a123 20ah soft cells

[whatever]

I recently had a power blackout for a week or so, I needed to use my a123 20ahr cells to power lights etc.
The cells were I think produced in about 2006 or 2007, korean a123 20ahr.
Unfortunately some of the cells went 'soft' during use ( the soft cells were all about 2.9v after use). My first reaction was, these cells are buggered, may as well throw them out, but I decided to try to recharge them. Interestingly the cells after recharge went back to a 'hard' state, there was no compression on the cells when being used for lights etc. The cells appeared to still have a reasonable ahr capacity.
To recharge them I started with a few hours on 500mA, then quite a few hours on 1amp, then the rest of the time to fully charged on 2amp.
I have not discharged the 'soft' cells since recharging them, I will put them under compression first otherwise i think they will go back to the soft state. I will post results with a view to seeing if the cells are in fact still useable at least for low amperage applications.

 

[dnmun]

pouch cells should always be in compression.

 

[whatever]

I think date of manufacture was 2005 ( korean date codes a bit different to usa codes), so 10 years old, and second hand when I got them, only good for experimenting to see if compression allows soft cells to be useable.

 

[whatever]

Just a note on what I think might be some safety concerns for testing cells that have gone soft ( even though back to a hard state after recharging):
1. hydrogen gasing: there is possibility of hydrogen gas being generated. I will put a small pinhole in the soft cells and cover with cellotape, if gasing occurs it will have an escape valve.
2. possibility of internal shorts: I would think the likelihood is pretty low, but I need to keep that in mind during the discharge tests ( discharge rate will be max 20amps).
I had two cells in extremely poor condition, where the graphite layer has delaminated you can feel/see lumps on the outer pouch, due to the internal flaking off of the graphite layer. None of the cells that went soft are showing external signs of delamination of the graphite layer, I would guess there is some delamination ( causing the softness perhaps) but since the cells have gone back to hard state after charging I'm guessing the graphite layer has re-attached to some extent. Just my guess, another cell that was in extremely poor condition I did a tear down on this thread
http://endless-sphere.com/forums/viewtopic.php?f=14&t=65770
This was on of the cells in extremely poor condition, wouldn't hold voltage and non-usable.

The cells are intended just for testing to see if there is any point in using compression to see if the cells are still useable at a 20amp rate.

Heres a link to a thesis that looked in detail at temperature and discharge rates, it includes some testing on a123 20ahr cells,
https://uwspace.uwaterloo.ca/bitstream/handle/10012/7936/Chen_Kaiwei.pdf?sequence=1
It goes into great detail on some of the causes of heat generation also.

 

[dnmun]

how can you feel lumps in the graphite layer if it is only microns thick? if you puncture the pouch after it has been sealed then how do you keep the moisture out? do you think cellophane tape will melt in the presence of the electrolytes? will the adhesive dissolve and be transported to the atmosphere as the electrolyte evaporates?

 

[999zip999]

Degassing a cell. Really. Hope not. Is degassing a word. Hope not.
Maybe use bubble gum and spit.

 

[whatever]

an unusable a123 20ah cell where the graphite layer has delaminated looks like this



I posted pics of internals on other thread mentioned previously

I've punctured pouch cells with small pinhole and cellotape in past, with no ill effects, no reaction with celloptape adhesive.
Its better for hydrogen to have a pathway of escape in case pressure builds up rather than have the pouch rupture with force,
its precaution in case gassing occurs. Sticky tape will keep moisture out no problems.

 

[whatever]

from this research paper
http://jes.ecsdl.org/content/157/4/A499.abstract

The performance loss of lithium-ion batteries with lithium iron phosphate positive chemistry was analyzed using electrochemical characterization techniques such as galvanostatic charge–discharge at different rates, ac impedance, and hybrid pulse power characterization measurements. Differentiation analysis of the discharge profiles as well as in situ reference electrode measurement revealed loss of lithium as well as degradation of the carbon negative; the cell capacity, however, was limited by the amount of active lithium. Destructive physical analyses and ex situ electrochemical analyses were performed at test completion on selected cells. While no change in positive morphology and performance was detected, significant cracking and delamination of the carbon negative was observed. In addition, X-ray diffraction analysis confirmed the changes in the crystal structure of the graphite during cycling. The degradation of the carbon negative is consistent with the observations from the electrochemical analysis. Ex situ electrochemical analysis confirmed that active lithium controlled cell capacity and its loss with cycling directly correlated with cell degradation. The relationship between carbon negative degradation and loss of active lithium is discussed in the context of a consistent overall mechanism.

