EVE LF280K cells initial impression.

[Dacian Todea]

Just got 16 cells I ordered from https://energiepanda.com/

I wanted to order the 230Ah but they were not in stock and Tim the owner offered me a discount so I was able to get the 280Ah cells at the same price but this will not influence my initial impression or subsequent reviews.

The prices are I think fair as they are as everything is included in that price shipping and taxes https://energiepanda.com/product/4pcs-eve-3-2v-280ah-deep-cycle-lifepo4-battery-cell-lf280k/

The Cells were shipped on May 23 and arrived on July 5 at the closest UPS store to my location where I picked them two days ago.

Here are the photo of the 4 boxes each containing 4 cells

Included with each cell is a flexible busbar and M6 x 10mm screws that are a bit short but Tim mentioned that his will source 12mm long screws that are a better fit much better

As now the busbars are 4mm the two washers are 3mm and so only 3mm of the screw will be in the thread while 6mm are available and the 12mm will be able to take 5mm out of the existing 6mm to leave some margin.

Of Course since I will have groups of two parallel cells I will need to get myself some custom size bolts anyway.

 Other than just taking all cells out of the box for visual inspection and testing a random cell for internal resistance I did not do any tests so it is just an initial impression.

In therms of cosmetics they looked great (I was assured that I got no special treatment) and the largest dent by far is this circular one on this cell in below photo. It looks like a cylindrical object maybe a bolt pushed on that side and it is even marked with two black lines and considered acceptable witch I will agree with. 

 

Below is the test I decided to test

My impedance tester is not good enough accuracy to measure such low value impedance and showed 0.3mOhm at 1kHz but it can be anything with the large tolerance. I do expect it will be around 0.2mOhm as the DC resistance that I could measure way more accurate was 0.4mOhm and that is usually at least about 2x the impedance at 1kHz

 So cells are more than excellent in this regards for solar energy storage.

They were excellently packed as there was no damage due to shipping. Each box is over 20kg as each cell is 5.4kg

The welded terminals are also excellent and together with the busbar make for insignificantly added resistance. The DC resistance when I made the measurement was 0.4mOhm at the terminals (the base where they are welded) and it was the same (not able to distinguish the rest) in the middle of the busbars (I had a busbar installed on each terminal) and measured voltage drop with a 10A current and so 0.004V the resolution of my multimeter.

I will be busy with some other things but before this winter I will have the cells installed 8S2P so a 14kWh pack and connected to a dual PV array made of 36x 255W panels.

And to this I hope to add also a 450liter stainless steel barrel as a 36kWh thermal storage. The 3.5kW inverter will also be installed on this same system and I will leave my current 4.5kWh A123 System battery in place to continue monitoring degradation past 5 years. Will do in a month or two a capacity degradation test after 5 years of full tyme use of that 4.5kWh A123 System battery.

In conclusion I'm very happy with the order and can not wait to have it put to good use.

[Dacian Todea]

Forgot to mention that based on date code all this cells where produced in December 2021 just random different days but all in the same months and as fresh as possible.

[Richard Bewza]

Bolts versus Studs for 280 amp hour battery cells.  

Dacian, it is much better to use studs for your batteries. I learned this lesson the hard way. Using bolts will either strip out with 1 bad mistake or fatigue over time and end up stripping anyway. Use 6 mm x 25 mm x 1 mm pitch studs with a flat bottom and an allen key receiver at the top.

Coat the bottom 6 mm of the studs with red threadlock and screw them in using the Allen key. Make sure not to tighten them much more than finger tight and allow 24 hours for the threadlock to fully cure. Bye anchoring the studs in with permanent red threadlock all future movement will be stainless steel nuts on stainless steel studs.

It is also important not to apply more than 4 or 4.5 newton-meters of torque. Make sure to clean the terminals around the studs, remove any burrs, and add antioxiding paste to all connections.

Please find attached some AliExpress suppliers and the appropriate materials to do this job correctly from the get-go.  I got the antioxidizing paste from Home Depot for around $6. 

Richard 

[Dave Festing]

Hi Richard,

Thank you the detailed procedure.  What you think about using Bellville washers to maintain the correct tension?

