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Planning and design issues - battery bank

pvdude

Solar Enthusiast
Joined
Feb 8, 2021
Messages
618
Location
Florida
Hi,
I am building a system for off-grid.
Florida, lots of Sun!

The electronics part is easy.
The inverter is a Schneider Electric Conext XW Pro 6848.
Four Panasonic VBHN340SA17 340 Watt Mono PV panels, to start with.

The battery bank is the puzzle. I need to design and purchase a 48VDC system.
I have been reading posts here and other places that lead me to conclude I should avoid
Lithium batteries for non-technical reasons.
Lack of quality control, vendor Ah capacity claims not delivered, BMS quality, etc.

Is there any source for quality Lithium batteries from a reliable vendor?

Possibly I am just freaked out by all the problems I have read about.

Would like to keep it simple.
I can purchase Trojan and Rolls FLA batteries locally.
Have a giant son that can lift them for me.
After reading the excellent Victron “Wiring Unlimited” document from the link in the FAQ’s, I can see that
cabling many 6v or 12v batteries to obtain 48v would be less than ideal.

The inverter can operate with a Battery bank range of 440 – 10000 Ah.
The Victron document suggests using 2vdc batteries in this situation.
I would need 24 of them in a simple series connect.

Looking forward to any and all advice and comments!
 
I can only see one question.

Yes. They are substantially more expensive than the commodity cells obtained directly from China. Note that you are only hearing about the problems. You're not hearing about the successes because that's how humans work. They complain way more often than they praise.

Concerning reduced capacity, personally, I'm cool with only getting 95-97% of rated capacity for 30-50% of the price for the same cells through authorized U.S. distributors.

You've provided only an inverter and the number of panels. There's very little to work with. What you need to evaluate is the amount of energy available and how much you plan to use daily.

"Florida" isn't very specific, so I can only make some rough assumptions about your available solar.

You have 1360W of panels. With a 48V battery system, you will charge:

1360W / 58.8V (typical peak voltage for Trojan/Rolls) = 23A

FLA/AGM/GEL batteries need to be charged within a certain current range for optimized life. That number is about 15% of rating, so your battery bank should be about:

23A / 0.15 = 154Ah.

10-20% charge rate is typically acceptable, so your battery capacity should be 115Ah to 230Ah

At 48V, that's 7.4kWh. With lead, you don't want to use more than 50% of your capacity, so your usable is 3.7kWh. That's enough to run a single residential fridge, a chest deep freezer and a little extra for something else.

Please re-read the Victron document. If you walked away from that with apprehensions about cabling batteries together, it's clearly addressed in that document. I have 8 Trojan T-1275 12V batteries wired for 48V with no issues.

When designing a system, it's important to establish how much energy is available (link #5 in my signature), and how much energy you need on a daily basis (link #1 in my signature). Once you have that nailed down, you can design an effective system.

A 6848 is a pretty beastly inverter. It is atypical for what one would expect with only 1360W of solar to feed the system. If you have very high power devices that will only be used for brief periods, that would make sense.
 
Thanks very much !
I have more reading to do, particularly on how to position the PV panels.
Disney World is a few miles South, I noticed they seem to have their PV panels pointed South-SW

I don’t really understand the “solar hours” calculations yet, will work on it.
I have room to add 4 additional panels, if needed.

I need to start & run a 1hp, 240VAC well pump motor. Off-grid.
The well pump runs intermittently.
Cycles only a few times a day, unless we are doing laundry/showering.

The pump is a Goulds 18GS10412C. 8.4 amps, running load.
Not sure what the starting amps is, probably at least 15A.
Location is central Florida.

Currently, when the utility fails, we run on a 21kw Perkins Diesel genset.
21kw runs everything on the property, if we manage the use of heavy loads
(heat pump, range, water heater, well pump)
This solar upgrade will run the well pump, instead of using the genset.
The genset will be reserved for times that solar is not available.
When the well pump is not running, the solar will run some lights & a refrigerator.

When the well pump runs, it will be about 2kw of load for 30-45 seconds,
plus the initial instantaneous motor start surge.
I’ll try to use that great spreadsheet or yours to verify numbers.
However, I plan to work the load “backwards”, for the lights.
ie: Turn off/disconnect everything, plug in just enough LED lighting to function.
Refrigerator load will be whatever numbers the Kill-A-Watt shows.

At that point, it’s just the well pump, refrigerator, some LED’s.

Thanks again!
 
The well pump completely explains it.

From this:


Looks like it has a locked rotor current of 41A. Your inverter needs to be able to handle that. I would expect it to.

A typical residential fridge will use roughly 2kWh/day.

I assume the pump is cycling to maintain pressure in a pressure tank?

Your solar and battery needs depend solely on how much energy your items use. I expect that you won't need more than the panels you already have, and the battery size I mentioned might suffice for pump/fridge/lights.

