ligwyd: Design my the perfect system

System

This discussion was created from comments split from: Matching Battery Bank Size to Panels.

ligwyd

Hi Tony,

Could you help me calculate to ensure my array/ battery bank are sized properly?

18x 260 watt panels = 4680 watts

48v system

12 - 12volt batteries - 3 strings of 4 batteries - 215 Ah each  - 645 Ah total. (I know strings suck - will do one string in the future)

Just read the whole post and have a question - what percentage of your systems total output should be consumed by charging batteries at 10%?

Is there a rule of thumb or is that simply answered with however you want design your system? I think it would make sense to have a big enough array to keep batteries happy and healthy (room for equalize) and also run you daily loads without generator (providing you have sun of course). It would seem that it would be nice to produce 10 kwh/day on top of battery charging demands.

Thanks John

Estragon

I assume 21ah is a typo, s/b 215ah?

Your pv output will normally be ~ 75% of STC,rating in most locations/climates in the better solar months (spring/summer). You can get a rough estimate of average daily output for your location at pvwatts.nrel.gov

As a first cut, making assumptions about location and loads; 645ah at 50%SOC is ~325ah +losses to recharge. 4680 x 75% = ~3500w ÷ 58v charging = ~60a (in full sun). Charging is fairly efficient up to ~85%SOC, so ~4hrs at full output into batteries, plus another 3hrs at diminishing current for absorb. In practise, you'd ideally want to get the batteries to absorb by early afternoon, then use extra potential pv current for opportunity loads.

Most locations don't get much more than 4-5hrs in *full sun equivalent* daily average, so some days you might get to full (with no loads), but probably not have 10kwh spare for opportunity loads. At higher latitudes with panels tilted fairly flat, you might see 10kwh extra in spring/summer.

ligwyd

Thanks Estragon,

So do you figure I am under paneled for the size of battery bank given that in good sun it would take everything the panels have just to bring the batteries up from 85% SOC? (ideally I would like to cycle the batteries down to 80% SOC) If so how much more panels would you recommend?

I would think it is the aim of solar in the sun, provided proper design, to provide enough power to bring up batteries through bulk, absorb and float and also power you loads in the day while the sun in shining ( in the sunny months of course - generator in the off season) and then at night draw from the bank to keep essential loads going and a little extra breathing room for whatever else correct?

Also, in your opinion, what is the perfect design? Obviously a loaded question but just looking for ball park figures for a battery bank Ah to PV watts ratio?

westbranch

Also, in your opinion, what is the perfect design?

one which allows me to  never have to turn on or hear the generator....  I wish....

ligwyd

True enough. That would be nice but very $$$. An optimally sized quality generator is nice no matter what size your system is ... just in case you need it. From all my calcs it makes sense to have the back up.

Estragon

There is no "perfect" design. The design starts with parameters (loads, location, etc), but also involves trade-offs within a wide range of "reasonable" designs, which are highly personal. Budget, ability to reduce and/or shift loads, willingness to run the generator, etc are some of these trade-offs.

I have about the same pv as you, and about 1/2 the battery capacity on my main system. It works well for me, but might not if I had a S.O. who insisted on running big loads regardless of weather, for example. Most of the year, I'll get to float shortly after noon on a reasonably sunny day with normal loads. I haven't run the genny at all since winter, but will need to in fall. The genny gets fired up if 50%SOC looks likely.

You don't have to get to 100%SOC every day though. With a bigger bank and relatively less pv, you might cycle more like 55-85% some days and get to 100% once a week or so, which is ok. 60-70% might take an hour in full sun, whereas 90-100% might take 2-3hrs.

ligwyd

I can get 400 watt 72 cell panels - 400 x 18 =7200 watts X .77 = 5544 watts. Maybe this would be a more appropriate array for around the 650 AH battery capacity range? Lots of learning:)

BB.

From a 5% to 13% rate of charge for battery bank calculation (not knowing hours of sun, WH of loads):

• 650 AH battery * 59 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 2,490 Watt array minimum
• 650 AH battery * 59 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 4,981 Watt array nominal
• 650 AH battery * 59 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 6,475 Watt array "typical cost effective" maximum

If you have 72 cell panels or Vmp~36 volts... Two panels in series would "work" for a PWM type charge controller.

For an MPPT charge controller, you really need 3x panels in series (Vmp~108 volts)--HOWEVER, you need to know which brand/model of charge controller you have and how cold it gets in your region. Vmp rises as temperatures fall--And if you are in a significantly sub-freezing region (during winter), Voc-array-cold could exceed the MPPT's Vpanel input max voltage (typically 140-150 VDC).

