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LFP overdischarge conflicting voltage limits

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Hans Kroeger

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Hi there,
Quote: Neither ISCr nor capacity fading occurs if the overdischarge is terminated before −12% SOC, where the first platform after the minimum voltage is located.
Unquote
I found this statement here: https://www.nature.com/articles/srep30248

From the above I concluded, that overdischarging an NMC Litium cell all the way down to zero volts does not harm the cell.
Wat about LiFePO4 cells?
Searching for papers I found only statements like: discharging LiFePO4 cells below 2 Volts will result in permanent damage.
I took a Headway LiFePO4 cell, discharged it to 0 Volts, left it there for a while, then recharged it. I could not find any loss of capacity afterwards.

So my question is: what is the effect on a LiFePO4 cell when overdischarging it to 0 Volts.?
thanks for any answer,
Servus Hans
 
Hans Kroeger said:
Hi there,
Quote: Neither ISCr nor capacity fading occurs if the overdischarge is terminated before −12% SOC, where the first platform after the minimum voltage is located.
Unquote
I found this statement here: https://www.nature.com/articles/srep30248

From the above I concluded, that overdischarging an NMC Litium cell all the way down to zero volts does not harm the cell.
Wat about LiFePO4 cells?
Searching for papers I found only statements like: discharging LiFePO4 cells below 2 Volts will result in permanent damage.
I took a Headway LiFePO4 cell, discharged it to 0 Volts, left it there for a while, then recharged it. I could not find any loss of capacity afterwards.

So my question is: what is the effect on a LiFePO4 cell when overdischarging it to 0 Volts.?
thanks for any answer,
Servus Hans
Click to expand...
Hello Hans, this question is similar to your BMS over voltage limit question.
I'll give you my opinion then everyone can disagree with me then you can make up your mind. lol

I have never discharged a cell to zero volts or 2V. But I think your confusion has to do with resting voltage again.
If you were able to get the cell down to a resting voltage of 2V, the cell would probably be damaged.

When you say you discharged to zero volts, if you would have stopped discharging, and waited for a long time, my guess is that the cell would have settled back up above 2.5V

That is just my opinion and like I said I never tried it.
 
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Most people charge them up quickly when they go below 2.5V.
So, if you test my theory it might damage your cell.

Even though you did it once without damage, it really doesn't make any sense to go below 2.5V because there aren't many Ah left.
You'd be lucky to get one full Ah from 2.5V to zero volts.
 
Do you have a bms without a LVD that would allow you to go to zero volts?
 
Hans Kroeger said:
Hi there,
Quote: Neither ISCr nor capacity fading occurs if the overdischarge is terminated before −12% SOC, where the first platform after the minimum voltage is located.
Unquote
I found this statement here: https://www.nature.com/articles/srep30248

From the above I concluded, that overdischarging an NMC Litium cell all the way down to zero volts does not harm the cell.
Wat about LiFePO4 cells?
Searching for papers I found only statements like: discharging LiFePO4 cells below 2 Volts will result in permanent damage.
I took a Headway LiFePO4 cell, discharged it to 0 Volts, left it there for a while, then recharged it. I could not find any loss of capacity afterwards.

So my question is: what is the effect on a LiFePO4 cell when overdischarging it to 0 Volts.?
thanks for any answer,
Servus Hans
Click to expand...
tru guys like you we will know more so i will say do it a couple of times all the way to 0v and see where you end up.
please keep a post on your experiment
 
There isn't any value in discharging that low anyways. What equipment will run at voltages under 2.5Vpc?
 
I made my first test power wall out of lifepo4 cells that came at near zero voltage. They worked amazingly well. They are pretty hardy cells.
 
Luthj said:
There isn't any value in discharging that low anyways. What equipment will run at voltages under 2.5Vpc?
Click to expand...
Thank you all for your replies!
The concern is, that Cell Modules (Protection and Balancing) dissipate power after an LVP disconnect. After an LVP disconnect, the remaining Ah in the cell is very low, as we all know. In such a case it can happen that this small current will overdischarge your cell, down to zero Volts, if the Battery is left unattended.
Let's take a 100 Ah Battery. Let's also assume that the 100 Ah cells have 0.2 Ah charge left, when being disconnected at 2.5 Volts. Popular cell modules being produced in Australia consume 0.3 mA at voltages below 2.5 V. It takes less than 30 days for a 0.3 mA load to discharge 0.2 Ah. It takes much less! time to get down to 2.0 Volts. A BMS module of the same manufacturer consumes 24 mA after the LVP disconnect. It takes less than 10 hours to overdischarge the cell down to 0 Volts.
I guess this explains why it is important to understand what happens to LiFePO4 cells if being overdischarged.

Summary:
A) if LiFePO4 cells must not be overdischarged beyond 2 Volts, you should
1. check the total current of your BMS (electronics + cell modules), and
2. set the LVP disconnect voltage level as high as your max current and lowest temperature allows.
B) If LiFePO4 cells can be overdischarged down to 0 Volts, it doesn't matter how much current your BMS consumes after an LVP disconnect. However, it might be a good idea to recharge your empty cells first with a very low current up to 2.5 Volts, before you switch on your normal charger.

After all this writing I hope, to get from you reliable data concerning this subject.

Thank you all,
Hans
 
I know of a couple cases where BMS with passive current consumption (mA range) discharged cells below 2V, where they sat for months. In both cases the cells would not recharge to normal capacity. In once case the cells bloated badly, and may have vented.

