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joelthewelder

I have 8) 100w panels, a mmp solar 3kw 48v to110v inverter and a 6kwh 48v battery.

My panels i want in the yard about 150ft out.   I have a shed at the mid way point that i could put the inverter and battery into. .

The inverter is rated for 60v to 115v input from the panels.

I hear the inverter should be close the the house? but the i hear its good to have the highest voltage on the longest run,, whether ac or dc. 

Should i split the difference and use the shed as a battery/inverter box.  (kinda like the idea)

Trying to decide on 4s2p on panels ( short run) or  series all the panels for 96v and put inverter and battery close to the house? 


First poast.  Thanks

joelthewelder

I can also move the shed out closer to the panels or move the shed closer to the house...

Any advice appreciated. 

joelthewelder

So i realize now my voc 22.6 is too much for all 8 panels.  My inverter says 145v max for voc.. .i will stick to 2 strings of 4.   

BB.

Welcome to the forum Joel,

The short answers are that you want the highest voltage to be the longest distance--That lets you use the smallest diameter cable (less copper, less costly cable). From a copper point of view, 60 VDC needs something like 6x heavier copper cable vs 120 VAC.

Sort of, the "best" run to make longer is the solar panel wiring. In general, there is no surge current, and placing the "battery shed" next to the house means it is easier to go out for a quick inspection of the hardware, batteries, etc.

While rules of thumbs gives us a nice starting point. Details matter. And, for some applications, more expensive charge controllers that support higher Vmp-array voltages can be nicer for those long(er) distance runs.

The other thing I suggest doing is looking at the hardware/setup you already have, and use some math and rules of thumbs to get the system "balanced" (matching battery bank to loads, solar array to battery bank, loads, hours of sun per day, inverter size, etc.).

For example, your present 3 kwatt inverter would drain a 6 kwatt*hour battery bank in less than 2 hours. For a normal off grid system, that inverter is probably too big for that battery bank for a "useful" off grid home power system.

However, perhaps you have special needs... "Welder"? Low duty cycle, high power for short periods of time. Starting a well pump. You have AGM or other high current batteries that "surge better" than flooded cell lead acid batteries.

Also, there are usually some hardware specifications/limitations. For example, a 48 volt battery bank needs to be charged at ~60 Volts. And a Vmp~60 volt solar array, on hot days, the output (Vmp--Voltage maximum power) can drop to 50 VDC or so--And limits the charging current to the battery bank. In general, a 48 volt battery bank should have a Vmp-array (standard test conditions) of ~72 volts minimum. And Voc-array-cold for a Vmp=72v array is very close to the 115 VDC max input (Vpanel max for charge controller) for your controller(?).

You can give us some more information (hardware brands/models, battery bank chemistry, nearest major city for hours of sun, what your loads per day are, etc.) and we can try to you design and build out a "balanced system".

In any case, the answers to your questions about cables. First you need to design the wiring to carry the current. A simple NEC chart is a good starting point:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

For an 800 Watt array (100 watt panels), Pmp~100w; Vmp=100w/17.5Vmp=5.71AImp (panel specs). A 4x * 2p array (starting point):

• 4 * 17.5 Vmp = 70 VDC Vmp-array (std)
• 2 * 5.71 Imp = 11.42 Imp-array (std)

Using the NEC, 14 AWG is heavy enough for current (using the correct wire--Outdoor/UV rated if in sun, wet rated if in conduit, etc.).

Next, what is the voltage drop for 150 Ft (one way run for this voltage drop calculator), 70 VDC, 11.42 Amps:
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=8.286&voltage=70&phase=dc&noofconductor=1&distance=150&distanceunit=feet&amperes=11.42&x=0&y=0

Result

14 AWG:

Voltage drop: 8.65
Voltage drop percentage: 12.36%
Voltage at the end: 61.35

Not good... Generally we look for ~1-3% voltage drop (fewer losses). Playing with the AWG number, find:
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.8152&voltage=70&phase=dc&noofconductor=1&distance=150&distanceunit=feet&amperes=11.42&x=62&y=18

4 AWG:
Voltage drop: 0.85
Voltage drop percentage: 1.22%
Voltage at the end: 69.15

Much more expensive cable....

