2 the price of ~12 volt panels per Watt).

A rough guide for sizing your solar array to battery bank capacity (5% is good for weekend/summer usage, 10%+ rate of charge is better for full time usage):

• 450 AH * 29 volts charge * 1/0.77 panel+controller derating * 0.05 rate of charge = 847 Watt array minimum
• 450 AH * 29 volts charge * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,695 Watt array nominal
• 450 AH * 29 volts charge * 1/0.77 panel+controller derating * 0.13 rate of charge = 2,203 Watt array "typical" cost effective maximum

Suggest that you typically discharge battery bank about 25% (to 75% state of charge) if daily usage... Weekend/backup power usage, 50% maximum discharge is suggested. And never go below 20% state of charge (or you can "kill" the battery).

If you want to better understand how much energy your system can produce... Let us know (roughly--nearest major city) is installed and we can go into more detail.

-Bill

Ironbull

I live in NW Tennessee, I get 3.1 solar hours for the winter low and average 4.9 hours for the year. I  came up with I need 750 watts of panels and 312ah batteries for the winter time when i have the least amount of sunlight. My watthour usage will be 1.3-1.4kwh so I rounded it up to 1.5. I added a little cushion for the batteries so I don't have to discharge them as much. 

I understand my panels aren't ideal, but I bought these with intention of making a smaller 12v system and it morphed into this.

Ironbull

I can get another panel to match if 3 strings would be better. 

littleharbor2

If you go 3s3p with your panels you will need a fused combiner.  At 4s2p you don't need any fusing on the array. Unless you need the extra 100 watts you might be better off $wise with 2 strings.

Ironbull

littleharbor2 said:

If you go 3s3p with your panels you will need a fused combiner.  At 4s2p you don't need any fusing on the array. Unless you need the extra 100 watts you might be better off $wise with 2 strings.

Could you elaborate a bit more on the fusing? I was prepared to get a midnite 6 bay combiner with breakers if necessary. Or do you mean something different? I'm still trying to wrap my head around what is needed.

BB.

Do you have a refrigerator or not? If you have a refrigerator, LED lighting, laptop/LED TV, smaller RV water pump, etc... I would be suggesting a 3.3 kWH per day system would be very nice and can give you a "near normal" electrical life (albeit with lots of conservation and use of alternative fuels for cooking, heating, hot water).

But, let us start with 2 kWH per day (a little buffer). And assume an efficient AC inverter for your larger loads.

The battery bank calculation would be (full time off grid and solar charging, I suggest you aim at 25% usage--Then on sunny days you can pretty much recharge the battery bank fully. If you go with 50% discharge per day, it takes almost 2 days, or more, of sunny days to fully recharge the bank (lead acid batteries are kind of "slow charger" vs Li Ion and some others):

• 2,000 WH per day * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 max discharge * 1/24 volt battery bank = 392 AH @ 24 volt battery bank suggested

Your 450 AH @ 24 volts is right in the groove... Any solar calculations within 10% (i.e., 440 AH vs 400 AH) is virtually the same (about the best accuracy you can expect from everything here).

And as above, the recommended solar array based on battery AH capacity:

• 450 AH * 29 volts charge * 1/0.77 panel+controller derating * 0.05 rate of charge = 847 Watt array minimum
• 450 AH * 29 volts charge * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,695 Watt array nominal
• 450 AH * 29 volts charge * 1/0.77 panel+controller derating * 0.13 rate of charge = 2,203 Watt array "typical" cost effective maximum

And then there is sizing the array based on your energy needs... 1.5 kWH per day--Is that a base load or a variable load (you can cut back on poor weather days)? If it is a base load, I would be suggesting a 1/0.65 (1.5x larger) to 1/0.50 (2.0x larger) "fudge factor" for base loads (say you need 0.5 kWH per day for computer and cell phone for work, LED lighting and water pumping, not so much).... Also, if you have streaks of "Cloudy weather", pretty much everyone would like more solar panels vs having to crank up the genset every 2nd day...

Your solar harvest potential:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Clarksville
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 54° angle:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
3.04	3.57	4.43	5.07	5.08	5.22
Jul	Aug	Sep	Oct	Nov	Dec
5.34	5.21	5.18	4.72	3.52	2.95

So, my suggested minimum planned usage of 2,000 WH per day--You pick the minimum hours of sun. If very little genset usage desired, pick December:

• 2,000 WH * 1/0.52 AC system end to end eff * 1/2.95 hours (avg Dec Sun) = 1,304 Watt minimum array for "Break Even December)
  

So--Tending towards 1,695 Watt array for a full time off grid system meets both the battery charging and your estimated usage pretty well--With a little bit extra solar (always a good thing).

Your present panels are fine--It is just simply a question of $$$/Watt and your wallet.