Due solar South is always best unless you have shading issues or you need to favor morning or evening charging.

Here's Orlando, due South, 38° Angle from horizontal:


1612900229899.png


Your worst month is Dec, so 4.16 * 1360 = 5.7kWh Enough for 2 hours of pump run time and a typical residential fridge.
 
A few things to READ first.


I started with Real Lead (Rolls Surette), abused & lived hard but maintained and still they "feel" it but work okay.
Decided to implement LFP as the cost to replace the Lead was just "Slightly lower" than going with LFP (LiFePO4).
Once my final shipment arrives I will have 1190AH/30kWh of LFP and 956AH/22kWh of Rolls Lead in the Backup bank.

The problems reported are not significant compared to the "No Problem" builds. The majority are User issues, making assumptions and conclusions based on those false assumptions. Maybe 10% of problem cases are a result of Poor Vendors. WE HAVE KNOWN GOOD Vendors and we know about sheisters too, it's all here.

I have been reading posts here and other places that lead me to conclude I should avoid
Lithium batteries for non-technical reasons.
Lack of quality control, vendor Ah capacity claims not delivered, BMS quality, etc.
^^^ Above = assumptions = conclusions. Don't fall into a trap with that.

Peruse the links in my signature, you will find more info there that will further help you and give you necessary into.
Do full research & understand the issues, benefits & trade-offs before spending another nickel on anything !
Impulse Buying will ALWAYS kick you in the butt ! (right where the wallet lives)
There are always Deals, there is no such thing as a once in a Lifetime, grab it now. If ANY vendor says something to that effect RUN don't walk.
Question & Verify everything, make no assumptions. Ask LOTS of questions - THE ONLY STUPID QUESTION IS THE ONE NOT ASKED ! Remember that kid in the back of the class who never asked anything because he was afraid that he'd look dumb and failed as a result...
 
Lead-acid can be viable. Operated according to specs, it could deliver the number of cycles documented to a particular depth.
Yes, it will last longer with 15% DoD than 60% DoD, but that won't necessarily give you many more amp-hours total cycles per dollar.
So you might choose to buy a smaller bank and wear it out faster (get your money's worth sooner.) Or, you might buy a bank capable of handling the occasional 3 days without sunshine, and it will last longer.

No reason to cycle the battery more than necessary by charging during the day and running a pump at night. Or a refrigeration unit. Make tanks full of water and freezers full of ice when the sun shines. Salt water ice can be used for a lower temperature phase-change material. H2O and NaCl are cheaper than either Pb or Li.

Lithium batteries as LiFePO4 are worth considering. They are generally safe. Other chemistries are known to burn; I saw that LG's high voltage RESU 10 packs were recalled last December.

I think Rolls has a reputation for particularly long life.

Still, batteries cost more than PV panels, so I think the goal should be to use as much as possible as power is generated, save as little as possible to draw from battery. A generator lets you get by with smaller battery. Could be just enough batteries for lights at night (and for motor starting surge), by way excessive PV panels so even on overcast days you have enough power. Panels are so cheap (as little as $0.12/W.)
 
This was very helpful in improving my understanding of the relationship between PV capacity and FLA battery bank Ah capacity:
“You have 1360W of panels. With a 48V battery system, you will charge:

1360W / 58.8V (typical peak voltage for Trojan/Rolls) = 23A

FLA/AGM/GEL batteries need to be charged within a certain current range for optimized life. That number is about 15% of rating, so your battery bank should be about:

23A / 0.15 = 154Ah.

10-20% charge rate is typically acceptable, so your battery capacity should be 115Ah to 230Ah

At 48V, that's 7.4kWh. With lead, you don't want to use more than 50% of your capacity, so your usable is 3.7kWh. That's enough to run a single residential fridge, a chest deep freezer and a little extra for something else.”

I have done more reading, and my antique, senile brain is beginning to emit smoke.
Seems as though I should also consider the LFP battery solution.
I read that LFP batteries can accept a faster charge rate than FLA.
If 10-20% of the Ah rating is the charge rate limit for FLA, I need to know the charge rate limit for a Battleborn BB10012.
Could only find this on the BB site “Can be charged up to 5 times faster than lead acid”.
So 23A / .8 = 29Ah ?????
Does this mean I can only charge 29Ah of LFP batteries w/ my existing PV capacity?
I don’t get it.
 
Almost all LFP can accept UP TO a 50% charge, BUT they don't care if you only charge them with 5%.

Best to assume system voltage when computing current, 1360W/48V = 28.3A, and the .8 factor only applies when you're looking at usable capacity vs. total cap.

So 28.3A/.05 = 567Ah and 28.3A/.5 = 57Ah.

Anything in the 57-567Ah range for a LFP bank is fine with your expected current.
 