The best option if passive BMS consumption is a risk, is to configure the low voltage disconnect to activate with some capacity left in the cells. Ideally enough to handle the BMS needs.

The other option is to use a BMS that doesn't use cell level modules, and thus can be easily turned off completely.
 
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Edit: Here is a research paper on very low states of charge. They conclude long periods of storage below 2.0V causes capacity loss.

www.nature.com

Transportation Safety of Lithium Iron Phosphate Batteries - A Feasibility Study of Storing at Very Low States of Charge - Scientific Reports

In freight classification, lithium-ion batteries are classed as dangerous goods and are therefore subject to stringent regulations and guidelines for certification for safe transport. One such guideline is the requirement for batteries to be at a state of charge of 30%. Under such conditions, a...
www.nature.com www.nature.com

"Capacity dropped by more than 35% after 30 days of storage at 0.5 V, which posed a safety risk"


" In contrast, the 2.0 V storage results exhibited a marginal increase in storage capacity post 30 days, rising to a capacity fade of 7.1%. After 90 days of storage at 2.0 V the final capacity fade was 11.1%. Cells stored at 2.3 V exhibited capacity increase after 15 days, although within the error bar (due to cell to cell variation). However, there was a confirmed overall rise of 2.6% in cell capacity after 90 days for all three cells stored at 2.3 V."


"The Ohmic resistance (Ro) includes both ionic resistance of the electrolyte and electronic resistance of the electrode and current collectors28,29,30,31. Under low voltage storage, it is reported that the copper current collector reacts with electrolyte components resulting in corrosion16,17,18, leading to higher Ro"

"More specifically, when cells are stored at low voltages over an extended period, the copper current collector attached to the carbon electrode is oxidised to Cu2+ and dissolves into the electrolyte14, 19, 32. The subsequent reduction in contact between the current collector and active electrode material manifests as an increase in Ohmic resistance. Although previous studies such as that of Jeevarajan et al.33 reported slight increases of cell resistance under over-discharge conditions, the onset and rate of copper dissolution is cell specific."

"Under over-discharge conditions, over-deintercalation of lithium at the negative electrode can cause decomposition of the solid electrolyte interphase (SEI). When the cell is re-charged, new SEI film forms on the graphite anode. The growth of the SEI film leads to degradation of the electrochemical charge-transfer processes at the electrode-electrolyte interface15, 34, 35."

"Furthermore, the decomposition of SEI leads to gas generation at the negative electrode. The generation of gases, typically CO2 and CO, cause swelling within the cell14 and consequently a resistance rise due to electrical contact loss thorough delamination."
 
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Some other interesting bits from that paper.

"Guo et al. found that the dissolution of SEI occurs within 0 to −10% SoC, severe copper dissolution then occurs below −12% SoC, with severe internal short circuiting occurring at or lower −20% SoC19. For the batteries used in this study, 0.5 V corresponds to −1.9% SoC and as such, in agreement with Guo et al.19, only SEI dissolution and gassing occurs."

The good news from this, is that while its very undesirable to discharge the cell below 2.0V, major damage between 2.0 and 0.5V takes a fair bit of time. If the cell is promptly brought back up, then capacity loss can be mitigated.
 
Luthj said:
Some other interesting bits from that paper.

"Guo et al. found that the dissolution of SEI occurs within 0 to −10% SoC, severe copper dissolution then occurs below −12% SoC, with severe internal short circuiting occurring at or lower −20% SoC19. For the batteries used in this study, 0.5 V corresponds to −1.9% SoC and as such, in agreement with Guo et al.19, only SEI dissolution and gassing occurs."
Click to expand...
This comes close to the paper I was refering to in my original posting. What I am trying to understand is the statement, that 0,5 Volts is equivalent to -1.9% SOC .
Still busy reading this paper.......
 
Hans Kroeger said:
What I am trying to understand is the statement, that 0,5 Volts is equivalent to -1.9% SOC .
Click to expand...

If the cell has 100AH, then to get from 2.5V to 0.5 requires an additional 1.9AH of power to be removed from the cell.

Obviously this will vary with each specific cell. In order to get to the rapid damage zone of -20% SOC, 20AH would need to be removed from the cell. This may only be possible if the cells which remain in the pack reverse charge the first cell to reach 0V. At that point the damage gets pretty extreme.

Though at 0.5V capacity loss seems significant at 1% per day.
 
Luthj said:
If the cell has 100AH, then to get from 2.5V to 0.5 requires an additional 1.9AH of power to be removed from the cell.

Obviously this will vary with each specific cell. In order to get to the rapid damage zone of -20% SOC, 20AH would need to be removed from the cell. This may only be possible if the cells which remain in the pack reverse charge the first cell to reach 0V. At that point the damage gets pretty extreme.

Though at 0.5V capacity loss seems significant at 1% per day.
Click to expand...
Hi Luthj,
when I revisited the paper in my original posting, it confirms, what you are saying.
Thanks a lot for your help and patience!
Cheers Hans
 
So, it seems like you need to stop discharging below 2.5V
Can't you set your charge controller to stop discharging at 3V so you have some power left until you charge again?
 
ArthurEld said:
So, it seems like you need to stop discharging below 2.5V
Can't you set your charge controller to stop discharging at 3V so you have some power left until you charge again?
Click to expand...
At high currents and low temperatures 3 Volts is much too high for my application. 2.7 Volts is the maximum disconnect Voltage in my case. That was the reason for me to start the discussion.
 
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