Clamping lithium cells.

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GregsGarage
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Clamping lithium cells.

Postby GregsGarage » Fri Oct 16, 2009 3:45 pm

Both Thundersky and Sky Energy recommend their cells should be clamped to prevent swelling. Hi-power say clamping is not normally required. I have a faulty Hi-power cell that Peter gave me, from his Insight project and decided to do some very crude testing on what effect clamping may have on the internal resistance of a cell.

This cell failed with a high internal resistance and some slight swelling of the cell is also apparent. The open circuit voltage is 3.32 volts, about what you would expect from a fully charged cell. Here is a pic of the cell with volt meter, sitting on my press.
Image

Now I am going to put a small load on the cell in the form of a 12v 3w automotive test lamp. The high internal resistance of the cell causes the voltage to drop to 3.06 volts. You can see the test lamp dimly lit in the picture. A good cell shouldn't have any measurable voltage drop with this small load.
Image

Finally I am going to clamp this cell between two plates, using the press. I haven't used very much force, about 2-3 pumps of the handle once the ram reached the plates.
Image
You can see that the internal resistance has dropped significantly just by applying pressure to the swollen cell. The voltage has now risen to 3.17 volts.

This would suggest that all large prismatic cells should be clamped/restrained against swelling and that cell life may be improved by keeping internal resistance down.
Greg Fordyce

Daewoo Matiz
http://www.evalbum.com/4191

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retepsnikrep
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Postby retepsnikrep » Fri Oct 16, 2009 5:08 pm

Very interesting Greg.

My 50 cells are sort of clamped being strapped to the carrying tray in blocks of 5 cells with 2x160kg strapping.

Does the faulty cell actually have any capacity now? Or is is just a rise in voltage we are seeing?

I'm not ignoring your e-mail i'm away and can't send mails at present.
Regards Peter

Two MK1 Honda Insight's. One running 20ah A123 Lithium pack. One 8ah BetterBattery Nimh pack.
One HCH1 Civic Hybrid running 60ah A123 Lithium pack.

GregsGarage
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Joined: Tue Apr 01, 2008 5:27 pm
Location: Galashiels, Scottish Borders
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Postby GregsGarage » Fri Oct 16, 2009 6:58 pm

Peter, the internal resistance is still to high to make the cell useable, but this shows that it has reduced some. I have left it on its side on a concrete floor with a couple 100ah gel cells stacked on top over the weekend. I want to see if 70kg pressing on it will have any long term effect.
Greg Fordyce

Daewoo Matiz
http://www.evalbum.com/4191

GregsGarage
Posts: 870
Joined: Tue Apr 01, 2008 5:27 pm
Location: Galashiels, Scottish Borders
Contact:

Postby GregsGarage » Mon Oct 19, 2009 8:16 am

GregsGarage wrote:I have left it on its side on a concrete floor with a couple 100ah gel cells stacked on top over the weekend. I want to see if 70kg pressing on it will have any long term effect.

It's monday morning and this has not made any difference to the cell.
Greg Fordyce

Daewoo Matiz
http://www.evalbum.com/4191


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