Off Grid System Upgrade

JYanke

Hello,

Considering upgrades to my offgrid system. Here are my ideas and just looking for some advice:

Current PV array is 3 panels wired in series 80 watts each (max PV Voc is 54 volts) 

My battery bank now is 2 flooded deep cycle 6v batteries wired in series for 12v 230aH total

1. Upgrade to a total of 4 deep cycle flooded 6 volt batteries 230 aH each. Wired in series/parallel combo to make a 12v 460 aH bank. 

2. Add panels to my array to accommodate new battery bank size. (3 more 80 watt panels making total of 6 panels in series with 108 volts max Voc and 4.5 amp max)

3. Upgrade charge controller from Morningstar Sunsaver MPPT to a Victron Energy MPPT 150/35 controller. 

Considering my present day system has had no issues keeping the 230aH bank charged and healthy for over 7 years now, Im simply thinking that the array would need doubled if I doubled the battery bank. Obviously will need to upgrade the charge controller as well and have been looking at these options from Victron Energy. Any thoughts on these controllers and am I on the right path with upgrade ideas? Thanks in advance 

Photowhit

I wonder what you are saying when you say;

JYanke said:

Current PV array is 3 panels wired in series 80 watts each (max PV Voc is 54 volts) 

The, I guess 80 watts each panel? and 3 in series for 240 watts? I think that's fine, what I don't understand is;

"(max PV Voc is 54 volts) " is that for a single panel? For a string of 3 panels it would seem low as the VOC would add and thaere aren't many panels that have a VOC of 18 volts (54/3=). 

Adding a parallel string is likely not a problem with the VOC but might be too high amps in some MPPT type charge controllers. 480 watts into a 12 volt system would max out a 30 amp MPPT type charge controller and some don't 'like' over paneling.

JYanke said:

Considering my present day system has had no issues keeping the 230aH bank charged and healthy for over 7 years now, Im simply thinking that the array would need doubled if I doubled the battery bank. Obviously will need to upgrade the charge controller as well and have been looking at these options from Victron Energy. Any thoughts on these controllers and am I on the right path with upgrade ideas? Thanks in advance 

The other consideration, is why go to the expense if it's not broken?

JYanke

Hi. The panels I have now are 80 watts each and are 18v each. I have all three wired in series which I typically get around 54v max and 4.5 amps max from the total array. Adding three more panels in series would give me around 108 volts max. Of course this has been enough to keep the two batteries charged but I am wanting more amp hour storage for times of increased power usage

BB.

The panels sound like Vmp~18 volts (typically Voc~21-22 volts)... There are Vmp~15 volts or so panels--They were originally made to be "self regulating"--Sort of charge and maintain a 12 volt lead acid bank without a charge controller.

Anyway... Start at the beginning. Sounds like your loads are fine for the existing battery bank and array--But lets do some math to size the "new system" configuration.

The 6 volt @ 230 AH batteries (aka "gold cart" type)--I like those for making a series parallel bank--Vs just using 4x 12 volt batteries in parallel... Easier to take a voltmeter to each battery to check health/state of charge... If any battery is higher or lower voltage than the others--Need to figure out why (wiring issue, bad cell, needing to EQ, etc.).

For a solar power system--5% rate of charge can work for weekend/sunny weather system. 10% to 13%+ rate of charge better for full time off grid use (aka "keeping the batteries happy").

• 460 AH (battery bank) * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 433 Watt array minimum
• 460 AH (battery bank) * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 866 Watt array nominal
• 460 AH (battery bank) * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 1,126 Watt array typical "cost effective" maximum

For a lightly used system--480 Watt array (6x80W) is OK... 80 Watt panels tend to be more expensive (if you have to purchase 3 more)... Looking at some new large format solar panels (usually used for Grid Tied systems--Many times, you can find installers with a few left overs that can be purchased cheap--Possibly to near $0.50 per kWH).

And there is also sizing the array based on your daily loads... Assuming fixed array near Akron OH:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Akron
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 49° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
2.54	3.13	3.80	4.60	4.87	5.23
Jul	Aug	Sep	Oct	Nov	Dec
5.38	5.12	4.86	3.85	2.48	2.17

If you were using (230 AH * 0.25 discharge= ) 57.5 AH per day... A 480 Watt array would need hours of sun per day of:

• 57.5 AH * 12 volts = 690 WH per day average
• 690 WH per day * 1/0.52 off grid AC system eff * 1/480 Watt array = 2.76 Hours of sun per day "break even"

That will take you from February to October pretty nicely...

