Experience purchasing cells from Shenzhen Basen Technology Co. Ltd.

Gizmo740

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In a hurry...skip to spoiler...last paragraph and pictures.

Been lurking in the shadows and dark corners of this forum for months now...I have to say the information and general knowledge, not to mention helpfulness, of you members is priceless. Would never have attempted this project without knowledge gained by reading this forum. With that said...wish me no fires!!! 🤷🏻‍♂️

I needed to replace an aging and tired "golf cart" storage bank in my garage and wanted something "safer" to be in human spaces. Been testing solar for several years now while building a conex container cabin for some very remote property I own in the Ozark foothills. Cabin is completely off grid so I want to know what my limitations are while on-grid in civilization. Reliable high capacity energy storage is the last piece of my puzzle.

Ordered 24 EVE 304AH cells from Emily through Alibaba. Paid with CC directly through Alibaba pay and the card I used has no foreign transaction fees. CC company confirmed buyer protection for 180 days from expected product receipt date and automatic refund if not received. Capital One if anyone interested. Payment was made September 27th (no issues with approval by CC or Alibaba) and received notice shipped October 12th...slow boat. I did have to request a tracking number mid October but really did not matter as there never was any updated information on FedEx site when 12 boxes showed up December 29th. Emily was responsive to inquiries and answered questions in timely manner...but I would not say proactive, more reactive. Freaked a little December 27th when Alibaba released funds and closed protection window as no updates for 90 days. Emily talked me off the edge...Capital One held on other line. Holiday and work schedule did not permit full inspection until this evening. Happy New Years Eve! All 24 cells appear to be brand new with no dents, dings, or noticeable bloating. All labeled with voltage and IR at time of shipping and random testing shows voltage spot on with .01 to .02 variances in IR. Cells came with welded studs, although I expected threaded terminals and grub screws. Welds appear satisfactory and I guess is an upgrade other than connection surface area considerably less than it would normally be. At least now I don't have to worry about stripping threads with grub screws. QR codes are stamped into aluminum top plate and are intact. Need to find thread on decoding date code to see when manufactured. More for information records and curiosity as capacity tests are more important.

Nerve racking experience only because of state of shipping now...93 days from payment to receipt. Very happy with condition of cells and packaging. I will be updating as top balance and capacity tests proceed over the next several weeks.

Happy and Safe New Year to all!!!

Gizmo

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Gizmo740

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One down 23 to go. Looks like going to average 17hrs per cell test. 311.8AH20220102_142527.jpg
 

smoothJoey

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@Gizmo740 you can get the equivalent of a parallel top balance by just charging each cell to 3.65 volts with a tail current of 15 amps.
As long as you charge each cell to the same voltage in the high knee with the same tail current its actually better than a parralel top balance because you can observe the individual cells settle voltage and self discharge.
 

Gizmo740

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Hadn't thought about that...good idea. They all started a 2.93 to 2.94. Ended test a 3.0 looking for settling close to origin for top balance. Longer charge but save time overall I think. Thanks! @smoothJoey
 

smoothJoey

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Hadn't thought about that...good idea. They all started a 2.93 to 2.94. Ended test a 3.0 looking for settling close to origin for top balance. Longer charge but save time overall I think. Thanks! @smoothJoey
Also since the Coulombic efficiency is so close to 100% with LFP you can capacity test the cells by charging them instead of discharging them.
So the sequence is...
Discharge to 2.5 volts and charge to 3.65 with tail current of $n amps.
 

RCinFLA

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One down 23 to go. Looks like going to average 17hrs per cell test. 311.8AH
How are you connecting the voltage sense on the EB tester?

Reason why I ask is looking at graph it appears initial voltage slump when applying 40 amp load is dropping from 3.378v rested no load to 3.292v with 40 amp load giving a 86 mV slump. This is a bit more than I would expect for 40 amps for a new cell. It is still pretty good but I would expect about 45-50 mV slump for 40 amp load. Less than 100 mV slump for 40 amp load should yield rated capacity.

I am guessing you stacked the voltage sense on top of the load cable lug on battery terminals. This might account for the extra drop.

Another possible reason is the battery is cooler than 25 degs C.

I also think the 304 AH cells are pre-lithiated anodes, instead of traditional charge forming process. I have not seen enough of these to determine if there is a particular difference in their initial measurement.

The reason for my interest in what might appear to be nit-picky question is I believe the terminal voltage slump for a quick three minute load test, near 25 degs C, can predict if a cell will meet its capacity rating. It can make checking a bunch of cells a lot faster than doing full capacity discharge curves. It is also the best way to match cells for a series stacking.
 

RCinFLA

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What is the preferred alternative?
Getting to aluminum battery terminal directly.