For larger systems, a good quality/high current (60/80+ amp) MPPT charge controller can really help (longer wire runs from array to battery shed/charge controller, smaller awg wiring for less cost, etc.).

Before you purchase any hardware--Go through a paper design or three and figure out all of the issues/costs/capabilities of the system will be.

It can be very difficult to put together a mix&match system when some of the components are not quite compatible with each other/your configuration needs.

-Bill

ligwyd

Hi Bill, Am I reading your math correct. Top example:

650 X 59 X .77 X .05 = 1477 ?

Hand me the sandwich I am missing from my picnic

Thanks,

John

BB.

Actually 1/0.77 (= 1.30 x ).

If that is your question.

I tend to use 1/xx as xx is a commonly used number (like solar panels and charge controllers are derated by 77% rather than using 1.30x (same derating factor, just one over reciprocal).

Saves me memory space in the old brain and explaining where I got the magic 1.30 number from and the 0.77 too.

I try to write my math like an English sentence. With standard calculator just start at the left. Technically there should parentises around the (1/0.77) to perform that calculation first...

Sort of make sense?

Bill

ligwyd

Cool Got it. Thanks Bill

650 X 59 X [1/0.77] X .05 = 2493

I know I have already asked this question one way and you guys have answered it several ways but let me try one more way just because......

If you all had to design a 48V system blind of the loads but were simply putting together a package that produces X amount of power and what you had to start with is 18 - 260 watts panels, what size of a battery bank would you through at it? Gut feeling based on your experience.

My thought, at present, is to have the batteries fully charged and into float by noon on your average sunny day. Not perfect sun but good sun. Generator is there for the winter and week of cloud cover.

I know this is shooting from the hip but the stage I am at now I am not sure I can afford to experiment on a bunch of 12v batteries paralleled up use them for a few years, find out they are not quite the right battery for me and still hope to sell them for a decent price. At least now that the batteries are still new and being maintained properly in a stable environment and the fact that I got them at cost I could stand a chance of breaking even now but not likely later. 

And lastly for this cart before the horse scenario, what batteries would you choose?

Thanks for your mind and your time guys

John

mike95490

To manage battery charging on a sunny day, you need at least 400ah of batteries,  Even then, you would be hitting them with quite a harsh charge rate. 600ah better match.

 But, many high end controllers can limit their output amps via some user programming, so you could, depending on the controller, get by with a bank of 8 golf cart (200ah) batteries in series.

BB.

John, I have moved this discussion to your own thread... I would suggest just keeping your "similar" discussions to your "main thread/topic". It kind of diffuses the discussion as things are answered in one thread, but not seen in another thread.

And asking a specific person for comment/feedback can slow things down too... Tony (Icarus) has not been visiting us very often in the last few years.

To try and answer your question--You really need to have some "fixed" target. ideally, your loads (seasonal if they vary). Or battery bank size, solar array size, or something else.

I will go ahead and give you a bit shortened answer based on your 650 AH battery bank. You are looking at having "more panels" than a "balanced system would suggest--But that can make sense as you are in BC Canada--Sun in scarce in winter, and fuel is expensive (solar panels are "historically cheap" and have little maintenance needs). Just run the numbers based on "my suggestion" as an optimum system design... The "Target". Not right or wrong, just a starting point:

First, assume 2 days of storage, 50% maximum discharge for battery bank... You are in cold climate--Either insulated battery box with some sort of heat--Or assume less battery usage in winter (less refrigeration/freezer needs)...

• 650 AH * 48 volt battery bank * 0.85 AC Inverter eff * 1/2 days storage * 0.50 maximum discharge = 6,630 WH per day ~ 6.63 kWH per day

That is a good amount of energy. Assume that you use other fuels for heating/cooking/etc. 3.3 kWH per day is very conservation minded and about the minimum energy needed for a "near normal" electrical existence. 10 kWH per day is a closer to normal electrical existence for an average family (no electric heat/cooking/water heating, etc.).

A 650 AH @ 48 volt flooded cell lead acid battery bank will support ~6,500 Watts of peak AC inverter load (and even a 3,250 Watt AC inverter is a good size for this installation--Do not oversize equipment if not needed), and a rough maximum solar array (need charge controllers that support remote battery temperature sensor(s)).

