Solar panel setup questions

Barkuti · 2019-10-21T01:25:10.000Z

Well, for now I can say this thread gained a lot of momentum. It's been a busy O:) weekend.

The_Fat_Controller:

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For example my victron is rated for 30amps , if i run a 12v battery bank my PV max will be 440Watts , but if i run in 24v @ 30 amps i can have upto 890w of PV and so on

…

Guess that arithmetic is maximum charge voltage times maximum current, yet these are maximums and you don't want to be pressing the pedal to the metal frequently, or do you? Long term reliability matters.

The_Fat_Controller:

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the MPT-7210A is 10amps max input or output ,so you'd need to go 48v (13S) bank and run your solar panels in series to give 36v that way you never go above 10amp on the charge controller.
…

Well, I discern the MPT-7210A is then not really well suited to handle 2 × 100W “18V” panels in parallel, as they can give out in excess of 5.5A each.

Given this and considering I may go with up to 4 × 100W panels, the better affordable solution as I see it is to choose 2x (Sunyima) CTK-EV-300 charge controllers, each one fed by 2 × 100W panels.

By the way, looking into this I found the following page with specifications for quite a lot of the devices discussed here: Product list @ Nanjing Longline Electronic Technical Co., Ltd.

I really like the DPS3806 boost-buck converter, affordable and flexible device. Found a video of it made by the same Julian Ilett fellow which talked about the MPT-7210A above:

SEPIC topology maybe? They're easily parallelizable.

Barkuti · 2019-10-21T04:01:28.000Z

The_Fat_Controller · 2019-10-21T11:30:50.000Z

[quote=Barkuti]

Guess that arithmetic is maximum charge voltage times maximum current, yet these are maximums and you don’t want to be pressing the pedal to the metal frequently, or do you? Long term reliability matters.

Well… in reality it is very very rare that you will ever see the maximum from a solar panel unless as someone else said it is Freezing cold and you have very very strong sun , the ratings they give for Pmax assume absolute perfect conditions, Most sensible controllers are also current limited so they will simple draw up to their max rating rather than self destruct

[quote=Barkuti]

Well, I discern the MPT-7210A is then not really well suited to handle 2 × 100W “18V” panels in parallel, as they can give out in excess of 5.5A each.

correct it would be 2 panels in Series… Parallel would be to much current also no head room for later upgrade , it is always better to get a larger controller if you can

There are some very nice Boost buck up & down converters around you dont need to spend alot of money to get the power out of the panels, the issue is as i said earlier going to be safely charging lithium batteries

Im not sure of the requirements for charging Lithium cells … for my Tubular Gel batteries they require a current limited 3 stage charge , Bulk Absorption & float and each voltage has to be temperature compensated (if you cause a gel battery to GAS) it is ruined and capacity lost so they must never be equalised

Im no expert on lithium but id imagine at the very least they will need Balancing, current limiting & over voltage protection , I think i also read somewhere they must not be charged if temperature below 0

I would have preferred lithium but cost was an issue and price difference big 80ah at 12v Lifepo4 , VS 110ah at 12v Tubular GEL …

Leoch GTP

Lifepo4

Barkuti · 2019-10-23T02:48:57.000Z

The_Fat_Controller:

…

I would have preferred lithium but cost was an issue and price difference big 80ah at 12v Lifepo4 , VS 110ah at 12v Tubular GEL …

Leoch GTP

Lifepo4

Lead acid batteries cannot offer anywhere close to their 20 hour ratings if discharged fast, much less the above 100 hour rating albeit this may be inconsequential due to battery size versus average power consumption. I also wonder about their actual lifecycle performance, but looks like they're still the most affordable technology.

I've already built a few small li-ion batteries with modern 18650 cells (LCO and NMC chemistries), going as far as binning and grouping cells to maximize series pack balance and lowering maximum charge voltage to boost cycle life too. No issues with them, guess it is too early to say anyway.

If you are willing to take a chance with some cells from china the price for li-ion can be lower, although it requires battery pack assembly skills. Take a look at my top choices at this moment:

https://www.aliexpress.com/item/4000197218968.html

This is a cell from a still unknown to me OEM, and rewrapped by some. Here's a review of it: Queen Battery QB26800 6800mAh 20A - the big guy! @ Thunderheart Reviews

By charging that cell to slightly above 3.9V still above 20Wh per can can be extracted at a low C-rate, which is pretty interesting.

https://www.aliexpress.com/item/32795814865.html

As far as I understand this other cell is from Heter, it is used by some Ukrainian fellows in their DIY e-bikes and unorthodox tests. Tester Henrik will soon publish a review (lygte-info.dk).