One way to do this is to have rigid plastic cupped bushing like what is used to insulate the hold down screw on a TO220 packaged transistor, and insulate the device drain or collector from heat sink, but larger 6mm diameter bushing. It allows the high current lug to sit on battery terminal contact shoulder while the sense wire lug and battery terminal screw goes through the high current lug directly into the battery terminal without touching the high current wire lug.
 

smoothJoey

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Getting to aluminum battery terminal directly.

One way to do this is to have rigid plastic cupped bushing like what is used to insulate the hold down screw on a TO220 packaged transistor, and insulate the device drain or collector from heat sink, but larger 6mm diameter bushing. It allows the high current lug to sit on battery terminal contact shoulder while the sense wire lug and battery terminal screw goes through the high current lug directly into the battery terminal without touching the high current wire lug.
Like this?
record.jpg
and this?
insert.jpeg
As in they both contact the terminal/platter instead of being stacked.
 

Gizmo740

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I need to start thinking outside of the box. Going to start experimenting with next few cells. In middle of second charge/discharge test now.20220102_191820.jpg
 

RCinFLA

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Yes, that arrangement is going to give you a little lower voltage reading for the sense voltage than actually on the battery terminals. When you want to detect 5-10 mV cell voltage difference with loading every little detail counts..

Would be a little better if you can get voltage sense alligator clips to grab on top of nut, better yet with an insulating washer between top of bus bar and nut. Most of high current will go from bottom of bus bar to battery terminal shoulder, leaving the nuts closer to actual battery post to battery post true cell voltage.

You can also check the battery terminal voltage at base of cell terminal with DVM and probes not allowed to touch bus bar. Can check if EB tester is reading same as DVM.

I think if you see yourself the no-load to 3 minute load voltage battery terminal voltage slump test compared to full capacity discharge test you will become a believer of the short 3 minute load test. You can get good indication with cell between 45% and 75% state of charge, even saving time not having to fully charge cell. I have gotten very close to the same slump voltage between that range of cell state of charge. Only thing you have to be careful of is letting the cell get cooler than about 18-20 degs C. The load voltage slump starts to increase when cell is cooler (as will capacity test results get lower AH at higher discharge current).
 
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Gizmo740

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@RCinSo is there an expected slump or will I create my baseline based on known good capacity cells? Also you think 40 amps is sufficient being less than .2C? I do see the necessity of more accurate lead positioning for this task. Caught this test at the end with current trailing off. 3.65 on EBC display and DVM was low by 1mv on shoulder of welded stud. Honestly still trying to wrap my head around the energy and sensitivity of these cells.
 

RCinFLA

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For 280 AH EVA's I expect about 45-50 mV slump for 40 amp discharge on a new cell. Older used cells can be 3 to 5 times greater so load test is also a good quick way to see if you got screwed by vendor sending you used batteries. These higher slump voltage cells will not make their rated capacity.

Problem is I don't have data for 304 AH cells which I believe are pre-lithiated. This process pre-loads the negative anode so the cell does not see the typical lithium loss from cathode during manufacturer charge forming. This new process would contribute to the capacity increase over traditional charge forming process used for 280 AH cells.
 

Gizmo740

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Looks like this one stabilized at 3.380 after 30 min then 40 amp load slump to 3.344. At 3.293 steady 30 min in. Can't zoom on plot until test completes. Will verify then.
 

RCinFLA

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Looks like this one stabilized at 3.380 after 30 min then 40 amp load slump to 3.344. At 3.293 steady 30 min in. Can't zoom on plot until test completes. Will verify then.
51 mV slump @ 40 amps is a good number for a new cell value. (y)

There is PC software for the EB tester you can download that will record the tabular data numbers to a file. Very good to keep the data so you can check cell performance again at a later date and compare it to original new cell data, to see how much the cell has degraded over time.

Another experiment you can do later or from the data is lookiing at the profile of the slump over about 5 mins.

Battery Diischarge Impedance.png

You will see an initial, almost instant drop in voltage slump when 40 amps is first applied. I am not sure how fast the turn-on step the EB tester is but on graphs of its discharge, the sampling rate on voltage sense looks to be about 2 seconds interval. Both rate of current turn-on of current and voltage sample rate may give the inital slump a lesser slope appearance than actual.

The initial immediate slump is to mostly due to battery internal connect I*Rs voltage drop and represent the foil layer metal resistance to the terminal connections.

LF280 overpotiential curve.png

Beyond the initial immediate voltage drop there is the kinetic overvoltage slump which has an exponential decay typically lasting a couple of minutes. This is the time lag caused by ion diffusion and de-intercalation of lithium ions from negative anode electrode. Its exponential decay gets much longer in time at less than 20% state of charge and at colder temps of cell.

LFP Over-potential Chart.png

At very low cell current there is little overpotential drop but is can be variable up to 20 mV. This is why you need to have a moderate load current to dilute this inital knee. This near no load current flat portion is also why just passively paralleling cells will not fully charge balance them, leaving up to 20 to 40 mV difference in cells open circuit rested voltage. This much open circuit cell voltage difference can translate to 20% to 35% difference in cell state of charge.

