Check my understanding (calling on folks who understand IR)

[Sharky722]

A controlled test would be hard to conduct, but some of the folks seeing slightly lower capacities might do well to commit the time to put one cell through a number of full cycles. It would be pretty interesting to see how repeatable the results are.

This is exactly what I did with my eve 280s.
2 of the 8 were 273 and 275ah which I was still happy with.
Upon separation of these cells i exercised them 4 more times from 3.65 down to 2.5.
They are now equal with the other 6 @ 282-285ah range.
I'm interested to learn how this happens and why at a chemical/mechanical level.
Maybe the formation of the SEI layer from factory plays an important role and preservation of this layer via not consuming Li+ in the repair by not abusing the cell.

 

[Sillyputty]

Nominal Capacity is a useful term if used properly and properly understood.

I was thinking about this while trying to replace the roof vent in my camp trailer - which allowed a "nominal" amount of water inside expressed as "zero" by the seller when I bought it, yet here we are. I do appreciate you taking the time to explain the current state of affairs in battery vernacular and how we got here and that it is nuanced, and I do realize that we've inherited a rather fuzzy definition of capacity in the particular market we're fishing in which requires discussions like this.

 

[Dzl]

This is exactly what I did with my eve 280s.
2 of the 8 were 273 and 275ah which I was still happy with.
Upon separation of these cells i exercised them 4 more times from 3.65 down to 2.5.
They are now equal with the other 6 @ 282-285ah range.
I'm interested to learn how this happens and why at a chemical/mechanical level.
Maybe the formation of the SEI layer from factory plays an important role and preservation of this layer via not consuming Li+ in the repair by not abusing the cell.

I know zero about chemistry, but I have a vague recollection of reading that there is no such thing as 'exercising' a cell, at least not in a positive sense. Could it have been that they were not properly balanced the first time around? Or some other factor (poor connection or contamination on the terminals or busbars, or low cell temperature maybe) was corrected in disassembling cycling and reassembling the pack?

Or maybe, there is a factor at play that I don't understand or misunderstand, that cycling could help. As mentioned I am not at all knowledgeable about the chemical side of things.

Whatever the case, I'm glad things worked out for you the way they did. And its good info for others who have cells that test low.

If you recall, what capacities did you record for the 4 cycles of the individual cells?

 

[nebster]

I wouldn't expect a large difference at those temperature differences but I really don't know. Like the graph you posted, all the data I have seen on temperature usually only shows big gaps in temperature (extreme low, freezing, room temp, hot). No indication of what ~5-10*C difference would make.

Well, I think there's a pretty clear indication. Look at the measured capacities at 0, 25, and 40C. Fit a curve to those values (pick your poison). You'll at least have an indication of what the temperature-related effect might be.

I really think everyone is splitting hairs here (3% on a 280Ah should be in the noise for us, or we are doing it wrong), but if you want to split them... it's time to get out the regression analysis tool and get to work. I guess?

 

[nebster]

I know zero about chemistry, but I have a vague recollection of reading that there is no such thing as 'exercising' a cell, at least not in a positive sense. Could it have been that they were not properly balanced the first time around? Or some other factor (poor connection or contamination on the terminals or busbars, or low cell temperature maybe) was corrected in disassembling cycling and reassembling the pack?

There are definitely chemistries that require priming, or warmup cycles, before they deliver their full reversibility.

There may be a small factor with LFP as well. It is the kind of detail that might not be interesting enough to make it into the public body of research knowledge. Only obsessive hobbyists with big, new cells and lots of time to spare might notice the behavior. Ahem.

 

[Gazoo]

If anyone knows, cell number 7 is the weakest cell in my 8S pack. Would it make any difference if I put that cell in the number 1 location? Cell number 1 measures 3.000 volts and cell number 7 measures 2.50 volts when the BMS cuts them off.

When I charge them to full, there is less than a 100mv difference between the highest and lowest cell. So they are fairly even at the top as expected since I parallel top balanced.

 

[cinergi]

This would be my guess too, but apart from broad user error or broad measurement error in the same direction, its the only factor I could see that would lead to tests skewing low.


I'm not 100% clear on that either, so I will let @cinergi clarify, so I won't speculate.

...me not speculate? did you honestly believe that? ? I have to indulge myself with at least a bit of educated guessing here...

