[JRP3]

Two recent cell deliveries that came with a large resistance spread, some sloppy looking cell construction, cells out of sequence, and questionable crating, have me wondering if CALB cell quality is not as good as it used to be. My order from three years ago was much more closely grouped and packaged better than these two orders:

http://www.diyelectriccar.com/forums...-59659p33.html

Has anyone else seen similar resistance spread and sloppy cell construction?

[steven4601]

Can you quantify your observations?

[JRP3]

It's all in the linked thread, basically my cell resistance range from three years ago was .28-.35 for 100 ah cells and the new shipments came in with twice as large a spread, plus some sloppy looking cell construction:

http://www.diyelectriccar.com/forums...&postcount=329

http://www.diyelectriccar.com/forums...&postcount=330

[steven4601]

My appologies for not reading the linked thread.

The resistance/capacity values being referenced to,is what they supply with the cells as a data log?

Not having read the whole thread that you linked to, Did you verify the data?

[JRP3]

My cells came with a sheet listing the serial number, tested capacity, and tested resistance for each cell. I also had the data sheet for the entire shipment of cells, it was an early order for EVComponents. All cells were more closely grouped than the two recently reported deliveries mentioned in that thread, and the resistance numbers were lower overall for the shipment three years ago compared to the two recent shipments in that thread. I also saw none of the reported sloppy cell construction that they mention, and my shipping crate was of good construction and the packaging was tight and proper.

[Elithion]

Originally Posted by JRP3

my cell resistance range from three years ago was .28-.35 for 100 ah cells and the new shipments came in with twice as large a spread

Just a moment, please.

The spec sheets list the AC impedance at 1 kHz (yes they call it "resistance" but that's NOT the DC resistance).

There is NO relationship between the AC impedance at 1 kHz and the DC resistance (at 0 Hz). I already knew that from reading scholarly papers, but I also decided to try for myself.

I measured the honest to goodness DC resistance in a batch of CALB cells; I then plotted CALB's numbers for "resistance" (AC impedance at 1 kHz) versus the actual DC resistance, and there is absolutely no correlation.

What matters to users such as you and me is the DC resistance: that's the only thing that affects voltage sag under load. The AC impedance only affects the amount of electrical noise generated. So what.

You may or may not be correct in being concerned about CALB's quality, but the spread in the "resistance" numbers in their spec sheet should not be it.

[mizlplix]

So, to be clear, you are saying CALB's "resistance" test is worthless and should be discounted as meaningful?

Then why was it even included?

Interested,Miz.

[Elithion]

Originally Posted by mizlplix

you are saying CALB's "resistance" test is worthless?
Then why was it even included?

It's not just CALB. Roughly, 70 % of Li-ion spec sheets give the 1 kHz impedance, 5 % give true DC resistance, and the rest give nothing.

Why AC impedance? Because cell manufacturers have ready access to equipment that can measure AC impedance.

Why at 1 kHz? because the impedance of Li-ion cells has a 2-lobe shape, and there is a dip at around 1 kHz, so the numbers look better there. Also, at 1 kHz the impedance is more consistent than at other frequencies.

Why not DC resistance? Because it varies a lot, from cell to cell, as SOC changes, as temperature changes, and as the cell ages. And because they may not know how to measure it, or do not have access to equipment that does so.

Why does CALB publish each cell's AC impedance? Because it gives the user a sense that the manufacturer is thorough and therefore it imparts a sense confidence in the user.

Why does CALB ship cells with such a wide variance of AC impedance? Because those values are all well below their self-imposed cutoff threshold.

How do I know? I did a lot of research in the process of writing the Li-ion BMS book. Still, there are some holes in my understanding, so please forgive me if I missed something.

Davide

[tomofreno]

Originally Posted by JRP3

Two recent cell deliveries that came with a large resistance spread, some sloppy looking cell construction, cells out of sequence, and questionable crating, have me wondering if CALB cell quality is not as good as it used to be. My order from three years ago was much more closely grouped and packaged better than these two orders:

http://www.diyelectriccar.com/forums...-59659p33.html

Has anyone else seen similar resistance spread and sloppy cell construction?

Looks that way. Wonder if it is temporary due to part of the production line down or similar. Quality was looking good with the shipments to diyguy and electriccar. Sorry to see this. I think I would ask to have some replaced to narrow the spread.

[lowcrawler]

I got mine from Keegan last fall and they were tightly grouped and well packaged.