 

[whatever]

page22 of this paper
http://drum.lib.umd.edu/bitstream/1903/12381/1/Williard_umd_0117N_12841.pdf
has some information of thickening of the graphite layer, a good illustration is shown also.
Its quite a good paper to gain understanding of the processes going on in lithium batts.

Its also contains some interesting results during their tests where during rest periods some increase in capacity occurred
on page74
( note: page numbers are given for adobe reader page number, the actual page number within the article will be different)

 

[whatever]

Just doing some calculations on compression for the tests.
I posted in this thread
http://endless-sphere.com/forums/viewtopic.php?f=14&t=52244&start=150
1 atmosphere pressure equates to 14.7psi
According to a123 document
( link by user CaptainKlapton)
http://endless-sphere.com/forums/download/file.php?id=157799

12psi is ideal compressive force ( see graph on page 32 of document above). The graph doesn't give what discharge current
was used for that ideal psi value to maximise lifespan. So will assume it is suitable for low amp discharge as will be used in my testing.

This brings up an issue for my testing. Since the cells that have gone soft have lost their vacuum low pressure internally ( due to a small amount of internal hydrogen gas release) which in a good cell equates to approx 14.7psi on the internal layers due to atmospheric pressure. I'm wondering if the 12psi ideal pressure, is taking into account the cells already have 14.7psi from vacuum bagging.
Perhaps in total I need to use 14.7psi and 12psi combined ( total 26.7psi)

If a new cell in good condition and good vacuum is compressed by 12psi externally, is that original 14.7psi still playing a role?
That is, a good cell being compressed by 12psi externally in fact has 26.7psi as the vacuum is playing a role?

How to calculate the external pressure added by faceplates:
I will be using two thick face plates with 4 bolts, the compression values can be calculated without the need for a compression measuring instrument by torqing the bolts to a pre-calculated torque.
There are calculators online that provide the necessary bolt torque values ( there are simple ones and more complex ones,
the simple calculator should be ok for my tests).
For example this online calculator will do the job:
http://www.engineersedge.com/calculators/torque_calc.htm

 

[whatever]

there are some good research papers viewable here
http://www.princeton.edu/~spikelab/research/mech_prop.html

In this paper from above titled :

Stress evolution and capacity fade in constrained lithium-ion pouch cells

http://dx.doi.org/10.1016/j.jpowsour.2013.06.165
They find a low pressure of 0.01mpa ( 1.45psi) gives the best lifespan for the lithiums tested ( cobalt oxide in this case).
Since the only data I've come across regarding a123 20ah cells compression values, is from the a123 doc previously post on es forum, suggesting 12psi is optimal, it might be worthwhile to take into account the lower value of 1.45psi given in the document above. Since both chemistries work basically in the same way, and the same mechanisms lead to capacity loss ( eg.
sei growth).

 

[whatever]

this paper
http://jes.ecsdl.org/content/161/11/F3065.full
titled:

A Model for the Behavior of Battery Separators in Compression at Different Strain/Charge Rates

They say

The expanding electrodes, constrained by the cell assembly, compress the thin porous separator leading to pore shrinkage and even closure

So too high compression may effect the lithion ion migration across the membrane to due the pores becoming closed.

 

[arkmundi]

Thanks for the information. I'm a hobbyist making eBikes, so the things that matter to me are starting with quality products and then assembling them in a manner that lends itself to the DIYer at home with commonly available parts & tools. Since I don't have, nor ever will have, any means to determine the compression on these packs, or for applying in a controlled manner, much of the technical info is of academic interest only to me. What does matter for those of us making battery packs using the A123 AMP20 prismatic pouch cells is a common-sense approach to pack construction that keeps the cells in compression during use. I believe I achieved that, using aluminium plate cut to size and duct taped together (no welding). So there is low initial compression. But any expansion is forcefully, orthogonally resisted.

 

[dnmun]

Thanks for the information. I'm a hobbyist making eBikes, so the things that matter to me are starting with quality products and then assembling them in a manner that lends itself to the DIYer at home with commonly available parts & tools. Since I don't have, nor ever will have, any means to determine the compression on these packs, or for applying in a controlled manner, much of the technical info is of academic interest only to me. What does matter for those of us making battery packs using the A123 AMP20 prismatic pouch cells is a common-sense approach to pack construction that keeps the cells in compression during use. I believe I achieved that, using aluminium plate cut to size and duct taped together (no welding). So there is low initial compression. But any expansion is forcefully, orthogonally resisted.

quality products? the pouch was obviously scrapped and had the tabs cut off to prevent it from being used. measured and labeled at -.3V, then this comment about how 200 microns thickness of carbon has deformed the surface to make it defective. like where is the reality in any of this?

