I accidently deep discharged my LiFePo4 prismatic cells

[merlin077]

Just a quick update: 8 out of 16 cells are charged to 3.2V now without any issues. The internal resistance meter arrived and all 8 cells are measuring 0.40mOhm each. Doesn‘t sound too bad?

Edit: The 8 uncharged cells rest at 2.1V and also measuring 0.40-0.44mOhm each…

 

[SteveCA]

My guess is the Heltec Baöan

I use Heltec balancers on my builds and have ever had an issue.

 

[merlin077]

So, all 16 cells are charged up to 3.2V and i‘m currently charging the first cell full at 1.2A Had to reset the iMax charger because it stops at 10 hours.

The swell didn‘t got worse, also the internal resistance measures the same. I am curious how much capacity is left in the cells

 

[Tim Tim]

Just curious, did you raise the voltage in steps and if so what voltage did you start charging at?

I have just collected my RV after 4 months and both my two 208Ah batteries have discharged. One to 10.8v and the other 9.8v with plenty of cell divination and the lowest cell down to 2.344v. I only had the Dally BMS connected with the inverter and solar controller both turned off.

When I left the van both batteries were at 13.3v with all cells at the same volatage. Idrive back home today so wil recharge them tomorrow.

 

[merlin077]

Just curious, did you raise the voltage in steps and if so what voltage did you start charging at?

I have just collected my RV after 4 months and both my two 208Ah batteries have discharged. One to 10.8v and the other 9.8v with plenty of cell divination and the lowest cell down to 2.344v. I only had the Dally BMS connected with the inverter and solar controller both turned off.

When I left the van both batteries were at 13.3v with all cells at the same volatage. Idrive back home today so wil recharge them tomorrow.

Hey, sorry for the delay!

Initially i set the power supply to 2.5V and 0.2A and let it rest until current drops. Then I ramped up the voltage to 3.2 and 0.2A, same deal… After the current draw reduces, i set the power supply to 3.5V and betweeen 0.8 and 2.5A current to charge them fully.

The first cells were charged with quite little power, which obviously takes forever. After i was more confident that nothing will catch fire, i used more charge current

Between the charging phases I took regular swelling measurements with a caliper and monitored the internal resistance. The swelling went back a few mm, which is good i think But still pretty obvious.

Next step is to discharge one of those cells with a 10A programmable load and hope we get a usable amount of energy out of them without setting something on fire…

 

[Tim Tim]

No need to apologise as it seems like you have enough on your plate. Hope it all works out for you.

I need to do some tests on the Daly BMS to see how much they drain the batteries and why they didn’t shut down when reaching low voltage. At 0.2A it will take forever on yours. As mine are 280Ah batteries I just put 2A in to take the individual cells up to 2.5v and then raised it to 5A to take them up to 3.2v. Next I will put the cells in parallel to take them up to 3.6v.

With the one battery, although it was only down to 10.8v the delta was 900mv. I put 12A of solar into it and was pleasantly surprised to see the BMS balancing and at the end of the day the battery sitting at 13.2v with a 7mv delta. After seeing such a variation in the cell voltages of both batteries I think I will top balance them again.

 

[nt40lanman]

That's very encouraging. You may still get some usefulness out of them.

Tim Tim, in the automotive biz, 200ma or 0.2a is too much draw. I would imagine it's the same here.

 

[merlin077]

Just a quick test of the chinese 150W load. Whoever wrote that „UI“ for the frontend must have a faible for Acid while at work ? Unfortunately it doesn’t provide RS232, so no logging and nice graphs.

And yes, i need proper cabling, because of the BBC quite significant voltage drop. Those flimsy gauge wires are not made for 10A continous power draw

The whole experiment will take place in the garage for three reasons:

The fan is quite a bit annoying, i don‘t wanna burn the house down in case something begins to die and last but not least there is a good motivation to clean up the garage

I think 10A as test current will be suitable as it‘s well within spec. If something goes wrong at this low current, then better under controlled circumstances.

I‘m really really curious how much capacity the cells can deliver!

 

[RCinFLA]

Just an fyi, 1 kHz battery impedance meter is not the best indicator of ionic migration resistance, which is primarily what electrolyte decomposition effects.

Mostly what you are measuring with a 1 kHz impedance meter measurement is conductive resistance of cell (mostly metal interconnects). The most prevalent damage item the 1 kHz impedance meter will show up is increased resistance due to electrode material delamination from the copper foil current collector on the negative graphite anode side and LFP cathode material from the aluminum foil current collector on the positive electrode side.



A little bit of bloating is not a disaster. Cells bloat during manufacturing charge forming process, but you typically do not see it because the manufacturer puts the pressure release port in after charge forming so the gas is allowed to escape before the cell is finally sealed up.

The visual gas bloating is not the really damaging part of electrolyte decomposition. The bad part of electrolyte decomposition is the hydro-carbon tars that are also biproducts of the electrolyte decomposition. These tars gum up the internal cell electrode material pores reducing the paths for lithium-ion migration. This results in a higher overpotential voltage (terminal voltage slump during discharge and terminal voltage bump during charging) necessary to get the lithium ions to move through the cells at a rate necessary to support the external demanded terminal current on the cell.

When you dissect a cell that has been severely overcharged with extreme bloating, when you unwrap the layers you will see the material which should be a uniformly flat black graphite colored electrode with a brownish haze color over it that is the hydro-carbon tar coating the electrode.

The cell wrap does not actually bloat. The top of the cell wraps are open to allow foil conductors to escape so gas just leaks out the top of the wrap laminates into the sealed outer metal container of cell. All you see is the bloated outside container. Usually, the bloating gas will eventually diffuse out through plastic terminal grommets in the metal casing. It may take a few weeks to a few months to totally deflate. You never want a burst pressure port however. That will allow electrolyte to eventually evaporate from cell.