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Have been reading and trying to understand the battery bank for my new system.
Feel confident about understanding batteries wired in Series.
Parallel-series is still making me a bit dizzy, please correct my drawing, if necessary.

Here is a list of what will be on pallets headed my way next month, so while I wait, can work on the battery plan.

Panasonic VBHN340SA17 340 Watt Mono Solar Panel - 40mm Black Frame
Schneider Electric Conext XW Pro 6848 6800 W 120/240 VAC 48 VDC Conext XW Pro Hybrid
Inverter/Charger
Schneider Electric RNW865101301 > Conext XW+ Mini Power Distribution Panel
Schneider Electric RNW865105001 > Conext SCP - System Control Panel for XW+ and SW
Inverter/ Chargers
Schneider Electric > Conext Gateway for XW and SW inverters
Schneider Electric > Conext Battery Monitor - RNW865108001
Schneider Electric Conext XW MPPT80-600 RNW8651032 > 80 Amp 12 - 600 Volt MPPT Charge
Controller
Schneider Electric RNW8651080 > 100A 125 VDC Single Pole PNL Breaker - 1-Pole
Square D HU361RB > 30 Amp 600 VDC Disconnect 3 Pole
PV Cable 44-0050 > 50 Foot MC4 Cable - #10 AWG
Midnite Solar DC Combiner, 3 Strings, Gray Aluminum, MNPV3
Midnite Solar MNFUSE15-600V > 15 Amp 600 VDC Fuse
Midnite Solar MNPV6 Fuse Busbar > Combiner Busbar for 4 fuse holders
Midnite Solar MNTS > Fuse Holder
4/0 Battery/Inverter Cable 5’ Black
4/0 Battery/Inverter Cable 5’ Red

BATTERY OPTIONS
Have looked @ a 48v forklift FLA battery, weighs as much as my car, but very durable. No way to handle that weight.
I can barely lift anything over 50 lb w/o a visit to and scolding from the orthopedic surgeon!

So at this point it’s going to be 6v FLA or 12v LiFePO4.
I can pick up the Trojan or Rolls locally (no shipping $$, just Diesel fuel and tolls)

CHARGING
Not sure the PV I have ordered can charge either of the FLA options.
I have 1360w of pv panels, am located in the part of Florida that the GHSI map shows 5.00-5.25, thus 5440 watt-hours per day.
I have room to add another 1360W of PV panels, so there could be 2720w available. Is this a good idea, or unnecessary?
I understand the 10-20% charge rate limit I have to use to charge the FLA batteries.
However, recently discovered (I actually like to read the manuals) the Inverter/Charger will charge the batteries from the grid.
So possibly there is a way to keep the batteries charged while the grid is available, even if there is not enough energy coming from the PV alone? Is this logical?
series_parallel_or_not.png
 
Possibly a simple decision:

If this is going to be cycled only when you have power outages, FLA all the way. They love to sit fully charged and ready for action more than they like cycling. LFP would need to be stored at 50% SoC waiting for a power outage, or they will degrade sitting at 100%.

If this is going to be cycled daily, LFP is a good candidate. I don't know that BB would endorse a bank of that size.

I do not see a KI6-HC on Rolls' website. I see a S6 L16-HC, which is one of their new lead carbon batteries with improved life. If those are the lead-carbon batteries for only $2800, that's the way to go IMHO. I have some old S-605 in a 2S2P (12V) configuration. I abuse them horribly, and they just keep going.
 
Or AGM, if you just want to install and forget (assuming electronics manages it correctly.)
What I have is 8x SunXtender 6V 405 Ah $5000.
Don't have to limit to 50% DoD; it is a trade-off vs. cycles. My system does load-shed at 70% DoD.
Lower cycle life than the quality FLA.

For mine, 0.2C is minimum desired charge rate, otherwise extended absorption cycle.
Max charge rate suggested is far higher, but no trade-off vs. cycles is quoted.
 
Rolls Surrette S6 LI6-HC 445Ah, I had a typed it incorrectly......
OK, off to Miami I go!
 
@Hedges makes a good point. AGM is more expensive, but it's maintenance free. You'll need to check your 24 cells for electrolyte level at least every month or two depending on frequency of use and conditions.
 
My neighbor is the same way. Pretty sure he's busier now that he's retired.

Dad just retired at 75. He now spends more time working on classic motorcycles than he did as a physical therapist.

I think that's the key to longevity. Stay busy.
 
I’m 71, retired a couple years ago.
Wife has to work 26 more months to be eligible to retire.
So I keep busy. Like to try new things.
Sold off all the classic hobby cars & motorcycles.
Still active in HAM radio (KJ4OLL).
Enjoy electronics projects, this one will not only be fun, but have actual, tangible benefits.
After she retires, hope to take the RV to BC and the NW territories, I have heard great things about the beauty and wildlife there.
 
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