[onegreenev]

No I did not test the resistance because it is not important. If the damage and loss of capacity has been done then checking resistance is moot. I would agree that you could keep some of the resistance down because the distance between the plates is kept to a minimum but the bloating is mostly outside the package. The plates are packaged tightly inside the cell and there is no indication that the gas bloating is causing any problem. The separation is caused because of mechanical changes within the anode/cathode and has nothing to do with gassing. Gassing is an indication you have screwed up. What I am going to do is take my most bloated cell from that pack that had been held tightly for 4 years and reclamp it and then open the top to release the gas and then tighten it back up and release the pressure again to see if the cell remains flat.

I do not know how to check resistance. If I can do that I will check before I reclamp and again after as long as the cell takes a charge. That I can and will do. Can you help me with what is needed to check resistance?

Pete

[Ziggythewiz]

Originally Posted by onegreenev

Can you help me with what is needed to check resistance?

Check voltage drop at a known current.

[dladd]

Originally Posted by onegreenev

I do not know how to check resistance. If I can do that I will check before I reclamp and again after as long as the cell takes a charge. That I can and will do. Can you help me with what is needed to check resistance?

Pete

You have a Powerlab 6/8, right? That will measure resistance for you.

[EVfun]

The quickest way to check resistance would be a load test. The catch being you need a pretty good load (1/2C to 2C recommended) and it has to be repeatable. So many loads have drift, which could mask changes.

An accurate load about 1/2C for 10 seconds with the voltage recorded right before load cut-off would discharge the cell so little that you should be able to check again, after clamping and giving the cell time to flatten, without needing to charge the cell. The SOC change would only be 0.2%. Of course, that would only work if the SOC was somewhere in the 30% to 70% SOC range where the discharge curve is quite flat.

These are some short quotes from the 2008 Thundersky document. For some reason I can't search, or copy, material from the it:

"Please use jigs and straps to fix the single cell or battery pack to avoid swelling for normal use!"

"The battery swelling usually happens when it is overcharged or over discharged."

"Discharge the battery pack to its standard minimum voltage and release the straps to replace the cell with new one of same capacity."

"Please use the renewer as the picture shows to make the case back to normal shape."

This is a pair of plates, heavier than the standard plates they shipped with, with threaded rods to pull the cell thickness down. It wouldn't fix swelling on the short ends or top or bottom. If the plates the cells shipped with are in place I'm not sure how one could swell. When would this clamp would be needed? This is an old document, so sometimes the translation leaves a bit to be desired.

This is an old document so some of the instructions are a bit excessive, belt and suspenders approach. We know now the cells are quite a bit tougher if you don't let them go over 3.8 volts or below 2.0 volts under load or 2.8 volts no load or allow them to get to hot. Still, my cells are strapped with the Thunder Sky hardware. It is a handy way to hold the cells down in the EV. When I re-bundled them I added threaded holes in the end plates (in case safe locations) so I could attach the blocks of cells by the end plates.

[onegreenev]

So once you collect the information then what formula is used to plug in the numbers? I am also aware of the manufacturers thinking for strapping but once the damage is done its done. Strapping really only does COVER/MASK the resulting damage. It is NOT a fix or preventive measures. To keep from swelling you stay off the top and bottom. That is the only way to prevent swelling. Once swollen the damage is truly done and clamping does not change that fact.


I'd like to see the cells vacuum packed within the plastic case and if any is left behind it should be nitrogen which is inert to help prevent any corrosion and to hold all internal things tight. A-123 pouches are vacuum packed. Vacuum compressed. Any gassing will puff them like a little puffer fish.

[EVfun]

Resting cell voltage minus loaded cell voltage will give you the voltage drop. The voltage drop divided by current will give you the cell resistance. It actually varies slightly at different loads because only one part of the sag is actual resistance. That is why a consistent load is needed.

As an example, my cells are right at 2.78 volts at 360 amps.

3.3 - 2.78 = 0.52 volts.

0.52 / 360 = 0.00144 ohms, generally referred to as 1.44 milliohms.

[bga]

The LFP cells are filled with an organic solvent in the electrolyte. At elevated temperatures, it has significant vapor pressure, so will swell the sides of the cells, particularly in the larger sizes.

The solvents are things like ethylene carbonate, dimethyl carbonate, diethyl carbonate. See http://en.wikipedia.org/wiki/Lithium...y#Electrolytes for more info.

They smell a bit like ether and have lowish boiling points, hence the need to physically contain the prismatic cells.

[skooler]

Originally Posted by bga

The LFP cells are filled with an organic solvent in the electrolyte. At elevated temperatures, it has significant vapor pressure, so will swell the sides of the cells, particularly in the larger sizes.

The solvents are things like ethylene carbonate, dimethyl carbonate, diethyl carbonate. See http://en.wikipedia.org/wiki/Lithium...y#Electrolytes for more info.

They smell a bit like ether and have lowish boiling points, hence the need to physically contain the prismatic cells.

Just gave you positive rep for that

Thankyou!

[Jesse67]

Back to the original question....

I have my cells strapped in groups of 6 and have noticed no problems so far, I can lift the groups out without much trouble but you have to make sure they are clamped tight to keep the middle cells from slipping out if you give a jerk on the handles!
The only tricky part is making up new straps and finding a way to tighten them, mine were shipped in groups of 4. I made up straps that wrap right around the cells and the end plates and tighten with a 1.5" machine screw and nut pulling one end of the strap to the other. I tighten the bolt so the strapping all plays about the same note, nice and snug. With a good clamping system I wouldn't hesitate to clamp 12 cells together, they just get heavier to move and harder to get lined up initially.

And the excellent debate,

Everything I've read about clamping the cells is to prevent the plates from separating due the gasses produced during overcharging or abuse, you will still damage the cell and possibly reduce it's capacity but if you can keep the plates from separating the internal resistance will not increase as much and the cell should still be usable. High internal resistance causes excessive voltage sag and increased cell heating.

This is one reason why cylindrical cells can generally take much higher current draws, the cylindrical shape is incredibly resistant to expansion due to the high tensile strength of the material and it's inherent shape, internal pressure puts the case in tension vs. mostly bending forces that are applied to the flat face of a prismatic cell under pressure.

In fact this discussion has made me realize that I have two cells which have higher internal resistance than the rest (more sag under heavy current draw than the average) I think I'll try tightening up those packs to see if I can bring them back down. I haven't noticed any swelling but perhaps some extra pressure will help out a bit.

Jesse

[EVfun]

Originally Posted by Jesse67

Back to the original question....

I have my cells strapped in groups of 6 and have noticed no problems so far, I can lift the groups out without much trouble but you have to make sure they are clamped tight to keep the middle cells from slipping out if you give a jerk on the handles!

I have my 60 amp hour Thundersky cells banded in groups of 8. The bundles are tight enough that no cell from the center will even move. Each bundle weighs 40 lb. They have worked out well so far.