Now, looking at hours of sun--Say a fixed array tilted to maximum harvest during the winter (because you want minimum genset use and keep array relatively snow free). Need to check numbers, but assume ~1/2 array pointed ~south east and other 1/2 pointed south west--Give you "more hours of sun" per day (virtual tracking without mechanical trackers). Lead Acid batteries hate "short time on charging".
http://www.solarelectricityhandbook.com/solar-irradiance.html

Kelowna, British Columbia, Canada
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 25° angle from vertical:
(Optimal winter settings)

Jan	Feb	Mar	Apr	May	Jun
2.36	3.49	4.27	4.59	4.43	4.41
Jul	Aug	Sep	Oct	Nov	Dec
4.90	4.98	4.69	3.45	2.41	2.07

Note that facing 1/2 panels SE and SW does not reduce winter harvest by much--But will increase hours of day on charge.

Looking at a 1/4 discharge (AC power) on a 650 AH @ 48 volt battery bank and 18x 260 Watt panels (and assuming you are not in a deep valley with lots of tall trees blocking sun in winter):

• 6,630 Watt*Hour load * 1/(18 * 260 Watt panels) * 1/0.52 system eff = 2.7 hours of sun per day "break even"

So--That means outside of November, December, & January, you can get away with little genset usage (assuming 6.6 kWH per day max).

Your genset should charge at around 10% to 20% rate of charge... So would need to be around (again, very rough numbers for starting point):

• 650 AH * 59 volts charging * 0.10 rate of charge * 1/0.85 AC charger eff * 1/0.80 genset derating = 5,640 Watt minimum rated genset
• 650 AH * 59 volts charging * 0.20 rate of charge * 1/0.85 AC charger eff * 1/0.80 genset derating = 11,279 Watt maximum rated genset

I do not live in northern climates, I do not design off grid power systems for a living, I am not your engineer....

Just sort of a nice sizing starting point. Others here can give you some real-life experiences to move forward.

Discuss.

-Bill

Dave Angelini

I do design/advise install from remote. It is my business model. The store here can also do this! 
What I have found over the years is that there are many who I call "armchair offgrid" They just want to think about it. Actually doing it is a different matter. If you are serious start small and learn it.

Bill has given so much to this forum I just am amazed by him

Discuss!

BB.

Dave, I hope that my information has been somewhat accurate as well. 

Anyone, please feel free to correct any issues my suggestions may have--I learn a lot from everyone here.

-Bill "no cat fights here" B. 

PS: I should add that "our store" is the host for this forum (they pay all of the bills, I am just a volunteer moderator). Our host is:

https://www.solar-electric.com/

Northern Arizona Wind & Sun
4091 E Huntington Drive, Suite B
Flagstaff, AZ 86004

Sales: sales@wind-sun.com

Customer service: windsun@wind-sun.com

Toll Free: (800) 383-0195

Local: (928) 526-8017

Fax: (928) 527-0729

aksala

I am still learning here and planning my first system. But I tend to simplify these things for my own way of thinking. If the question is really what battery bank would you throw at a given array, I'd still run the given array through something like PVWatts to determine what that array may give you for your location. Then consider what controller you'd use, and then you can determine what size bank makes sense for charging with what you now have available. If PVWatts says you generally have enough to run a 60amp CC at near maximum, say 3500W, then you can figure you can charge at near capacity of the charger. So if you're charging at max 60a and sticking to a rule of thumb 10% rate of charge, then it's rather easy to say roughly 600ah of battery bank should make things happy.

Yeah, this is overly simplistic and general but so is the question right?

What I've found by looking at the PVWatts data for my location is that weather plays a huge role in production, as does temperature since my location is in Alaska. So I am going to over size my array by about 30% in order to compensate for both weather as well as latitude and low elevation angles for much of the year. With two 60a CC's that max out at 3500W I'm going to put 9.3KW of panels up. Over paneled at STC but not in what I believe will actually be produced, especially in the fall. I'm also going to have a rather large 48V bank, but this is as much as I can afford to put up.

Dave Angelini

There are 2 kinds of people, those who want solar and those who want more. You can't have too much up there. Just make sure you limit the current for summer to the battery max or less.

Bill, I think you are very accurate!

BB.

Thank you very much Dave for your kind thoughts.

-Bill

Mangas

Bill,

Mine too.

Thank you!

ligwyd

Thank you everyone for your time. Look forward to giving back.

John

ligwyd

Re: 650 AH * 59 volts charging * 0.10 rate of charge * 1/0.85 AC charger eff * 1/0.80 genset derating = 5,640 Watt minimum rated genset

       650 AH * 59 volts charging * 0.20 rate of charge * 1/0.85 AC charger eff * 1/0.80 genset derating = 11,279 Watt maximum rated genset

Is it better to have a generator to charge at 10% or 20% or is is just fine anywhere in between?

