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Discussion Starter #1 (Edited)
The 18650 - 14s10p Project - 48V x 34Ah

This is to chronicle a project to construct a 1.6 kWh module from 18650 cells. The configuration is 13s10p (edit: upgraded to 14s10p) using 130 (140) "Grade A" Panasonic NCR18650B (edit: unprotected) li-ion cells. These cells are nominally rated at 3.4 Ah and guaranteed to 3.25 Ah.

The first order of 100 cells (at US$3.28 each - $272/kWh) was received from Shanghai today and I have started testing individual cells, and plan to test all cells. Shipment was by air and arrived in 4 days. Shipment adds another $1 to the price, and with transaction costs amounts to $1.24 a cell. I was told it is not possible to ship via sea, and must be by air.

It has been observed that cells purchased from some suppliers in China have contained some fake mislabeled re-cycled cells or fake low-quality low-capacity Chinese cells. I plan to test for capacity, weight, impedance, and thermal behaviour during charging.

Photo of shipment - each cell has an individual white paper box with a safe handling warning, and a pair of these boxes are inside a green box with the same warning. These boxes were not from the original manufacturer (Panasonic / Sanyo). Each cell had a sticker that covered the manufacturer's label that says NCR18650B without giving the capacity. The sticker says "18650 3400 mAh 3.7V". There are also lot numbers on the cell's wrapping and probably on the steel casing, which may give a clue to the origins of the cell.
 

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Discussion Starter #2 (Edited)
The cells arrive with 3.51V charge or about 25% SoC - a bit low.

The first cell tested provided a whopping 3,462 mAh of capacity (272 Wh/kg, relaxed 4.20V - unrelaxed 2.86V at -0.25A discharge current). This is rather surprising as I have never seen any cell, including new brand name laptop cells, that could exceed its marked capacity, even at discharge rates as low as 0.1A. The manufacturer's own capacity rating is 3350 mAh (typical), 4.20V - 2.50V at -0.65A discharge.

I would now need to test the same cell at 0.5A to get a better idea of its charge capacity.

At 45.8 grams each, the cells are about 2.7 gr lighter than spec, which puts them at 3.67 kg/kWh, probably the lightest storage available.

A second cell results in 3,466 mAh under the same conditions.
 

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i hope those are real cells and not fakes, that added label is kinda flaky and there appears to be no marking as "Panasonic". It is hard to find info on whether or not these cells are internally protected or not--there are numerous websites advertising them, some say yes others say no...

Panasonic datasheet indicates rated capacity at 3200 mAh, but it varies so much with both temperature and discharge rate that almost any value could be quoted.

i wonder if the datasheet has transposed the digits of the weight, then your measurement matches exactly.

 

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Discussion Starter #5 (Edited)
Hi kennyhobby - the cells are unprotected, at least according to the order, and the price reflects that. The labeling is standard for Panasonic. The standard label neither mentions the manufacturer or the capacity. That is why the distributor affixes the capacity sticker.

The 3200 mAh "Rated capacity" on the data sheet is confusing. In another spec, it says "Nominal Capacity (typical) 3350".

The spec weight is 'max 48.5'. I suppose different lots may have some weight variation, or that overtime, they manage to reduce weight.
 

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Discussion Starter #7 (Edited)
Hi Karter2 - thanks for the links to the 18650 testing site.

Yes, that is exactly the cell I have received. Here is an image.

The label at the bottom of the cell wrapper says "E6 7130".
 

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Discussion Starter #9
Can you carefully measure the overall length (without shorting the ends)?
They all measure 65mm plus a tad over. About 65.2mm.

There are now two batches. E6-7130 and E6-6X21.

I have measured the capacity of about 10 cells (once each). If measured at 0.25A, they are above 3400 mAh with one at over 3500 mAh. If measure at 0.5A, they are above 3350 mAh.
 

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The "normal" ( most commonly used) discharge rate for capacity testing, is 0.2C. Hence why the spec sheet states 0..65amps.for those cells.
 

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Discussion Starter #11
Karter2, do you know why China or the shipping companies require cells to be labelled for capacity, when they know a lot of cells exported are fakes and have unbelievably wrong capacities? Is there a promotion subsidy for high capacity exports?
 

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Discussion Starter #12
A quarter of the cells have been tested, and all have passed. The variation between cells is slight and the physical appearances are identical.