-Bill

Ironbull

The more I read, the more confused I get. Ok so now I think I'm looking at this all wrong. My thinking was I needed around 45 amps from the panels to adequately charge my batteries. But now I'm reading that the mppt controller takes the excess voltage and converts to amps. Or am I misunderstanding? I just don't know where to go from here. 

mcnutt13579

You can feed the FM80 all the panels you have in any combination [that it will accept] and it will transform them into the right output for the batteries very efficiently.

There is more to it than that, but that is a good starting point.

Ironbull

So as long as I have enough wattage, the controller will do the rest? And the advantage of higher voltage is the ability to use smaller wire vs higher amperage? 

So if I'm understanding correctly I should be good with 2 strings of 4 panels since I will be below my controller 150v limit. 

Now the question remains, how do I tie it all together? Combiner box?

mcgivor

With 2 strings a combiner with fuses or breakers is not required but is useful during diagnosis of the system, breakers in particular allow switching under load, my recommendation is use a breaker combiner if at all possible, a single fuse/breaker would still be required so why not two.

BB.

Think of an MPPT controller as being the equivalent of an automatic transmission for a car... The transmission figures out the right gear ratio/slip to allow the engine to operate at its optimum RPM vs the 0-100 MPH of the tires (and throttle).

MPPT: Power = Voltage * Current.... 80 volts * 10 amps from array = 800 Watts = 14.75 volts * 54.2 amps into battery  (excluding losses)

A PWM controller is sort of like a single speed bicycle. At the "right speed/rider match", it is quite efficient. Outside the optimum range, not so much.

PWM: Imp-array = Battery Current (excluding losses)

So if I'm understanding correctly I should be good with 2 strings of 4 panels since I will be below my controller 150v limit. 
...
8 12v 100w panels
Vmp 18.9v
Voc 22.5v
Imp 5.29a
Isc 5.75a

Yes... But you need the details (solar/electronics/electric code is all about the details):

• 4 x 18.9 Vmp = 75.6 volts (for 24 volt battery bank Vmp-array should be ~35 to 100 VDC at standard temperature)
• 4 x 22.4 Voc = 89.6 volts (voltage at 75F; must never exceed 140-150 volts depending on your controller specs)
• 4 x 5.29 Imp = 21.2 Amps (your wiring need to carry this current continuous with less than 3% voltage drop)
• 4 x 5.75 Isc = 23 Amps (your wiring must carry without overheating)

Higher voltage from Array to charger+battery bank+shed for MPPT type controllers means you can use much smaller AWG copper wire (and save money on copper). More or less, PWM controllers have issues much over 10-20 feet (very heavy cables required).

MPPT controllers can send current at higher array voltage 100 feet or more (again details and needs matter).

The combiner protects a shorted solar string from being over current-ed by the other attached strings... In general, you only need a combiner panel+fuses when you have 3 or more parallel strings (2 panel strings feeding a shorted panel string--Fuse or Breaker on shorted string pops and helps reduce the chances for fire from overheated wiring).

A combiner box is sort of a "reverse connected" main panel... Current goes in through breakers and all current is collected on the bus bars to send to charge controller on two cables (+/-)

https://www.solar-electric.com/lib/wind-sun/PVcombiners-explained.pdf

There is a lot to learn, and I suggest you do it in stages... Do a complete paper design (or three), and figure out what will work best for your needs (cost/benefit).

Once you have the paper system designed (basic array size, voltages, current, battery bank, etc.)--Then you start looking for the pieces to put it all together.

-Bill

jmb

I have 9 panels like you describe and have them wired 3s3p.  I have a 15 amp in-line "solar fuse" on each serial string and the three strings into combiner cable.

BB.

Sounds good.

Your array is a bit on the smaller side... But if you keep an eye on your battery bank state of charge and are being careful not to over discharge the system (especially in winter/during bad weather) without utility power or genset backup (if you need the energy)--You should be fine.

Any other questions or details to discuss?

-Bill

Ironbull

Yes I would Bill.

I've decided to get one more panel and do 3 strings of 3. I got 6 string combined box with breakers from Midnite solar to connect it all up. Now I'm having trouble getting a clear answer on what size wire to run from the combiner to charge controller. I've used a couple different calculators with different results. The combiner will be approximately 100' away from the CC. The best I can figure, I will need 4awg wire. Can anyone confirm?

From my research on grounding, some say use a ground at the array and some don't. And some say everything needs to be grounded at the same point. This will not share anything connected with the house. I do have an existing ground rod approximately 20' from CC, inverter etc that the house ac system uses. But I've read that invites lightening into the house.

Speaking of lightening, I was looking at suppressors to attach to the combiner box. There are several different sizes. How do I determine what size is needed? And should I install anything near my other components  also?

I want to thank everyone for helping me sort this mess out.

BB.

OK, one more important piece of information--100 feet from array to MPPT FM60 charge controller...