 

[whatever]

to arkmundi:
do you use bolts at all?
to dnum: 0.3v is not marked on the cells, that was a sticker saying c130, some sort of labelling from the supplier.
I bought the cells probably 3 or so years ago. The cells all arrived with good voltage ( 3.2v or over), I discharge tested all the cells and capacity was between 18 to 20ahr.

There were 17 cells in total. 2 cells have failed, they failed whilst being used uncompressed on an ebike at 48v 20amp.
The aim of posting here is to determine if the cells that have now gone soft recently ( 7 cells ) will be useable in any way after compression is used. As I'm researching a bit before doing the tests I thought some of the research papers I've come across might be of interest to others. Considering the price of the cells at around $8usd each and getting a number of years use from them before any problems, I think a worthwhile experiment. It will be interesting to see if the soft cells ( that went back to hard state after charging) will have any decent capacity and ability to deliver current.

 

[whatever]

came across this paper:
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA575499
Its studied the expansion of the internal layers during charge/discharge ( using xray and a neutron beam)
Interestingly one of the cells tested was an a123 cell ( probably the 20ah pouch since it was made for electric vehicles).
One interesting result was:
the graphite layer caused most of the expansion when it absorbs lithium during charging.
They found expansion occurred during from 0%soc to 40%soc, then no expansion, then again expansion from 90%soc to 100%soc.
( soc =state of charge). Also they used 12psi compression via springs on the a123 sample.
So it might be possible to prolong the longevity of a123 cells by keeping them between 40% and 90% soc, as that is the area where expansion of the graphite is minimal.
Also their test method used springs where the deformation of the spring gives them a value for the compression amount in psi.

 

[whatever]

so there are two fairly simple ways to measure the compression:
1. use bolts and torque wrench, torque converter for bolts online to get psi
2. use springs ( better method as it allows for the expansion of cells): measure force in pounds of spring to compress by given amount ( use simple/cheap spring scale to get the value),
both methods are pretty simple and not expensive.
Need to calculate total area of the face of the a123 cell being compressed to get psi value.
a123 20ah cell face:
15cm by 20cm = 300sqcm
or
5.9 ( 5 and 29/32) by 7.87 ( 7 and 7/8) = 46.433 sqinch

To get 12psi you would need:
12*46.433 = 577.196 lbs
If using 4 bolts then each bolt or spring would need to provide:
577.196/4 = 139.299 lbs ( approx 63kg)
Thats approx the weight of a small thin person just to give it some context. If using springs to get some idea of how much
'spring' they need, a small thin person standing on the spring should not compress it fully.

 

[999zip999]

I believe charging to 3.45~3.5v and discharge to 3.1~3.0v. I also believe in aliens. I think just a light compression is all that is needed or mainly to control expansion. I just mega wraped ( tm ) mine. Do you believe ?

 

[whatever]

I made a slight error reading that last paper,
they found linear relationship of soc to thickness of graphite layer up to 40% soc ( for the a123 materials tested), expansion of graphite layer stopped after 40%soc, so fully charinging, and discharging only down to 40%soc would mean minimal expansion contraction of graphite layer.
Since delamination of graphite layer on the copper anode is a problem in longevity of lithiums ( including iron phosphate type),
its worth keeping it in mind in order to get max life out of the batts.
Too much compression causes 'creep' of the separator membrane i.e. closes up the pores ( blocks flow of electrolyte and lithium between plates). So looks like 12psi or less is way to go.

Expansion and contraction of the material can lead to fracture of the electrode and
eventually capacity loss as particles are no longer electrically connected to the current collector,each
other,or the carbon matrix in which they are suspended. The carbon anode material is
known to expand upon intercalation of lithium into the host structure that occurs during charging of the battery.

When I opened up a failed a123 20ah cell ( pictures posted in another thread), it was the graphite layer which had delaminated from the copper anode, the iron phosphate layer on the aluminium cathode was in extremely good condition.
So its seems the expansion/contraction of the graphite layer is a major contributor to cell longevity and failure, the paper cited above its sole purpose was to investigate this. They give references to other studies which show same results. I think its a very informative paper to read, and it gives detailed data on their methods/results.

 

[whatever]

Just some other info thats included in that last research paper on a123 internal layer dimensions:

The LiFePO4active material is 54 μm thick applied to each side of a 20 μm aluminum current collector,
yielding a total thickness of 128 μm for the positive electrode. The carbon active material is 39μm thick applied to
each side of a 10μm copper current collector, yielding a total thickness of 88μm for the negative
electrode. The separator is 25μm thick.