I am using 3 different discharge testers and at varying discharge currents. So it has been difficult to arrive at a testing standard. After some compensation I get the following at 0.25A discharge:

Minimum capacity 3,300 mAh
Maximum capacity 3,500 mAh
Average capacity 3,444 mAh

Testing is from 4.17V relaxed to 2.8V unrelaxed.

At 0.5A discharge, I get about 4% less capacity: 3,310 mAh on average.

In order to verify these cells, I don't think it is necessary to capacity test more than 50% of the cells.
 

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Discussion Starter #13
How does one test for cell DC impedance?

When the cell discharges, the voltage drops within 5 seconds (unrelaxed state), and then when the current is cut, the voltage returns back to almost its original level (relaxed state).

So I believe in addition to drop due to impedance, the electrochemical process also results in drop of voltage.

How does one separate the two? Just measuring the drop in voltage on a 1A discharge will be the combination of the two effects, and not just due to the DC impedance.

Any ideas how to measure just that?
 

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The classic method for "DC Internal resistance" ! (DCIR), is just as you discribed.
Discharge at a known current (1C, or 2C, 5C, etc)...measure the steady state voltage under load, then remove the load and measure the relaxed voltage .
Accurate measurements needed,..then Ohms law.
Note DCIR varies significantly with temperature, and state of charge , so be careful to standardise test conditions if you want to compare cells.
You may find this useful...
https://endless-sphere.com/forums/viewtopic.php?f=14&t=73701&hilit=Dcir+testing
Another test recommended is the "charge retention" test..
Charge all cells to a common voltage, say 4.17 (easy if you group cells between two contact plates and charge in parallel)
Then remove the charge contacts and allow the cells to stand for several days before checking individual voltages to see if any self discharge or any voltage variation.
You only need one bad cell to mess up a pack.
You should also match cell capacity as close as possible within parallel groups.
 

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You should also match cell capacity as close as possible within parallel groups.
Just curious, given a limited number of cells, why not distribute the cell capacities so each level has about the same capacity (within reason)?
 

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Just curious, given a limited number of cells, why not distribute the cell capacities so each level has about the same capacity (within reason)?
Cells in a parallel group cannot be individually monitored for voltage or SOC, or protected against over discharge, so need to be well matched.
Whereas, strings of Parallel groups of different capacity can be monitored and protected by a good BMS.
One reason why its a good idea to assemble a pack with strings of parallel groups.
 

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Perhaps a good way to measure internal resistance would be to measure the voltage difference for current of 0.5C and 1C. That would eliminate the effect of the unloaded (relaxed) condition.
 

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Kar2,

I never use a open circuit cell or battery voltage in the DC resistance measurement and calculation. I always use two different load points. Like 1C and 5C, or 0.5C and 1C. The zero current voltage can introduce errors.

And cells most likely are labelled with Ah due to shipping and transport regulations.

major
 

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Yes, agree guys, for the optimum result its "Delta V"..
And it should be a "4 wire" method also
...but i have never noticed a significant difference for normal applications.:cool:
 

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Discussion Starter #20 (Edited)
Thanks to all for the suggestions.

Hmmm ... as dcb says, I am not sure if it is a good idea to put all weak cells in the same group. For simplicity, assume discharge not more than 1C, and the configuration is 10s5p. Assume 45 of the cells are 3000 mAh to cutoff and 5 of the cells are 2500 mAh to cutoff (the weak cells).

If the 5 weaks are grouped together, then cutoff for the group happens at 2500 mAh per cell (i.e. when 5*2.5A has been drained from the weak group). This means the other 9 groups each has 5 * 0.5 = 2.5Ah of unused capacity. So total dead capacity = 2.5 * 9/10 = 2.25Ah.

On the other hand, distribute the 5 weak cells, one per group. So cutoff will happen at (4*3000+2500)/5 = 2900 mAh. There will be 5 groups each with 100 mAh of dead capacity per cell. So total dead capacity = 5 * 0.1 * 5/10 = 0.25Ah. Which is much less.

When a weak cell is grouped in parallel with four strong cells, I don't think it is the case that cutoff will happen at 5 * 2500 mAh. Rather, cutoff will happen at 4*3000 + 2500. Being in parallel, the stronger cells will supply more current proportionally while the weaker supplies less, and thus the weaker cell cannot get over-discharged. Their voltage will always remain equal. If the current is cut, and the five cells separated, the five in parallel will have same voltage. One weak cell cannot disproportionally reduce the voltage when in parallel. In series, it will disproportionally reduce the voltage, due to the "first past the post" cutoff.
 
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