A problem I face with cell balancing is that commercial balance boards are designed to do it at very high voltage, above maximum permitted voltage. This is not ideal, in my honest opinion. It probably is better to install switchable voltmeters at each battery stage together with pushbutton enabled bleeding resistors (inexpensive flashlights with linear drivers would probably be a nice idea :-D for such role), and do balancing by hand from time to time.

By the way I've noticed that DC solar cable can be somewhat pricey. I wonder if some beefy silicone wire could also do good…

The_Fat_Controller · 2019-10-23T14:09:07.000Z

Lead acid batteries are indeed wasteful both in discharge due to Peukert Law but also during charging much energy is wasted for example to recharge 1kw/h used will generally take 1.5kw/h there is another problem as battery state of charge increase rate of charge decrease so they only really fully use the panels when they are at 50% dod this is why my system is Hybrid & sends the surplus power to the inverter when batteries are no longer pulling full power from the panels it is also important that the batteries are always fully recharged to avoid Sulfide buildup and capacity loss. (memory effect)

Life cycle wise , Floaded & AGM have very short life 300 cycles maybe… But Deep cycle GEL has long establish high service life many many cycles if Depth of discharge is kept to 50%

I totally agree Lead acid is not the best technology for this application but the cost of large lithium battery bank and all the additional technology needed to charge it safely is still very high

I generally keep my discharge rate to 20H giving a theoretical 45AH per battery assuming a 50% DOD totalled up this gives fully consumable 90AH at 26v (2340 W/h) note that 24v would be fully discharged….

When i built system a few years back the cost of good (3ah high drain) 18650 was 10 bucks each! and i calculated i then needed 220 of them to make up the same 2300 watt hours and to make matter worse cycle life was only 200-500 depending on discharge rate!

Where as the manufacturer of my Gel batteries quote 2000 cycles with my above setup and gave me a 6 years guarantee (I will be counting)

But now you can buy 5ah 26650 from Liitokala for just 3 bucks… Perhaps tables have now turned it really depends on the cycle life

Those Liitokala 3.7V 26800 7200mAh you posted look very good, I wonder if anyone has tested them properly?? if that cycle life & capacity is true then it is certainly a better option and may even work out cheaper than lead acid! I may have to order a few for testing

The_Fat_Controller · 2019-10-26T21:25:28.000Z

Ordered a few of those Liitokala 3.7V 26800 7200mAh you posted will let you know if they are any good… and if they are i may have to make the switch from Gel to Lion

id still need 98 of them assuming A 7AH capacity to match my current setup so its a not without costs

Barkuti · 2019-10-27T01:16:29.000Z

The above 26800 cells are made by a good chinese OEM this is nearly for sure as far as I know (it may as well be Heter, but God knows).

In order to get high cycle life out of li-ion certain life-prolonging practices are due. Check BU-808: How to Prolong Lithium-based Batteries @ Battery University. The article is somewhat old and, while still valid, it may not be required to restrict the maximum charge voltage as much. I say this because I get excellent cycle life out of my smartphone batteries, charging only up to ⅔ of max or ≈4V with minimum load according to 3C Battery Monitor Widget (yes, I give 'em thousands of cycles), and also from what I can fathom out of documents like this Samsung INR18650-35E datasheet from Orbtronic (check page 14/19 table, Electric Storage Systems/Uninterruptible Power Supplies). Could select 3.95/4.2V just to be safe, still 20+Wh out of each of the above 26800 cells.

Yes, you will have to solder cells for a while.

snakebite · 2019-10-28T16:22:05.000Z

whatever you calculate for panels just go ahead and double it.
or triple.
i have solar setup here and its not uncommon to get long stretches of low production.
upsizing the battery has its own set of issues one of which is cost.
i overpanel and find something to do with any overproduction.
like heat water.

volchyonok · 2019-11-14T16:41:26.000Z

I am also interested in these 26800. In this store, QB26800 can be bought much cheaper than asking for Liitokala 3.7 V 26800 7200mAh. However, QB promises 6800mAh against Liitokala, which promises 7200mAh. In appearance of the “positive” output, these are elements from one manufacturer and I think that Liitokala is simply lying about such a large capacity, because the QB26800 shows less results (when discharging a current of 10A to a cut-off voltage of 3 volts, the best batch reached 7000mAh, and The same ‘the big guy! @ Thunderheart Reviews’ with a similar discharge current of 10A received only 6568mAh). Where can there be 7200mAh from? Is this another fantasy from a famous liar named Liitokala?