I believe what you are looking at is two parallel tracks of testing:
In post #111 (Capacity test #4) is a full pack capacity test (limited by the lowest cell to 262Ah). What you are seeing in the bottom right of the computer screen is the Orion-BMS measuring each cells individual voltage as the pack discharges
In post #112 (Cell test #3) may be a single cell capacity test (though I'm less clear on this, hopefully @cinergi can clarify)

Going back through the thread I think these are the results of the pack level capacity tests:
Capacity test #1: 252
Capacity test #2: 261
Capacity test #3: 261
Capacity test #4: 262

Does that look correct @cinergi

I don't have my spreadsheet in front of me but yes - one post was the pack and the other post was the cell. The battery pack level tests are all around the numbers you quoted plus one more which clocked in at 268.

 

[Sharky722]

I know zero about chemistry, but I have a vague recollection of reading that there is no such thing as 'exercising' a cell, at least not in a positive sense. Could it have been that they were not properly balanced the first time around? Or some other factor (poor connection or contamination on the terminals or busbars, or low cell temperature maybe) was corrected in disassembling cycling and reassembling the pack?

Or maybe, there is a factor at play that I don't understand or misunderstand, that cycling could help. As mentioned I am not at all knowledgeable about the chemical side of things.

Whatever the case, I'm glad things worked out for you the way they did. And its good info for others who have cells that test low.

If you recall, what capacities did you record for the 4 cycles of the individual cells?

I had no idea at the time the issue a poor connection/contamination could have (but now i do).

Originally I thought being at the end of the pack had something to do with it, so they were put in the middle (parallel) on the next charge. This had the same result <280.

There was a definite 2ah approx increase on these cells when cycled individually on successive charges/discharges each time.

I admit I wasn't very thourgh documenting this on my maiden lifepo4 outing.

Have just taken delivery literally 15mins ago of my next lot of eve LF105s from Basen I will be sure to be more thourogh this time round.

 

[nebster]

I'm interested to learn how this happens and why at a chemical/mechanical level.
Maybe the formation of the SEI layer from factory plays an important role and preservation of this layer via not consuming Li+ in the repair by not abusing the cell.

My (wild) guess is that maybe the electrolyte wicks down out of the jelly roll membrane a little bit, reducing the active surface area.

I say that because new cells sometimes make a sloshing sound when you first handle them, but after a while they don't really seem to make as much noise.

 

[Dzl]

Well, I think there's a pretty clear indication. Look at the measured capacities at 0, 25, and 40C. Fit a curve to those values (pick your poison). You'll at least have an indication of what the temperature-related effect might be.

Yeah, I understand the general trend.
But the granularity of the chart you posted and others I've seen don't really give enough detail to assess whether there is a meaningful difference between the standard test temperature of 77*F and 67*F or 60*F

Based on your chart, one thing you can see is that the difference is not linear. The 15 degree difference between 25 and 40 is near zero (hard to say exactly maybe 1% or less?), the 25 degree difference between 25 and 0 is maybe 4-6x the difference between 25 and 40.

Can we assume that the 15* difference between 25 and 10, is equally as small as the 15* difference between 25 and 40, I don't think so, but I can't say for sure. If I were to guess, I would think a test between maybe 65F and 85F would not have a big impact on capacity.

 

[Dzl]

I really think everyone is splitting hairs here..

Its a technical DIY forum full of people strange enough to find the idea of building a battery interesting... what did you expect ?

I will admit, I get a bee in my proverbial bonnet, when sellers overpromise and underdeliver, or are otherwise disingenuous.

 

[nebster]

Can we assume that the 15* difference between 25 and 10, is equally as small as the 15* difference between 25 and 40, I don't think so, but I can't say for sure. If I were to guess, I would think a test between maybe 65F and 85F would not have a big impact on capacity.

Yeah, definitely a small impact. But well within the realm of possibility if 3% is the difference people are observing in different places.

 

[Dzl]

Yeah, definitely a small impact. But well within the realm of possibility if 3% is the difference people are observing in different places.

I agree, it could certainly be a contributing factor, hard to say how much.

Though as noted a page or two back, measured temps by the Chargery and the Orion in Cinergi's testing show 23 and 24 Celsius, and STC is 25 +/- 2, the measured capacities were 261 and 262 (or about 93.5%)