pressure or not?

[ltosolini]

Experts,
after ending up with 12 badly swollen cells out of my 20 pack of Wina 50Ah LiFePo4 I have learned the hard way that this type of cell must be mechanically contained in order to put a uniform pressure to avoid swelling.
I wonder if I should do the same for winston 40 AH or Sinopoly 40AH LiFePo4?
Thanks,
Luca.

 

[Sunder]

You need to compress LiFePo4 with 1-2psi of pressure if ypu go anywhere near the C rating. So if the cells are 40ah, and you're drawing anywhere near 40ah you should compress them.

This can be done by putting plastic, wood or similar at rhe ends of the battery, clamping them with some gclamps, then using a strong fibre or cloth tape to bind it.

 

[Instigater]

Now let's talk about Lipo soft pouches. Is it OK to contain them in rigid container that doesn't allow for expansion? I know it may break container and I'm willing to take the risk of making a new one after a while. I'm going to build a battery in very tight space where every milimeter counts. My idea is to apply fiberglass directly on top of new packs.
My concern is about fire hazard if/when cells try to swell but container doesn't allow to do so. Battery pack will include BMS so it should not be stressed.

 

[Sunder]

I asked this question about 3 months ago... Like LiFePo4 2, LiPo benefits from being constrained, although the consensus is not as clear as to the pressure. RC guys say that tough shrink wrap or even cloth tape is sufficient. Others have said that a hard surface with higher pressure similar to the LiFePo4 cells is better.

If LiPo expands with any significant force, the cells are already dying. The concern shouldn't be about starting a fire due to the pressure, it should be the pressure being that high!

 

[Instigater]

Thank you for quick response! Then I'm building container directly on top of packs.

 

[ltosolini]

How can it be explained why the LiFePo4 cell excersise pressure at high current?
Is this related to charge, discharge or both?
Thanks,
Luca

 

[Sunder]

I don't know the exact physics and chemistry, but obviously for the power to be generated, a chemical reaction has to happen. In this case, from what I've read, during a more rapid form of the chemical reaction (i.e. high current), the transfer of ions can be so rapid, delamination can occur - the chemical layers can separate, meaning a loss of capacity in the future. Keeping the layers within the cell pressed together with a small pressure, can keep the two layers in contact with each other, preventing the capacity loss.

 

[Hillhater]

The cell electrolytes have volotile components that "off gas" if they get too hot...such as high discharge rates or over discharging them . Those gasses cause the internal pressure.