Cheers,

Dave

[Dacian Todea]

Richard,

Not a fan of studs and also not a fan of glue.  I will buy proper sizes bolts so that about 5mm of the tread is engages and I have a torque wrench that I will set at 4Nm

These are not drilled cell terminals there are proper welded aluminium nuts.  Likely some of those drilled terminals in the past where improperly taped or messed up when tapped or people used screws that where to long or overtightened studs maybe by not leaving 1mm or so at the bottom or not using the hex key when tightening the nuts.

Since this welded nuts are taped before being welded to cells any rejects where already removed so all are properly taped and can easily handle 4Nm if you engage at least 4 to 5mm of the thread. They will also not be damaged over time and you need some sort of quality spring washer like the included screws.

This included screws are just not the proper length but the seller mentioned that he will remedy that. He used to deliver the cells with thinner busbars  likely 2mm and this are 4mm thick where they connect and they have about a 50mm^2 cross section witch is plenty even for high current applications.

The top that makes contact seems to be decently flat to not need any manual intervention and I will be using a small amount of vaseline to prevent possible corrosion.

Likely I will be searching for galvanized steel or zinc plated steel bolts as that will be closer than stainless steel and my batteries will leave indoor in a controlled environment with low humidity and non corrosive.

For outdoor or close to Ocean maybe stainless steel will be better tho not sure.

[B0bD]

fwiw, this is a link to how I connected my similar cells, and the math behind the decisions.

https://diysolarforum.com/threads/3p8s-840-ah-build.24602/#post-297016

Bob

[Dacian Todea]

Bob,  Thanks for the link.

Initially I read from the post where the link opened but the info about connections was above that.

There is some level of thermal stress and mechanical stress due to changes in dimensions based on SOC. Thus the reason I use flexible connections.

I could use aluminium bolts but a zinc plated steel one is close enough so that there will be no corrosion. I have no decided on the type of washers but I tested some similar washers not to long ago and their pressure is not consistent over time but those where not genuine washers so maybe material they were made out of was not great.

Cell compression is actually fairly essential if you want long life out of the cells and the charged/discharge rate has little if anything to do with that.  Just look at you cells thickness when fully charged vs when say below 50% SOC

This cells are almost like puch cells as the case is super thin aluminium thus not much to keep the cell from bulging when fully charged. 

[Vojko Rajh]

Hi.

You mentioned cell compression. 

What is the proper way of compression? Do you also put isolation between cells? When to tighten the bolts regarding SOC? 

Thanks, Vojko

[Dacian Todea]

Vojko,

There is that blue plastic wrap but it may have imperfections or it may get cut while compressing so it is a good idea to add some electric insulation material like silicone (soft) between the cells

The case is connected I think to positive terminal.

The compression needs to have elasticity so no matter the cell dimensions (that do not charge much while under compression) the pressure is about the same or at least there is some pressure.

So is the simplest to compress cells when they are at lower SOC say below 50% that way you tighten less say the springs  or some spongy material between cells (that is usually used in automotive batteries) less and leave some travel so there is still compresion when they are at high SOC.

Below is a photo of my A123 system battery with the red compression springs. Will do something similar for group of 4 cells with the new battery but with just 6 bars, 3 on each side no need for the other two at the bottom. 

 

[Vojko Rajh]

Thanks!

BR,, Vojko

[B0bD]

Let me know if the details of the bolts, nuts, spring washers and methodology are worth posting.

The solution was engineered, not trial and errored.

[Marinepower]

I have the EVE 280 cells and have been using them for 2 years.  

So far they have been very good all around and balance has been good.  I have never done a capacity test, so cant comment on that.

In a stationary application, they seem ideal.  More of a challenge in a mobile application, as they are delicate.  I do not have the welded terminals you have, and I really wish I did.  That is much needed upgrade ( as the terminals are weak without this)  and was not available to the market when I bought my cells.

I tried a couple of busbar solutions (solid tin copper, CALB flexible).    I found these to be the best so far.  They are made by a company called Weber and there seems to be lots of sellers on Aliexpress.    See example here:  

https://www.aliexpress.com/item/1005003478098106.html?spm=a2g0o.store_pc_groupList.8148356.34.e2c6748aThx5WX&pdp_npi=2%40dis%21CAD%21C%24%206.64%21C%24%206.50%21%21%21%21%21%402100bdd516583489633227915ef991%2112000028506276634%21sh

[Dacian Todea]

Yes those busbars look great but the included ones are also excellent and super flexible. It is just braid tinned copper.

