[dtbaker]

I am wondering why the LiFePO4 cells are irrecoverable after over-discharge. I've had a few fail (all my fault for not stopping when pack voltage started dropping).

Both times I've had a cell fail, it has been just one cell in the whole pack, but once its dead, its really dead and cannot be charged, or kickstarted with a power supply or even a completely dumb external power source like a 12v battery with no controls.

any guesses as to what happens inside the cell? makes me want to take it apart to see, but not enough to actually do it.

It does beg the question though as to how low a voltage a cell can be run until it becomes irrecoverable. and why? i.e does it make a difference if it happens under moderate load while driving, or very gradually due to self discharge or parasitic discharge from controllers while sitting in a garage without periodic charging?

 

[Kevin Sharpe]

any guesses as to what happens inside the cell?

I think it's dendrites that finally 'kill' the cell... here's some interesting background reading;

Preventing Dendrite Formation in Lithium Batteries

What are dendrites, and why do they cause fires in lithium batteries?

 

[dtbaker]

I think it's dendrites that finally 'kill' the cell... here's some interesting background reading;

Preventing Dendrite Formation in Lithium Batteries

What are dendrites, and why do they cause fires in lithium batteries?

Dendrite formation looks to be primarily over time, and more commonly with Li-ion. My question is specificly related to LiFeSO4 cells (like CALB or Thundersky) suffering from over-discharge while under load.....

This seems to be a catastrophic failure, no flames or anything, but certainly kills the cell in one 'event' and they do not seem to be recoverable. I'm wondering if something inside fuses, or open circuits, or what?!

 

[john61ct]

No, just physics, rather nature of the chemistry.

Seems crazy to go so bare cell after multiple such events, you don't even have LVD protection?

Personally I'd use an AH counting BM, start alarms at 20%, absolute cutoff at 6-8%.

Obviously only stands a chance of accuracy if reset at every cycle's 100% point via measured endAmps.

LVD can then act as a failsafe backup, if only using pack level, cut off when overall voltage gets below say 2.85Vpc average

If monitoring cell levels, when **any** of the cell voltages get <2.7V

 

[dtbaker]

No, just physics, rather nature of the chemistry.

Seems crazy to go so bare cell after multiple such events, you don't even have LVD protection?

Personally I'd use an AH counting BM, start alarms at 20%, absolute cutoff at 6-8%.

Obviously only stands a chance of accuracy if reset at every cycle's 100% point via measured endAmps.

LVD can then act as a failsafe backup, if only using pack level, cut off when overall voltage gets below say 2.85Vpc average

If monitoring cell levels, when **any** of the cell voltages get <2.7V

while I do not have BMS at the cell level, I do rely on both the controller, and an ah meter to monitor at the pack level. The Zilla has decent LV parameters for both warning and limp-home voltages, and I do reset my ah counter after charging so I have a decent idea what's going on....

There are challenges to this approach to be sure. as the pack ages and sags more under load, especially in cold temps. What I ended up doing over time was moving the LV 'warning' down to avg of 2.0v/cell so the controller wasn't clamping down on me during sag under heavy load. This means not a lot of time between when pack voltage starts dropping and when it falls off the cliff. The ah meter is pretty good in the summer... but knowing how to 'de-rate' for cold temps is always a guessing game. For instance my cells in the eMiata are 130ah, and I generally avoid going below 100ah consumption like the plague. In the winter however, empirical evidence suggests to me than 70-80ah would be a lot safer

twice now in 8 years I knowingly pushed too far into the 'limp-home' range, and have paid the price both times. I was stuck too far from the nearest plug in rural NM, and risked it when I should have pulled into a strangers home and begged for a charge.

All that aside, my original question is not how to avoid cell death, I know how to do that.... just avoid over-discharge.

My question is what happens physically that makes the cell non-recoverable.

 

[Kevin Sharpe]

Mechanism of the entire overdischarge process and overdischarge-induced internal short circuit in lithium-ion batteries

 

[pdove]

Hey Dan,

A LiFESO4 battery is a Li Ion battery since it still works by transferring Li Ions between terminals to produce current.