QB26800 (price)

The_Fat_Controller · 2019-11-14T16:50:32.000Z

Well i ordered 3 as a sample, I cant do accurate test but i will certainly post what i get from my charger

(When they eventually arrive)
I think they are being shipped by sea turtle

volchyonok · 2019-11-14T18:03:23.000Z

The_Fat_Controller:

Well i ordered 3 as a sample, I cant do accurate test but i will certainly post what i get from my charger

It will be interesting if you do not have a simple household charger (with very low currents), but there is such a high-precision four-wire electronic load (discharge current up to 10A), with which, for example, the QB26800 test was performed on this forum QB 26800, 6800mah, 30A - Страница 3

When discharged with a current of 10A and before the cut-off 3v, this person received a maximum of 6900-7000mAh, but the results for other people were somewhat more modest, since his party with very fresh elements was the most successful QB 26800, 6800mah, 30A - Страница 4

Such a discharge device can also be purchased on ali-express 30.09€ 27% OFF|150W Konstante Strom Elektronische Last 60V 10A Batterie Tester Entladung Kapazität Tester meter 12V 24V 48V|discharge capacity|battery testerconstant current electronic load - AliExpress

Barkuti · 2019-11-14T22:15:48.000Z

volchyonok, be sure both QB26800 batteries (datasheet) and LiitoKala 7000/7200 (cough!) are manufactured by the same original equipment manufacturer. The OEM sells his cells to whoever can buy the MOQ (Minimum Order Quantity) which can be a few thousand pieces, but other than this it is a relatively simple thing to do for a business or society (like Vapcell or LiitoKala whoever it may be).

The effective capaciy for the 26800 cells may be a little bit higher when using BMS systems with lower than 3V cutoff; this is common and I really doubt it would in practice be really detrimental for the batteries.

P.S.: to be accurate any testing device needs some sort of calibration. I, for example, took the time to properly average all my Lii-500 channels and obtain channel correction indexes. In my experience Lii-500s tend to very slightly underestimate capacity, at least mine does.

volchyonok · 2019-11-15T07:59:04.000Z

Barkuti:

I, for example, took the time to properly average all my Lii-500 channels and obtain channel correction indexes. In my experience Lii-500s tend to very slightly underestimate capacity, at least mine does.

This is not a matter of calibration, but of the strength of the discharge currents, which are mandatory as an orientation for choosing the suitable elements, which then will have to work under high loads in the battery for electric vehicles. Given the estimated number of parallel cans in each cell (3P …. 5P, etc.), one should bear in mind the ability of each element to hold a load in the region of 10 amperes and know exactly what capacity it can give to the ‘common boiler’ (“Joint boiler”). Your household charger, even if it is treated with high-grade pure gold, does not work with discharge currents greater than 1 ampere, so its informational value in the case of selecting batteries for a powerful battery has no value. A discharge device is required with the ability to load an element with a current of at least 10 amperes, even if its main load is a current in the region of 5A.

The_Fat_Controller · 2019-11-15T11:06:05.000Z

The cells for a large solar battery bank do not have to be identical, they do however have to be protected & fused individually , it is inevitable that in a pack containing a few hundred cells some will fail over time and degrade at different rates so redundancy and protection has to be built in to accommodate it

This is why i ended up going with Tubular Gel, The costs involved even if you build your own Lion bank by the time you have factored in all of the extra bits you need to make it safe , the cost of the cells BMS fuses & protection boards etc you end up spending just as much money and life time of the pack is really no better, The real advantage for lithium in a solar system is that it accepts high charge rates , there is no tail off like there is with lead acid

volchyonok · 2019-11-15T15:22:41.000Z

For an additional guideline, whether the QB26800-6800 and LiitoKala-7200 are the same product, it is necessary to clarify their actual dimensions (diameter and length of the case), which should be measured with a fairly accurate meter. Yes, they look almost exactly the same “positive” contact, but this is just iron, which can also be ordered from a third manufacturer, which produces only component shells, but not the inside. If there is a difference in size, then QB and LiitoKala can easily be not the same OEM product, so their characteristics can also have a significant difference.