Also, the gen I am considering can put out 27 amps prime 240V. When I use a AC to DC calculator:

http://frontierpower.com/wp-content/uploads/2016/01/KS750-T4-dwg.pdf

https://www.batterystuff.com/kb/tools/ac-to-dc-amperage-conversion-run-through-an-inverter.html

I get 150 Amp DC at 48 V. If a 650 Ah battery can be charged at 10% isn't that 65 amps? This would be max charging amps at 10% correct? (minus inefficiencies of course)?

Last time I charged each individual string of 4 - 12v batteries the display on the inverter and scp briefly hit approx 20 amps through but shortly after reduced to much less. Granted I am only topping up the batteries that are currently being stored and bulk charge would continue for longer if the batteries where at 50%. Now multiply the 20 amps I experienced X 3 as it will be when the 3 strings are paralleled up and should hit around 60 amps or so.

And lastly if you want to equalize at approx 64 volts when necessary, you may need a little more?

Just looking for general critical feedback on my understanding/ misunderstanding? I'm sure much will come into focus the more I use the system.

Thank you all for you patience and guidance.

John

BB.

5% rate of charge (and no other battery/genset loads) is probably the minimum.

10% rate of charge is what some battery vendors recommend as a minimum.

13% rate of charge is a point where it is good to have thermal monitoring of the battery bank (FLA batteries can run hot without forced cooling, and/or a remote battery temperature sensor--As FLA batteries get "hot", their absorb voltage set point should drop--It is possible to get in a thermal runaway where battery charging voltage drop, charger tries to pump in more current, batteries get hotter, etc.).

20-25% is the region you do not want to exceed for charging FLA batteries....

FLA batteries do not "like" high charging currents--So a slow and steady 10% or so charging current will probably give you the best life.

You are in a cold climate, so batteries are kept pretty cool anyway (assuming not insulated/heated battery box at the same time).

FLA batteries have variable chariacteristics... Below 80% State of Charge, they are nearly 100% efficient at charging. Over 90% SoC, they are probably closer to 50% efficient charging). At ~100% SoC (equalzing), they are near 100% losses (~1/2 goes into heat, and ~1/2 goes into generating hydrogen+Oxygen).

During equalization, a 2.5 to 5% rate of charge is appropriate. And with a 100% SoC battery, even a 2% rate of charge can overheat and cause major battery damage or worse.

A good starting point for off grid charging, charge the battery bank between 50% and 80% and let solar manage the last part.

If your solar is poor (winter/low sun angle), recharging to >~90% once a week is good.

When to choose to equalize a battery bank... Some vendors say when cells are ~-0.015 to -0.030 SG units between low and high cells.

Other recommended equalizing once a month. Tall cased batteries probably do better with once a month equalizing (better mixing of electrolyte to prevent stratification--heavy SG at bottom; light SG at top. Gassing helps mixing).

Only run EQ cycles when needed. Do not run long EQ cycles--Only long enough to bring all cells up to max SG (log, temperature corrected) and no further (measure every 30 minutes)--Then stop.

Long EQ cycles are sometimes required if batteries have overall low SG. May take hours. Sometimes even have to stop and restart next day if batteries run hot (let cool). Very rare and most people do not need to do multi-hour/day EQ ever.

As an example, if you charge your battery bank from 50% to 80% with a genset, a 10% rate of charge will be ~3 hours of genset runtime. Is that good or bad for you?

Also, you may plan your charging/genset runtime around your other optional winter loads (vacuuming, washing, cooking with electricity, shop tools, pumping water, etc.). If so, then you will want to support both charging + optional loads when rating genset. Battery charging is a "difficult load"--As you can see it can pull maximum rated (charging current) and provide into batteries for 3+ hours very easily. Most of larger home loads run only pull heavy current when starting... Or even ovens/heaters will cycle the power once up to temperature. Batteries just draw as much current as you can feed them until this are in the ~80-90%+ range state of charge.

For gensets, running them around 50% to 80% loads is most fuel efficient. Desiels can be fuel efficient down to 40% of rated (engine) load--But some (older?) diesels may wet stack, glaze cylinders, carbon buildup. Running >50-60% can reduce those issues.

I remember somebody here said the modern diesels should be able to run at 40% or so loading without above problems.

I am not a diesel mechanic or engineer--Others here can give you better answers (from experience) than I. The above just just a framing of the basic issues as I see them.