8 12v 100w panels
Vmp 18.9v
Voc 22.5v
Imp 5.29a
Isc 5.75a

3s x 3p array:

• 3 * 18.9 Vmp = 56.7 volts
• 3 * 5.29 Vmp = 15.87 amps

Normally, you would aim at 3% to 1% voltage drop... Using a simple voltage drop calculator with 100' cable run, copper wire, and trying different values:

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=56.7&phase=dc&noofconductor=1&distance=100&distanceunit=feet&amperes=15.87&x=65&y=21
8 awg, 100 feet:
Voltage drop: 1.99
Voltage drop percentage: 3.52%
Voltage at the end: 54.71

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.8152&voltage=56.7&phase=dc&noofconductor=1&distance=100&distanceunit=feet&amperes=15.87&x=67&y=23
4 awg, 100 feet:
Voltage drop: 0.79
Voltage drop percentage: 1.39%
Voltage at the end: 55.91
Voltage drop: 1.33
Voltage drop percentage: 1.41%
Voltage at the end: 93.17

And this is where high voltage for your solar array is your friend... Just to give you some options... By adding 1 more panel and a bit different wiring: 10 panels 5s x 2p.

Vmp-array=94.5 volts
Imp-array=10.58 amps

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=5.211&voltage=94.5&phase=dc&noofconductor=1&distance=100&distanceunit=feet&amperes=10.58&x=59&y=23
12 awg, 100 feet:
Voltage drop: 3.36
Voltage drop percentage: 3.56%
Voltage at the end: 91.14

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=94.5&phase=dc&noofconductor=1&distance=100&distanceunit=feet&amperes=10.58&x=74&y=12
8 awg, 100 feet
Voltage drop: 1.33
Voltage drop percentage: 1.41%
Voltage at the end: 93.17

I hate to keep jacking up the size of the array on you... But, you now have the choice of a larger array and less copper from array to charge controller--A major advantage of using a higher end MPPT charge controller.

If you install the larger cabling 8 AWG with 3x3 array now... You can always install another panel larger and rewire the array to 5s x 2p or even 5s x 4p for 20 panels total if loads/wallet works out for more panels down the line.

5s x 4p:
Vmp=94.5
Imp=21.16

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=94.5&phase=dc&noofconductor=1&distance=100&distanceunit=feet&amperes=21.16&x=48&y=31
8 awg, 100 feet
Voltage drop: 2.66
Voltage drop percentage: 2.81%
Voltage at the end: 91.84

-Bill

Ironbull

At this point another panel wont hurt my wallet anymore than it already is. I do want more room for expansion if the need arises in the future. Thanks again. 

BB.

Ironbull said:

From my research on grounding, some say use a ground at the array and some don't. And some say everything needs to be grounded at the same point. This will not share anything connected with the house. I do have an existing ground rod approximately 20' from CC, inverter etc that the house ac system uses. But I've read that invites lightening into the house.

Speaking of lightning, I was looking at suppressors to attach to the combiner box. There are several different sizes. How do I determine what size is needed? And should I install anything near my other components  also?

I want to thank everyone for helping me sort this mess out.

And to address the rest of your questions... Lightning is a big issue for folks that are in those areas.

I like Midnite's approach to surge suppressors:

https://www.solar-electric.com/search/?q=midnite+surge

Not cheap... But better than the spark gap+sand that you see in many others.

More or less, you have to ground the frames and any metal work for supporting the panels. Lightning is a "radio frequency" event and does not follow copper wire like DC and 60 Hz AC does... You want the ground rod right at the base of the array (outside of the foundation if roof mounted). And no sharp bends (the increased "impedance" of sharp bends will cause the lightning to jump to another path). And you want to run the cabling from the array (including the DC cables) outside the building to the entrance box, where you would install the suppressor and lightning rod at the base of the wall. You don't don't want to bring the wiring down the inside of the building--Lightning wants to "leave" the center of the building and go towards the outside walls (self generated magnetic field, and find any way to move sideways it can).

It also helps to have surge suppressors on your inverter's AC output suppressed too... Years ago, the founder of our forum (NAWS, since retired) had seen that the most common failure of a lightning strike (or nearby hit), is the AC inverter's output stage.

Midnite's surge suppressors are sized based on the working voltage of the lines being protected.  Clink on their' "Documents" Button for more details..

http://www.midnitesolar.com/productPhoto.php?product_ID=283&productCatName=Surge Protection Devices&productCat_ID=23&sortOrder=1&act=p

Some links to read:

BB. said:

Re: Working Thread for Solar Beginner Post/FAQ

A couple threads about Lightning:

Off Grid Grounding Technique?
Another Question, this time about Lightning

Note, the above are discussions, not a do A, B, and C--and you will be "safe". There probably is no such thing with lightning. Several different techniques are discussed--and a few of those posters even have experience with lightning. :cool:

And our host's FAQ:

Lightning Protection for PV Systems

From other past posts here, Windsun (admin/owner of NAWS), he said that most of lighting induced failures he saw were in the Inverters' AC output section.

Towards the end of this thread is a very nice discussion of proper generator grounding.

-Bill