Now do the same thing for a 3,000 Watt @ 120 VAC inverter:

• 3,000 Watts / 120 VAC = 25 Amps

From the NEC,10 AWG is the minimum wire AWG. Drop calculator 120 VAC, 150 feet:
 https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=3.277&voltage=120&phase=ac&noofconductor=1&distance=150&distanceunit=feet&amperes=25&x=60&y=24
10 AWG:
Voltage drop: 7.49
Voltage drop percentage: 6.24%
Voltage at the end: 112.51

Still too much drop... Try with different AWG:
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=1.296&voltage=120&phase=ac&noofconductor=1&distance=150&distanceunit=feet&amperes=25&x=51&y=17
6 AWG:
Voltage drop: 2.96
Voltage drop percentage: 2.47%
Voltage at the end: 117.04

Somewhat better (note that 3kW inverter >> 800 Watt array; 120 VAC is only a bit higher than 70 VDC).

So, in your case, because AC inverter >> Solar array --- There is not a "big" advantage of AC run over a DC run.

HOWEVER, this depends on what you really need... Do you need 3 kWatt 120 VAC? Or less? Will you need a >> 800 Watt array (battery bank size, your daily loads, your hours of sun per day, etc.)...

I highly suggest we start at the beginning (loads, location, your needs such as weekend/backup power, full time off grid or what). At this point, your AC inverter is huge vs the solar array wattage, and your battery bank is small for this size AC inverter).... For example, an off grid home/cabin with a 6 kWatt*Hour battery bank--I would be suggesting a ~600-800 Watt AC inverter, not a 3,000 Watt inverter (drawing huge amounts of current from a "smallish" battery bank).

Your thoughts?

-Bill

joelthewelder

Hey Bill, thanks for the help.  My battery is a lithium ion 120ah. I will be building 3 more batteries in parallel soon.  (24kwh Over the next year)  i will be adding more panels too over time.   This will be for nightime use for now.  Lights.and 400w pellet stove. 

Still trying to get through your post.  Wow...

joelthewelder

Battery info. Its a 14s pack lg chem.  N2.1

joelthewelder

Here is the specs on the right inverter, sorry about that. Mine is the 3048lvmk

joelthewelder

To the rest of your questions,  i am in marysville washington.  The hot water tank is my biggest load,  but im not worried about that or the washer and dryer as of yet.    We have a fridge and freezer and a dishwasher.  

No, im not going full time off grid anytime soon. 4 year plan perhaps.

joelthewelder

I am pretty impressed with usa mpp solar as a seller..  I ordered the inverter 2 days ago and its here already with free shipping.  

The feature that i like when i was researching it is that i can hook it to ac, charge the batteries, then run off the battery until its empty and it switches the load to ac power instantly.  

I can also buy another one and parallel them in single phase up tp 9 units for 27kw max

Or 9 units can work together to make 3 phase or split phase..  

Thanks again bill for your examples and calculators.  They are very helpful.

joelthewelder

I ordered parts to build a second lith battery so i will soon have 12kwh 240ah battery bank.  

BB.

Sounds intersting Joel.

I would suggest you aim for 3,300 WH per day (3.3 kWH per day or ~100 kWH per month). That will support an Energy Star full size fridge, LED lightning, washing machine, solar friendly pump, laptop charging, cell charging... A pretty near normal "electrical life" with lots of conservation (using other sources of energy for heating, hot water, cooking, etc.).

I can run the system design math for you, if you would like.

-Bill

joelthewelder

You have been most helpful already Bill.  On the number of panels on a string question, from what i can tell based on the inverters pv 145v max and one of the calculators you sent. It looks like i could get 5 panels on a string and still be below the max volts .  (90v vpm and 113v voc)  ?  Is that too close for comfort?  