I will use the included one to parallel the cells (8s2p) and will use tinned aluminium lugs and flexible pieces of silicone insulated copper wires to connect the cells in series.

My requirements are just about 120A peak both for charge and discharge so just 60A per cell.

[Marinepower]

Great, well looking forward to seeing your progress as you test and build out the cells. 

[Philippe Damour]

BobD

Hello 

I would recommend black burnished steel stud. STHC 6mm x 20mm, hex 3mm socket and flat end.

-Steel because stainless steel has a electrical conductivity.

- Stud to use the full length of the machined threads in the terminal and such avoiding to damage those.

- No thread blocking glue because that is an insulator.

- Black burnishing for its moderate corrosion resistance, low friction thanks to the smoothing effect and good electrical conductivity.

- I like also the Belleville washer to provide an homogeneous axial force. Belleville washer comes also burnished.

- One drawback: it is recommended to put an electrical insulation protection system on top of the battery because studs stay higher above celles than screws.

To be found by many distributors of bolting, or ebay/Amazon   

Regards 

Philippe 

[B0bD]

Not sure why steel.

When I built my battery a year ago, one aim was to make the connections as reliable as possible

After a bunch of research:

-avoid dissimilar metals - Plain steel forms a cell with aluminum and is prone to corrosion. 

                                              Zinc plating is close to aluminum, the thickness of zinc on commercial bolts is very thin. 

                                              Hot dipped galvanized is much better but very hard to find in M6-1.0

                                              Aluminum bolts are available and not terribly expensive - I went with these:  https://www.ebay.ca/itm/184700470042  60 bolts cost about $60; same seller also sells nuts

                                               There are a lot of sellers offering 6061 - but 7075 has higher strength.
-contact pressure in the connection when dealing with aluminum needs as high as 30 N/mm^2 for minimal resistance. Having anything above about 7.5 N/mm^2 provides a significant benefit. The contact pressure                                                     needs to be maintained through mechanical and thermal cycles.

                                              my battery terminals are 18mm diameter with a 6mm hole; the force needed to get the recommended pressure is 1500N, or 335 pounds.

  - belleville spring washers to develop and maintain the contact force. Rather than installing a belleville washer and tightening it flat, I selected a spring washer that in a stack of three provides the target force when                                                    deflected by 0.5mm, conveniently a half turn on the threads.

                                              since the spring is not fully compressed, it can accommodate any small movements in the joint without losing pressure

                                              this method also protects the terminal thread by only applying the force necessary

                                              These are the washers I used

                                              :BI-146412, from International Springs at http://www.internationalsprings.ca/en/downloads/BI-Series-Belleville-Washers.pdf.   $36 for 200, plus tax and shipping

-bolting - ideally the bolts would be heat treated but the non heat treated 7075 bolts are adequately strong ( by calculation and by testing)

                                             rather than try to get the exact length of bolt, I bought slightly longer bolts,, and installed a nut above the spring stack.  The bolt is hand threaded the full depth of the terminal, then the head of                                               the bolt if held while the nut is turned to compress the springs.

 -the terminal surfaces need to be cleaned of any oxide prior to assembly. Aluminum oxidizes nearly instantaneously; the references recommended using a product like NoAlOx ( which contains particles that pierce                                                   the oxide layer) and an abrasive ( I used scotchbrite) to thoroughly clean the surfaces. Do it once, wipe the junk off, then do it again and leave the NoAlOx in place while assembling the joint.                                                  The NoAlOx will squeeze out, forming a seal around the periphery of the joint.

The bolt and nut need to be lubricated; I coated everything with NoAlOx.  If the fasteners are dry there is a risk of galling the aluminum.

 

The picture shows the spring washer orientation.

In the year since I installed the battery, there has been no change in the resistance of the terminal ( using a xxxx resistance tester) nor has there been any heating of the terminals, loading the battery to 0.2C.

My battery is a 3P8S setup using 280 AH cells, so a total of 48 connections.

There are as many ways of making connections as there are battery builders, but I didn't encounter many posts that were other than opinions rather than based on some research and data.

Hope you find this interesting and maybe even useful.

Bob

[Philippe Damour]

Hello Bob

Thanks for your detailled explanations.

Using long high strength aluminum 7075 screw, as you did, is a very good solution, equivalent mecanically to a stud. 

When I recommended steel, it was in comparison to stainless steel,  very often delivered with cells by chinese suppliers and also mentioned by Dacian.!! 