Depending on the size of your array--A quick calculation for an MPPT solar charge controller:

• 480 Watt array * 0.77 panel+controller deratings * 1/14.5 volts charging = 25.5 Amp minimum MPPT controller rating (suggested)

Your thoughts?

-Bill

JYanke

Thanks Bill. Awesome information as always. You are correct, I meant each panel has 18v vmp max. 

According to your calculations my current system (3 panels at 80 watts each) is capable of roughly a 5% rate of charge for the 230 AH bank. 

230 AH * 14.5v * 1.298 * 0.05 = 216 Watts min

I think the 5% rate of charge will be okay for my use of the cabin. (As it has been so far for the 230aH bank) Would love to make it more efficient but can’t justify the extra cost at the moment. 

I also want to try to utilize the three panels I already have, and sourced the same ones from a place called Sunelectronics where I bought the original 3. The panels would be about $74 a piece plus shipping. 

The Victron Energy 150/35 MPPT has the following ratings which I think would work?

Rated Charge: 35 A
Nominal PV Power: 12v:500W
Max PV Short Circuit Current: 40A
Max PV Open Circuit Voltage: 150V coldest 
Absorption: 14.4V adjustable 
Float: 13.8 adjustable 
Max Efficiency: 98%

BB.

I don't see any reason why the Victron 150/35 would not work... Unless you want to add capacity in the future (would need a new/second charge controller for larger array).

You have the choice of 2s/3p, or 3s/2p, or 6s/1p (series/parallel) connections for your array... I did not see an efficiency curve for the Victron--But typically the most efficient is about 2x Vbatt (2x 15 volts = 30 Volts Vmp-array)... However, going higher is not going to cost much (perhaps a lost of 1% or so in efficiency).

There is an issue with 6 panels in series--If you happen to be a region that gets terribly cold--6 panes in series may exceed 150 VAC.

You can use this Midnite calculator for min/max array voltages vs temperature:

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

Probably the simplest would be 3 series x 2 parallel strings... Still reasonably high voltage for the array -- And can use smaller AWG wiring. Depending on how far your array is from the battery charger/shed... Looking typically fro 1% to 3% maximum voltage drop from array to charge controller.

 https://www.calculator.net/voltage-drop-calculator.html

If you don't want to run a another array wiring string (or rewire with heavier cables)--6s x 1p is probably OK (I don't think Ohio gets that cold...).

-Bill

JYanke

Thanks Bill. Would you mind describing what you mean by 3 series x 2 parallel strings “3s/2p”. Trying to wrap my head around how the 6 panels would be wired in that configuration. At the moment my panels are not far from the controller and battery bank and have the following:

panels to midnite solar 150vdc 15 amp breaker: MC4 4mm^2 wires

breaker to charge controller: #12 AWG

charge controller to battery bank: #12 AWG

*would also have to upgrade the breaker for the new array

BB.

This does an OK job:

https://www.explorist.life/solar-panels-series-vs-parallel/

Basically, solar panels--you connect + of panel A to - of Panel B; then + of panel B to - of Panel C... The - of A and + of C are now your three panels in series (3x Vmp).

You connect both 3 panel series strings together (- to -, and + to +) and that adds the current (Imp) from each string to 2xImp.

The first is 3s x 1p... The second is 1s x 3p... Solar panels are (more or less) a bunch of 0.5 volt cells in series... A Vmp~18 volt panel has 36 cells in series. You would just take two strings of the left drawing and combine the outputs together (+ to + and - to -) for a 3 series * 2 parallel strings.



Note that batteries are just a bunch of 4 volt cells in series (a 12 volt Lead Acid battery has 6 cells in series). Your 2 volt AGM cells are 24 in series for a 48 volt battery bank.

Please make sure you understand the wiring... In some cases, miswiring can toast your solar panels or worst.

How long is the cable run from your panels to the charge controller?

You presently have a Midnite controller (what model)?

-Bill

JYanke

Thanks Bill. Im following you now. 

The panels are wired in series using short lengths of the MC4 4mm^2 wire. From the panels I have roughly 10 feet of MC4 4mm^2 wire to the Midnight Solar “Solar Array Combiner” model MNPV3 with a 150vdc 15a breaker. 

