Compressing a swollen LFP battery

[billbrach]

I did a bad thing. I have 16 new 280AH CATL cells from Docan (Thanks Lily Zhang!!). after top balancing all 16 batts, i decided to load test 4 batts out of the bunch.

First battery made 301 AH, with a cutoff voltage of 2.5V. On the second battery, the damn cutoff voltage was somehow set to ZERO, instead of 2.5V !! NO INSTRUCTIONS come with these inexpensive electronic load testers, and there are subtle differences between brands, and how they operate.

After about 20 hours on the load tester at 20A constant current, I noticed the batt voltage was 1.5V instead of 2.5V and it has still NOT shut off !! It's then I noticed the cutoff was set for ZERO, instead of 2.5V. Sorry for the long story, thought it might help answer the question.

So, I'm recharging this individual cell, back to about 3.3V.

Several questions:

1. Will the case reflatten itself, as it recharges ?? I suspect the answer to this is NO. It is aluminum, it has deformed.

2. If the case will not reflatten itself, can I slowly "clamp" the battery back to having flat sides ?? After all, it is an aluminum case, and I'd think it would be possible to reflatten it, by clamping.

3. If I try to reshape the case by clamping, should this be done fully charged or discharged ?? I'd guess discharged, as there is less energey in the batt in case something shorts internally.

4. Will this over-discharge affect the life or capacity of this single cell ?? I would guess YES.

Incidentally, I will ask these same questions of Docan too.

Thanks for reading all of this and thanks for any inout !!

Bill

 

[RCinFLA]

It takes time for overdischarge damage to happen (days). Just charge at a low current rate until cell gets above 3.0v then you can increase charging current.

You did damage some of electrolyte. Electrolyte is stable from about 0.1v to 4.3v of cell voltage. The bloat will eventually diffuse out the plastic terminal gromets. May take a few weeks. Don't try to force it excessively as you can pop the vent plug with too much pressure. A little pressure is okay to get metal to flex back in shape as pressure is relieved.

The bloating is primarily just the surrounding metal case. The actual cell wrap is open at top of wrap and gases escape top of wrap into metal container cavity.

Gas bubbles in the laminate wrap will eventually work their way to the top and escape into metal case cavity. The electrolyte has viscosity similar to diesel fuel, so it can take a little time for bubbles to fully migrate out of wrap. A gas bubble creates a separation gap between graphite and LFP electrodes so no lithium ions will flow across the bubble gap causing a slight increase in cell impedance until bubble migrates out of laminate.

When manufactured, cell electrolyte is injected while holding cell cavity under a vacuum to fill the laminate without leaving too many bubbles remaining. During the electrolyte injection process some electrolyte may run over top of cell wrap down the side. You will sometimes hear some of it sloshing in metal case bottom.

The physical bloating is not the primary degradation due to electrolyte breakdown. Other products of the chemical breakdown of electrolyte are tar-like hydrocarbons that remain as blocking coating on graphite and LFP electrodes of cell. The tar coating creates some blocking resistance to lithium ion flow.

 

[billbrach]

Cell voltage did not go below 1.5V. So does than mean the electrolyte is still OK ??

Also, the cell did NOT vent, so I'm guessing this is a good thing too.

And, I *think* you are saying a can re-clamp it back to a flat state, but I ought to do that slowly over a couple of weeks, a bit at a time ??

 

[RCinFLA]

If cell did not go below 1.5v I highly doubt you would have any cell bloating. Manufacturers give a 2.5v min voltage discharge spec to have some margin to 1v which is really where damage can happen. There is very little cell capacity between 2.5v and 1v so it does not take much more discharge to go from 2.5v to 1v. Just cell self-discharge rate can take a cell from 2.5v to below 1v in a couple of weeks.

1.5v will not cause any damage to cell as long as recharged soon and not left at that state of charge for days.

Leaving a cell at a very low state of charge for days will grow lithium metal dendrites which can short out cell and cause some corrosion of copper anode foil surface increasing cell resistance.

If you pop the vent port due to overpressure you open the case to atmosphere. Eventually the electrolyte solvent will evaporate out.