Just trying to get more voltage for the long run..  i will try it with 4 and see if it works ok first.  Best to leave some safety room.

BB.

The problem is that solar panel specs are with the panels at 75F/25C ("flash" testing of panels--Just a few seconds of test light to confirm Voc/Vmp/Isc/Imp/etc.).

I am guessing that it does get a bit below freezing there at least a few days a year... Vmp and Voc rise as temperatures fall--So you have to look at the Voc-array-cold value.

In theory... Say 25F is your minimum temperature (-4C). Standard temp is 25C. Voc rise is -0.45% per C (typical)... So, your array should max out around:

• 5 * 22.6 Volts Voc-std = 113 Volts
• -4C min temp -25C std temp = -30C spread
• -30C spread * -0.0045 ratio/V offset =  0.135 ratio increase
• 113 Vmp-array-std * 1.135 increase = 128.3 Volts Voc-array-@25F

So, it looks OK with 5 panels in series (assuming my minimum temp guess is right).

-Bill

joelthewelder

Thanks Bill, the calculator was not specific to my inverter but thank you much for clearing it up for me.  

One of the solar temp maps says the record low is 15f for marysville so your not far off.  

Would the mppt in the inverter care either way as long as it does not go over 145vor under 70v.

BB.

You do not want Vmp-array to fall below ~72 volts (hot array, lower Vmp-array-hot). As you need almost 60 volts to charge the batteries, a bit for voltage drop, voltage drop across the charge controller (panels to batteries), etc...

Otherwise, not really any issue if Vmp-array is in between. MPPT charge controllers tend to get a bit less efficient the higher the array voltage (by 1-2% typically) the higher the array voltage--But that is a small loss of energy (you could not really measure without laboratory equipment).

Just keep the inverter/electronics in a well ventilated area (hot electronics die sooner vs cool electronics).

-Bill

joelthewelder

Panels were delivered yesterday.

Mppsolar also suggested to sticking to 4 panels per string.  

Time to weld up a solar tracker stand..

joelthewelder

I finished my solar tracker and finished putting my battery together.

Fired up the inverter and it started charging at 10 amps last night at the end of the day.

joelthewelder

When i went to work at 2am my battery (6kwh) was at 35%, by 9am it was at 50% and by 12:30 it was at 86% and slow charging at .2a

I think i need to put my other battery together now.  Awesome...

BB.

Sounds good... Use a good DMM to measure the voltage at the battery bank and at the output of the charge controller (if connections and wire size/length is OK, the voltages should be near identical).

Charging to 90% or so for Li Ion batteries--Generally they will last longer. Over charging is not good (neither is over discharging). So review the battery requirements vs what you are feeding them.

-Bill

joelthewelder

Thanks bill.  I was doing some load testing last night and playing with settings.    I had  a 750w heater running as the load,  i had about 700w of solar coming in and about 50w leaving the battery to make up the difference. 

I switched to 1500w load and i started pulling 800w from the battery.  

So cool.  Exactly what i was hoping for..

I really like this inverter

This is with no ac power in yet.  

joelthewelder

Yesterday, I installed a second breaker box for critical loads next to the main box.  Then moved kitchen lights , dining room lights, pellet stove, internet router,  porch lights and the laundry room lights over to the new box.  

With all the lights on and the pellet stove running for testing, i was at 750w load.  

Once i fininsh my second battery and upgrade to thicker battery cables i will switch over more lights but for now it's working really well.

Still playing with my inverter settings but so far solar first, then battery, then Ac as backup works great.  

joelthewelder

My battery started solar charging at 5:22am and sunrise was at 5:31am today.

9 minutes of tracker magic.

joelthewelder

So the most volts i have seen on my 2 strings of 4 panel so far is 82v, if i added a 5th panel to each string and upped the voltage a little (still in safe inverter range), would the system perform better on a grey days as far as charging?  