And here for some reason they go not like 7200 (mAh), but already as 7000 (mAh)
https://aliexpress.com/item/33048536823.html?spm=a2g0o.detail.1000014.7.34496ab7vrse35&gps-id=pcDetailBottomMoreOtherSeller&scm=1007.13338.146400.0&scm_id=1007.13338.146400.0&scm-url=1007.13338.146400.0&pvid=e560c318-4cb8-4c6d-b86b-76289663f3cc

The_Fat_Controller · 2019-11-15T18:57:30.000Z

volchyonok:

Page Not Found - Aliexpress.com

4. Actual capacity : 5500mah I will be most dis chuffed if the ones i ordered only make 5.5ah

cells

Barkuti · 2019-11-15T20:58:10.000Z

volchyonok:

…

This is not a matter of calibration, but of the strength of the discharge currents, which are mandatory as an orientation for choosing the suitable elements, which then will have to work under high loads in the battery for electric vehicles. …

I can perfectly understand what you mean, don't worry. My Lii-500 serves me well to obtain accurate capacity measurements at low discharge rates. With regards to power delivery or discharge rate estimates, I have what I need to obtain accurate DC internal resistance measurements with a hack of my own. With this data I can more or less easily estimate maximum discharge rates.

Discharge curves are an excellent asset which standard discharge testers do not support. Here are a couple devices from ZKE Tech which can do discharge curves with PC software:

https://www.aliexpress.com/item/32856429623.html EBD-M05, a small little marvel for up to 5A discharge which recently someone recommended.
https://www.aliexpress.com/item/32806617908.html EBD-A20H, a more professional device for up to 20A discharge rate.

Just taking a quick look at some standard discharge curves, 1A and 5A for example, lets me discern battery internal resistance and expected maximum discharge rate.

The_Fat_Controller:

… it is inevitable that in a pack containing a few hundred cells some will fail over time and degrade at different rates so redundancy and protection has to be built in to accommodate it

…

I can't completely agree with that, I think cell life can be largely extended and mishaps avoided.

In any case, affordable 1S BMSes can be obtained which, for a very large pack, could be installed for each pair or trio of cells:

https://www.aliexpress.com/item/33025760150.html I see a hundred pieces of these 6x 8205 BMSes for ≈€43.99 shipping included.

volchyonok:

For an additional guideline, whether the QB26800-6800 and LiitoKala-7200 are the same product, it is necessary to clarify their actual dimensions (diameter and length of the case), which should be measured with a fairly accurate meter. Yes, they look almost exactly the same “positive” contact, but this is just iron, which can also be ordered from a third manufacturer, which produces only component shells, but …

…

And here for some reason they go not like 7200 (mAh), but already as 7000 (mAh) :FACEPALM:
https://aliexpress.com/item/33048536823.html

Battery factories are not hot dog stands. They cost large sums of money and resources which only large corporations can afford, namely if they're geared for large volume scales.

The 26800 cell is a new entry in this market, and I really doubt there is another OEM manufacturing such cells right now, i.e. all 26800s with the “QB26800” triple leg/lobe top pole shape come from same brand factories.

Source: https://www.thunderheartreviews.com/2019/01/queen-battery-qb26800-6800mah-20a-big.html

Fri, 11/15/2019 - 20:26

volchyonok · 2019-11-15T20:36:58.000Z

Barkuti:

all 26800s come from same brand factories.

There are different ones, and not only 26800 with another ‘plus’ output, but even 26700 26700-26800 — Yandex Disk

Barkuti · 2019-11-15T20:58:05.000Z

Mmmkay volchyonok, you convinced me to correct myself. Edited my above post.

Barkuti · 2019-11-15T21:12:27.000Z

The_Fat_Controller:

…

4. Actual capacity : 5500mah :| I will be most dis chuffed if the ones i ordered only make 5.5ah

cells

Your cells are expected to clock around 7000mAh at a low discharge rate. In the advertisement from the shop where you bought them with tabs, 7200mAh can be read as Nominal Capacity in Specifications:

I was told somewhere that is what matters, so test your bunch and file a dispute if they fail to meet ≈7Ah with very high consistency.