Maybe a foot or so of #12 AWG running from breaker to the Morningstar 15A Sunsaver MPPT

Few more feet of #12AWG from the Sunsaver to the batteries 

JYanke

So looking at the 3s/2p configuration I would have 480 watts and still have the 54 volts Vmp but would double my imp from 4.5 to 9. 

I think I would still have to change the controller. The Sunsaver specs says it handles up to 60v but shows max pv wattage is only 200 w on 12v

BB.

You usually do not need a combiner box for 1 or 2 parallel connected strings. For 3 or more parallel strings, then the combiner is used. One (or two) breakers/fuses per string.

The series protection fuse/breaker is there to prevent the other paralleled panels feeding current to a shorted panel/string. The fuse/breaker for the shorted panel/wiring trips and stop current flow (and reduce the chance of starting a fire).

If you are using a combiner--It sounds like you have a 1 series x 3 parallel panels? And are (probably) for the MorningStar MPPT (sunsaver) controller?

Placing 3s x 1 parallel string would b maxing out the Vpanel input for the Morningstar (Voc-array 60 volts maximum).

Have to be clear Voc (voltage open circuit--no load on array), and Vmp (voltage maximum power--Pulling the "optimum" Imp current maximum power from the solar panels). For your panels, Vmp~18 volts, and Voc~21.5 volts or so... These specifications are based on the cells at ~75F... When panels get cold (overnight, cold environment), Vmp and Voc rise... When panels get hot (typical under full sun warm weather, not much wind), both voltages fall.

So--I am a little bit confused at the moment. 

Regarding wiring... 4mm^2 wire is roughly 11 AWG. If you want to send 9 amps for 10 feet @ ~54 VDC, then the voltage drop calculator would be:

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=4.132&voltage=54&phase=dc&noofconductor=1&distance=10&distanceunit=feet&amperes=9&x=9&y=2

Voltage Drop Calculator

Result

Voltage drop: 0.23
Voltage drop percentage: 0.42%
Voltage at the end: 53.77

So voltage drop is not a big issue (you can calculate each length of cable+current. Note this calculator assumes 10 foot one way run = 20 feet of actual wire run).

And you can use the NEC table for look up max current for 12 AWG wire (US "odd AWG" wiring is not commonly used):

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

And 12 AWG is good for 20 amps typical. So you can double your array and use the same 11 AWG cable to bring power from the array to the charge controller Vpanel input.

-Bill

JYanke

Thanks Bill. To be honest I installed the combiner box mainly as a means to mount the breaker and have an easy way to disconnect the PV array. Also considered potential of expanding the system one day. However, all thats in the combiner box is the one 15a breaker. My panels are just three 80 watt panels wired in series and route to that breaker (3sx1p)

I will digest all this great info and let u know if I have any questions. Appreciate all the help

joe

JYanke

Good Morning Bill,

when you reference “rates of charge” such as 5%, 10%, 13% etc. could you elaborate a little more about this? What exactly does a 5% rate of charge entail? Thanks

BB.

For our needs, we use the battery AH capacity at the 20 hour discharge rate;... There are other rates uses from 6 hour to 100 hour discharge rate--But 20 hour is pretty standard with many battery manufactures. 

That means:

• 100 AH / 20 hours = 5 amps (for 20 hours from 100% to 0% state of charge)

So, when we talk about 5%-13% or so rate of charge--That is based on the battery 20 hour capacity:

• 100 AH * 0.05 (5%) rate of charge = 5 amp rate of charge
• 100 AH * 0.10 (10%) rate of charge = 10 amp rate of charge
• 100 AH * 0.13 (13%) rate of charge = 13 amp rate of charge

Of course, we are usually talking about the total bank AH capacity--When you have batteries in series, the voltage adds. When you place batteries (or strings of batteries) in parallel, the AH capacity adds.

If you have 4x "golf cart" batteries that are 6 volt @ 200 AH and want a 12 volt bank, you put 2 batteries in series for 12 volt (2 * 6 volt) string of batteries, and put 2x strings in parallel for 400 AH (2 * 200 AH). That gives you a 12 volt @ 400 AH battery bank. A 10% rate of charge would then be:

• 400 AH * 0.10 (10%) = 40 Amps charging

And the array to supply that amount of current for a 12 volt bank would be:

• 40 Amps charging * 14.5 volts nominal charging voltage * 1/0.77 Hot panels and charge controller deratings = 753 Watt array nominal (for 10% rate of charge)

-Bill

706jim

If you had a battery rated at 100 ampere-hours a 10% charging rate would be 10 amps. 5% rate would be 5 amps etc.