-Bill

ligwyd

I am still scratching my head as to why my supplier ordered two mppt 60 150's over 1 mppt 80 600? I have discussed already, and  agree that back up is good should one CC fail?

Doing the basic math. One 600 V CC (Schneider) can out put 4800 watts. My array of 4680 watt X .77 =3603 watts so the 600 would be a perfect match and I would only 4 PV wires to deal with instead of 12?

Obviously I will work with what I have and it will be fine, but in the future, at present I think I'd go with 600V in the future.

Is it true that higher voltage CC are less efficient due to having to take a high voltage and change it into a lower voltage ie heat. The closer the CC voltage to your battery bank charging requirement the more efficient correct? (enter in PWM CC's)

Sorry to post two question  so close together. I can't wait to finish project and start another.

Thanks

John

BB.

More or less, the disadvantage of the "600 Volt" MPPT charge controllers is this cost ($$$) when compared to 150 volt controllers and the "extra" cabling.

You have to show additional "respect" for the higher voltages too (can be more difficult to find breakers/fusing/etc. for higher voltages, especially DC voltages--If needed).

Could also be your supplier has more experience/more comfortable with the lower MorningStar unit (solar and installed a lot of them). Some folks love vendor A, and others hate vendor A....

There is always the "first time"....

You have to look at the specifications and charts for the different units. It can be a complex relationship between efficiency and loading/current flow. In general, I don't think any difference between efficiency of the two high vs low voltage units is enough to make it a decision point.

Generally distance from Array to controller/battery shed is the major question (and the cost of copper to span that distance). Longer distance favor the higher voltage units.

-Bill

Dave Angelini

I would not have let you make that mistake.... General advice, the mppt80-600 is 96% efficient on a 48V nominal. I did the electronic field test for it in 2010.

ligwyd

................I think I am best to work with what I have now? If I go with 600V I gotta change out all breakers in combiner/ PDP and rewire rw90 from combiner/ CC. It will work. My cost is $615 for each 150 CC and $1410 for one 600V CC. Pretty close to a wash either way so It shouldn't matter in the end. Just a little more wiring..... Agreed? I guess I don't have much choice at this point hey

Estragon

IMHO, if the array is reasonably close (say a hundred feet or so), 150v should work and be cost effective. If thousands of feet, the wire cost gets you.

ligwyd

The XW+ 6848 is pre set to do 1 hour EQ. I think I did up to 3 or 4 separate 1 hour EQ charges on a couple of the strings and watched (measured) the SG slowy come up to 1.260. I think I can increase the EQ but won't for now. I like that feature being set at 1 hour for starters. I made sure temp did not get out of control and I will have the batt monitor adjusting volatge when its all set up.  20' containers are 2x4 framed, 2" closed cell spray foamed walls/ roof/ underneath 4x8 sheet hardy boarded inside and batter box has 2" styrospan and 5.5 " Roxall under sub frame of batt box. Noticing even without heat pump to control climate, the high and low temps are mellowed out quite a bite. Will have to watch for thermal runaway in the summer and turn on the AC in the unit. Won't take a lot to cool 160sq.ft white container in the shade of a big solar array.... hopefully

I will find out where my little diesel gen will run at its highest efficiency and run it there for bulk. I never knew that FLA's were that efficient at taking charge below 80% SOC. I am learning more from you guys in 2 weeks than I have in the last two years.

Next build I'll ask for an entire equip quote from you guys and crunch the numbers. Most everything I use, with the exception on kinetic racking made her in Ontario Canada, comes through the US anyway. Hopefully I can get finished this project and get on to the next.

No words can express my appreciation of this forum.

Thank you all.

John

ligwyd

Would you guys turn down 19 strand and wait for 7 strand PV wire or is it not that big of a deal? Don't want to break any wires terminating. Most of the suppliers up here all are quoting me 19 strand and thats all most of them can get. I would think MC4 connectors would like the 7 strand better as well? If 7 is the way to go (don't really need the increased flex of 19 strand) then I will wait for it and get on to something else in the meantime. I have a roll end of each and they both feel comparably manageable. My aim also is to keep the OD to min. (non-direct burial non double jacket etc)

ligwyd

Estragon said:

IMHO, if the array is reasonably close (say a hundred feet or so), 150v should work and be cost effective. If thousands of feet, the wire cost gets you.

Yep the array is attached to the shipping container. Yeah!! One point for having two 150 V CC's.
Thanks. Makes me feel a wee little better about not having the one 600V CC