 

[RCinFLA]

If I read the load test screen correctly it looks pretty bad with 3.13v and only 51 AH's out of cell at 6 amp load. Problem you have is the tester does not have remote voltage sensing, so the voltage drop on the wires are significant to what the tester reads for cell voltage. Check voltage at battery terminals with a DVM compared to what tester says. You will likely find at the cell terminals the voltage will be significantly higher voltage.

I use a DL24P with four wire connections to bring remote cell voltage measurement directly to cell terminals and prevent the load current carrying wires voltage drop from impacting cell voltage reading on the tester.

6.2 amps discharge rate is not enough to show much overpotential voltage slump on a 105 AH cell. You really need at least 20 amps discharge rate for a good test on cells. I have modified my DL24P with two more MOSFET load/heat sinks to achieve 50 amps load test for 280 AH cells.

This graph is for a new 280 AH EVE cell but you can scale it by C(A)%. 105AH/280AH load current for same terminal voltage slump. A cell will increase in overpotential slump terminal voltage by 3x-5x over its lifetime.

 

[Hans Kroeger]

If they keep a charge over time, and stay at a pretty constant voltage after settling, they could be fine. LiFePO4 is, for a lithium battery, quite forgiving. I would still go through the trouble of testing each and every one of them for capacity. Also, the cells that are bloated: you can try to slowly charge them to 30% or thereabout, then disconnect them and let them sit for a few months. I've noticed they can de-bloat substantially over time.

That said, getting a cell below 2.5V is not good at all and you should probably consider them very damaged. Don't keep them in the house; put them in a shed in a metal box or some other place where if something catastrophic were to happen, they can't cause more damage.



That's why I'm a fan of the JK BMS: it can shut itself off when cells get below a certain value preventing draining the cells to power the BMS.

I am confused: I know that my findings are contrary to all experts opinions. A while ago, I have discharged a Headway 15 Ah LiFePO4 cell down to 0 Volts, kept it short circuit for mor than 12 hours. After recharging the cell it seemed to be OK, no loss of capacity.
Recently I replaced 24 Winston 100 Ah cells, which were left discharged for more than a year at < 1 V.......
After replacing them by new ones, I checked 8 of them, all of which were around 110 Ah capacity. They showed no increase of internal resistance...
Last week, I received 4 Elerix 100 Ah LiFePO4 cells, which were discharged below 1 Volt. A customer had returned them to the supplier, because of "unknown" problems. I charged these cells, starting with 1 Amp, slowly increasing to 40 Amps.
The final Tests revealed a capacity of 107 Ah. Again, they showed no increased internal resistance.
I remember reading a scientific paper, which explained that LiFePO4 cells will be damaged only if they are operated with reverse polarity of 1.2 V or more.
My conclusion ist, that LiFePO4 cells may be discharged below 1 Volt without "damage". However I have no idea If these cells may impose an increased risk of short circuits by dendrites.

 

[upnorthandpersonal]

Just because it can cause damage, doesn't mean it always does or is always immediately noticeable. Some effects are only visible over longer periods of time. For example, when you over discharge, you start to form copper metal dendrites.

Radware Bot Manager Captcha

This leads to a shortened lifespan.

 

[Hans Kroeger]

Thank you.
My question is:
what is the worst case of damage resulting from such a short . Would you recommend to stop testing this kind of cells in my testlab?
Would you consider further tests to be of an acceptable risk, but recommend not to use these cells in an RV or boat?
Your opinion is highly appreciated, thanks!
Hans

 

[Hans Kroeger]

I am confused If I read the paper you refer to "radware bot Manager Captcha", I am questioning how to overdischarge by 120 %. From my experience one cannot discharged an empty cell (at 2.5 V) by another 20 %......

 

[upnorthandpersonal]

what is the worst case of damage resulting from such a short

Due to dendrites? Dead battery cell. They're not exactly low resistance shorts, so nothing spectacular, but they will puncture the separator and make the cell unusable.

Testing this is fine, but I wouldn't want to use them in a position where you rely on them in the middle of the ocean or something. If a cell breaks in a multi-parallel setup, one can just replace the cell(s) - but this might be hard to do on the road/sea.

I am confused If I read the paper you refer to "radware bot Manager Captcha", I am questioning how to overdischarge by 120 %

They likely use that % number in reference to the cell voltage, not capacity. Also, the title of the paper is "Failure Investigation of LiFePO4 Cells in Over-Discharge Conditions" - the captcha thing is just the 'title' generated by the site when direct linking to the content.

 

[Hans Kroeger]

They likely use that % number in reference to the cell voltage, not capacity. Also, the title of the paper is "Failure Investigation of LiFePO4 Cells in Over-Discharge Conditions" - the captcha thing is just the 'title' generated by the site when direct linking to the content.

 

[Hans Kroeger]

Thanks again for your answer, very helpfull.
Concerning the paper you mentioned I checked the original study. As far as I unterstand they discharged the cell down to 2 Volts. Then they further discharged the cell and finally they applied a reverse voltage to push a Charge of up to 20% Ah total through the cell......
This case may happen when you discharge a battery of several cells, one of which is empty, the others are charged. Ok! These other cells can push the empty cell to a reverse voltage.
My point is dealing with a single cell being discharged through a small load current down to a voltage below 1 Volt. I have seen no tests to prove that the cell will suffer from permanent degradation. IS there anyone wo can point me to paper dealing with this Subjekt?
Thank you for an answer, Hans

 

[upnorthandpersonal]

https://www.sciencedirect.com/science/article/pii/S0378775316316998