Clamping lithium cells.
Posted: 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.
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.
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.
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.
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.
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.
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.
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.