But stainless steel electric conductivity is poor, 1.6 compared to carbon steel 5.9 ×10^6 Siemens/meter

High strength aluminium alloy are much better, in the range of 12 to 18 ×10^6 Siemens/meter, but the aluminium anodisation is like a ceramic layer with very poor electrical conductivity, making the 7075 screw as poor as a stainless steel screw in term of electrical conductivity.!!

Consequently, using carbon steel, is beneficial, because current can flow, not only beteen upper surface of terminal and lower face of the bus bar, but also through the stud, nut and washer to the upper side of the bus bar.

Regards

Philippe 

[Dacian Todea]

Philippe,

The electrical conductivity of the screw is irrelevant. The only contact is the one between the busbar and terminal. You will normally use washers and those are usually made from some sort of  flexible steel or stainless steel.

I will be using zinc plated steel as zinc is very close to aluminium galvanically (way closer than stainless steel).

I ordered this exact ones and they already arrived but I need much more parts before I install the battery  https://canadabolts.ca/collections/metric-flanged-bolts-serrated-yellow-zinc/products/flanged-bolts-serrated-yellow-zinc-m6x16mm?variant=9812695253039

They are quite expensive but I also got this to compare and they cost quite a bit less and are much more commonly found at local stores https://canadabolts.ca/collections/metric-hex-bolts-full-thread/products/hex-bolt-full-thread-zinc-plated-m6x16mm?variant=7487207833647

I also got zinc plated lock washers from the same store and also flat washers to adjust to the height of the bolt depending on what else I need to connect there.

Bob's installation is great and the only thing I'm not a fan of is the thick aluminium busbar that has zero flexibility. Copper is much softer and if tin plated it will make a great contact plus the flexibility of the busbar allows for both vibrations and thermal cycle movement plus the change is size of the cells with SOC even when properly compressed.

[B0bD]

Dacian's point about the conductivity of the bolt is spot on. The area of the terminal is 226 mm^2 ( 18mm diameter with a 6mm hole); the area of the bolt is 28mm^2, or 12%.

Have to balance that against the potential for galvanic corrosion, and in any case the terminal thread will be oxidized and will reduce that effect.

The info I provided applies to any bus bar connection.  You mentioned a light coating of vaseline - i'd strongly recommend a thorugh coating of NoAlOx or similar, including into the terminal threads, to break the oxide coating and to seal against moisture.

However, regarding my choice of solid bus bars, after reviewing the documents provided by my battery manufacturer, I decided to forego the benefit of compression in favour of making sure the connections are stress free.  My battery cells are spaced about 3mm apart.  From data provided by the cell manufacturer and my usage pattern, the estimated non-compressed life is 32 years, so I made the decision to not bother with compression.  Not a decision everyone would or should make but thought I'd provide the rationale. 

[Dacian Todea]

No battery will last 32 years at least none of the ones I know about.

The most significant degradation is not due to cycling but calendar aging witch is around 1% per year depending on brand and use conditions mostly ambient temperature (a 10C increase in ambient temperature will double the rate of calendar aging).

So I will expect any LiFePO4 to be good for 15 to max 20 years depending mostly on that calendar aging and if you are OK with 80% or 70% of original capacity by the end of life.

I will measure the A123 System battery capacity degradation this year (will be 5 years since install) but is a bit more than 1% average degradation per year since now and is in ideal temperature condition 18C to 26C as it is used indoors and it is properly compressed (spec for compressed cells 6000 cycles at 100% DOD). The cells are almost never below 40% SOC and most days are not below 60%

I do not know exactly the role of compression for calendar aging but I suspect it will not be great.

They almost never test for calendar aging as they do fast test usually no more than a few months when they do thousands of cycles so calendar aging plays almost no role.  But in real applications calendar aging is responsible for majority of the degradation.

Maybe you do some capacity tests over time and let us know how things progress.

[B0bD]

Exactly - my battery will age out long before it cycles out.  

Daily the battery discharges to 75-80%, assuming a reasonable amount of sun.  I am heavily overpanelled, and generally reach full charge daily, unless it is raining.  

This fairly gentle cycling seems to minimize the cell's "breathing" - there is little if any change in cell dimensions.

I definitely will monitor for any changes in capacity but I suspect we will all be much older before there is a noticeable change.

BTW, the combination of 4 DSSR20's and my existing Midnite charge controller is working well.