They are correct. Lithium-ion batteries operation at low-temperature and charge cause lithium plating, which can deplete the cell capacity, diminish cycle life and may lead to dendrites that can pierce the separator causing shorting within the cell safety incidents such as fire.

Rapid Lithium plating in Lithium Cobalt Oxide cells begins at voltages over 4.17 Volts. Not sure what it is for your chemistry.

My advise is to use voltage (high and low) to cut off the power to the charger or car instead of using Amp Hours. Voltage is what kills a cell.

Most commercial vehicles will shut off based on a cell being too high or a group of cells but not the entire pack.

If you don't monitor cell voltage then drop your pack charge voltage a little at least in the winter. I wouldn't use amp hours to control charger or low voltage shut off.

 

[boekel]

Please read Kevin's linked document.

what happens is a reversed voltage over a discharged cell, this is what damages it. low voltage only is not good, but doesn't destroy a cell at once.

 

[dtbaker]

Please read Kevin's linked document.

what happens is a reversed voltage over a discharged cell, this is what damages it. low voltage only is not good, but doesn't destroy a cell at once.

I did read it.... but the article didn't really cover what HAPPENS when over-discharged under load, and it was more for computer and phone battery chem, not large format prismatic LiFeSO4 cells like calB, Thundersky and the like which we use in our cars.

I'm guessing that yes, they short, but I'm wondering if there would be any way to open them up and repair/recover.....

 

[pdove]

I did read it.... but the article didn't really cover what HAPPENS when over-discharged under load, and it was more for computer and phone battery chem, not large format prismatic LiFeSO4 cells like calB, Thundersky and the like which we use in our cars.

I'm guessing that yes, they short, but I'm wondering if there would be any way to open them up and repair/recover.....

Well I am speaking from experience. It the voltage goes too low they will short. I have done it. No there is no way to recover it. I cut one open and copper sheet were coated with a black substance.

 

[dtbaker]

Well I am speaking from experience. It the voltage goes too low they will short. I have done it. No there is no way to recover it. I cut one open and copper sheet were coated with a black substance.

the black goo I think is the Li electrolyte... I'm just super curious to know if the shorting is between the copper/AL foils, or perhaps just at the top where they all get collected at the terminals.

secondary question is whether or not there is any place we can or should ship dead cells to for recycling LiFeSO4 cells?

 

[boekel]

I did read it.... but the article didn't really cover what HAPPENS when over-discharged under load, and it was more for computer and phone battery chem, not large format prismatic LiFeSO4 cells like calB, Thundersky and the like which we use in our cars.

This paper investigates the entire overdischarge process of large-format lithium-ion batteries by discharging the cell to −100% state of charge

Cells are connected in series in actual battery packs, and a cell voltage lower than 0 V can occur when a cell is overdischarged.

when a cell is empty, and the rest of the cells are not, the voltage over the empty cell reverses because all the current has to flow through this cell. This is what kills them fast. And hopefully painless.

 

[pdove]

the black goo I think is the Li electrolyte... I'm just super curious to know if the shorting is between the copper/AL foils, or perhaps just at the top where they all get collected at the terminals.

secondary question is whether or not there is any place we can or should ship dead cells to for recycling LiFeSO4 cells?

The three primary functional components of a lithium-ion battery are the positive and negative electrodes and electrolyte. Generally, the negative electrode of a conventional lithium-ion cell is made from carbon. The positive electrode is a metal oxide, and the electrolyte is a lithium salt in an organic solvent.

I believe this deposit to be the carbon pulled through the separator by high current from over discharge. I am sure it’s reacted and formed some mixture of carbon and lithium salt.

You could cut them open and separate the aluminum and copper sheet and recycle them at a metal recycling plant. The amount of lithium is very small and not worth anything.

 

[john61ct]

Note that the term "lithium ion" covers dozens of widely differing chemistries, LiPo, NMC, etc.

If your attempts to satisfy OP's theoretical curiosity are specifically about LFP, then please say so.

There may be some general statements that can be made about all LI types, but I doubt that is true for the specific details.

 

[pdove]

My experience is with LiFePO4 (CALB Bestco and HiPower) and LiCoO2 (18650}.
Although I believe the failure mechanisms are universal.