Just thinking about all the grey days ahead. 

BB.

Power = voltage x current

Once you are in the mppt operating range, it does not really matter. 2x voltage OR 2x current is the same.

Playing with series and parallel connected panels allows you to do things like adding one panel.

2sx2p= 4 panels. 3sx1p= 3 panels...

Bill

joelthewelder

BB. said:

Power = voltage x current

Once you are in the mppt operating range, it does not really matter. 2x voltage OR 2x current is the same.

Playing with series and parallel connected panels allows you to do things like adding one panel.

2sx2p= 4 panels. 3sx1p= 3 panels...

Bill

Thanks Bill.  I get what your saying. What im asking not so well is if on one grey day the mppt is on the edge of charging at say 60-62v.   And the next day is a bit darker and it wont charge at all at say 59v.  

If i switch from 4s2p to 5s2p my voltage should be then well above 60v on grey days.

All day slow charging vs no charging , if that makes sense?    

BB.

The Vmp voltage is based on Cell construction and wiring, and temperature of the cell/panel.

For any useful amount of energy harvest, the Vmp-array is not really connected to light levels.

Once you have weak but direct sunlight falling on the panel(s) (and even back scatter from the sky), Vmp and Voc for the array will be obtained.

If you see Vmp falling below (say 80%) of the Vmp-std for the panels/array, then either A) there is not enough sunlight for the MPPT controller to "qualify" the actual Vmp-array voltage (too little sun, too little current), or you have another issue (miss-configured array, problems with MPPT controller, etc.).

More or less, for a MPPT controller, you want the Vpanel input voltage to be greater that Vbatt+a few volts to operate correctly. For example (made up worst case numbers--i.e., a very hot day, EQing battery bank), the minimum Vmp-array-std (standard test condition) voltage should be:

• (60 Volts battery charging + 2 volts controller drop) / 0.80 hot panel derating of Vmp = 77.5 volts Vmp-array-std minimum suggested rating (desert climate, cool FLA battery bank, etc.).

Note that Vmp of the array is not a "peak", but a more "rounded" curve... So you will not see much difference in current/power at 77.5 volts +/- a couple volts.

And you have the other MPPT limit--Typically a maximum input voltage for the controller... For a ~140-150 volt Vpanel-max controller in a sub freezing climate, A Vmp-array-std of around 110 Volts will give you around Voc-cold of 140-150 volts--The maximum input voltage allowed for a typical higher end MPPT solar charge controller.

There is exact math that you can use to calculate the Vmp and Voc based on your local ambient temperatures, and some Manufacturers have a website or downloadable program to do the calculations quickly. For example Midnite's:

http://www.midnitesolar.com/sizingTool/index.php

In summary... Generally the array is designed to range from Voc-array-cold (maximum voltage open circuit minimum local temperature) to NOT OVER-VOLTAGE the Vpanel-input of the MPPT controller, and...

Vmp-array-hot (minimum array voltage at hot ambient temperature +35C rise from sun heating) to ensure that the array MINIMUM FULL POWER OPERATING voltage is high enough to properly supply to the controller+battery bank+voltage drop on the hottest days and charging a near full battery bank/EQ charging.

-Bill

joelthewelder

 Today i added one more panel in series to each string to make 5 panels per string with 2 strings in parallel. 5s2p.

 My voltage is now 88v on a totally grey day and its now slowly charging my battery.

I think im in good shape now.  

I will keep an eye on voltage when its cold and sunny.  

joelthewelder

I got my second battery finished, balanced  and online in my system.  Im now at 12.4kwh/ 240amp hours.  The last few days have been raining but my system in now charging on grey days.    Very cool.  The extra 2 panels made a huge difference.  

I got my inverter hooked to the wifi so now all my data gets logged every few minutes and i get email alerts if anything happens like overload trips or power failures.

This makes it easy to monitor the pv voltage from anywhere.   

I might upgrade to watchpower down the road so i can make inverter changes online but this works for now.