As I mentioned temp of cell effects performance. Ion migration slows down increasing cell impedance and creating greater terminal voltage slump with load current. At 25 degs C temp, the cell impedance is fairly constant between 40% and 85% state of charge. So you do not need to be closely matched in cells' state of charge to do the load current voltage slump test, but do have to pay attention to cell temp.

Cell impedance vs temp and capacity impact.png

Open Circuit Voltage vs Temp.png
LFP capacity vs temp.png

As a cell ages, lithum gets consumed in degrading chemical byproducts that reduces available free lithium for desired cell operation. These degrading chemical processes have a secondary negative side effect of blocking ion flow within cell which increases cell impedance. This is what to look for to determine condition of cell. When they say a cell has dropped 80% in capacity is getting near end of longevity life its not just the capacity loss that is the issue. It is the increase in cell internal impedance that causes so much voltage slump with moderate discharge load that the cell becomes unuseable. An aged used cell will perform at room temp like a good newer cell looks at cold temps.

Like AH size cells put in a series string should be matched which is best tested to have similar voltage slump for same 3 minute moderate load test. 40 amp discharge for a 280-300 AH is marginal test load but that is limit of EB tester. Would prefer 0.2 to 0.4 CA load current.

This is also the reason doing capacity testing at low discharge current is pretty much a waste of time as it doesn't show the terminal voltage slump when moderate load current is applied At low discharge current, an old used cell may show a respectable capacity. It is also why doing the quick moderate load, slump voltage test will immediately expose an older used battery from a vendor. I have seen some really garbage cells shipped by vendors claiming grade A cells.

Attached is a Youtube video that shows this, although I disagree with his conclusion that one of the two cells tested is garbage and other is good to ship. In my opinion the amount of voltage slump for what he considers good to ship is also garbage and definitely should not be called a grade A cell. He is not using remote cell terminal voltage sensing and the four foot pair of #2 cables have too much voltage drop causing the instrument to show more voltage slump.

 
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Gizmo740

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@RCinFLA Awesome information. Thank you for taking time to share! Going to revamp my approach to quick testing each cell and implement @smoothJoey test to top charge/balance. Quick math will save ~5 hours per cell. Currently testing to 3.65V with tail of 15A (value per factory data sheet) but considering lowering tail current for more absorb as factory specs are testing at 250A charge/load and I won't have near that capacity until packs are built at higher voltage. Thus far cells are settling to 3.38X after aforementioned charge and 30 min rest. Thoughts?

Second cell results at 315.X AH. Back to work so wife is getting crash course in angry Pixies! Have to wait to overlay plots. Hoping she saved data correctly :confused:
 

smoothJoey

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Currently testing to 3.65V with tail of 15A (value per factory data sheet) but considering lowering tail current for more absorb as factory specs are testing at 250A charge/load and I won't have near that capacity until packs are built at higher voltage.
I think consistency is important to get all the cells equally charged.
I'm not sure what the optimal tail current is for your charge current and charge voltage but since you are only doing it once or twice 15 amps is probably close enough.
Its only 1 or 2 cycles out of the thousands of cycles of the battery's lifetime.

The full resting voltage seems a bit lower than I would expect.
Not sure what it means.
How long did the cells rest before measurement?
How long can you let the cells rest?
There is not a huge amount of experience on the forum with the 304ah cells yet.

Just to confirm, your batch process is discharging to 2.5 and then charging to 3.65 and measuring capacity based on amps in as opposed to amps out?
 

Gizmo740

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Revised process: Quick 3 minute 40A test on each cell logging and comparing voltage sag. Looking for any anomalies indicating cell issues to investigate. Will use sag values to group cells for packs. Then if all cells are in acceptable range, discharge each to 2.5V, rest 5 min, charge to 3.65V with 15A? tail current. Record AH charged as close capacity...will start with 2 that I have tested original way to check variances in discharge and charge capacity. Once all cells charged to 3.65 will 8p them by comparable voltage sag values recorded earlier and charge again to 3.65V. This charge should be quick and finalize top balance. Build 3 8s packs
 

smoothJoey

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Revised process: Quick 3 minute 40A test on each cell logging and comparing voltage sag. Looking for any anomalies indicating cell issues to investigate. Will use sag values to group cells for packs. Then if all cells are in acceptable range, discharge each to 2.5V, rest 5 min, charge to 3.65V with 15A? tail current. Record AH charged as close capacity...will start with 2 that I have tested original way to check variances in discharge and charge capacity. Once all cells charged to 3.65 will 8p them by comparable voltage sag values recorded earlier and charge again to 3.65V. This charge should be quick and finalize top balance. Build 3 8s packs
It should be very interesting to see how the quick test correlates to the charge capcity test.
If it correlates well(I would not be surprised if it does), then you might be able to skip the discharge component to the later part of the batch.
 
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