Capacity test resulted in the BMS cutoff with 1 cell at 2.5v and the rest at 3v?

[MilkMan]

I have 8 new aluminum case 120ah lifepo4 cells for a 24v system that I bought off Aliexpress. (Same seller that Will recommended for another size) They arrived at 3.25v. I paralleled all 8 batteries and put them on a charger a 3.65v with the goal of top balancing before performing a capacity test. After waiting a week I put them in series and charged the pack to about 28v. After the charger finished I swapped them back to parallel at 3.65v for about a day.

The pack was still charging at 1.2 amps when I removed the charger. I think my power supply voltage reading may be inaccurate as each cell was showing about 3.67v. There's a little swelling on most cells and one cell as wide as 1/8". The pack in series read 29.3v. I read something the other day after the fact that said I should not charge such a large pack at such a lower current because of possible over charging which is what I unfortunately did.

I performed a slow capacity test pulling 80 to 100 watts until the BMS cut off at 2.5v. I did not add the BMS until after the above balancing. 1 cell reached 2.5v and the rest were about 3v when the battery cut off. The cell at 2.5v is the cell 1/8" swollen. With that said, my capacity test resulted in 122ah. I've had the pack back on the charger for about 50 minutes and all cells are 3.24v and rising with only a 0.008v difference. (BMS still attached) The power supply/charger is CC/CV.

As someone very new to DIY battery building and with a plan to install the battery in my RV, should I be concerned about the 1 cell? I've attached a picture of the cells and some of my gear. The left 2 most batteries are the most swollen with the low 2.5v reading being the far left/positive terminal. Thank you for the input!

EDIT: Here's a high quality image upload. https://i.imgur.com/iBhcxXm.jpg

 

[Luthj]

So the cells swelled after the top balance? Is it possible you accidentally pushed the cells past 4V?

It sounds like the one cell has lower capacity than the others. But all of them delivered more than 120AH. For longevity sake I would not discharge to 2.5Vpc.

 

[MilkMan]

So the cells swelled after the top balance? Is it possible you accidentally pushed the cells past 4V?

It sounds like the one cell has lower capacity than the others. But all of them delivered more than 120AH. For longevity sake I would not discharge to 2.5Vpc.

Yes, this was my first time charging the pack. The first 2 definitely swelled some from the top balancing. There may have been a very small bulge when they arrived but hardly noticeable. They did reach full capacity discharge which I was glad to see. My current plan is to set the low voltage cut off at 24v for regular use. Down to 2.5v was for capacity testing only.

 

[Ampster]

I would not worry about it if your pack capacity is not affected. I would clamp that pack to take the strain off of your terminals.

 

[Dhopewell]

I’m going to develop a reputation as a curmudgeon but this issue around low voltage limit keeps coming up and I want to understand why. For EVE LF280 cellls, the big prismatics which are prevalent on this board, the charge cycle for determining a 4000 cycle lifespan is 3.65 to 2.50v at 1C discharge and 25C temp. Full to empty in two hours at room temperature. The manufacturer states that cycle provides the full rated life. It says nothing about what happens if one cell in a pack is low like your case, but if that cell stays at or above 2.5v at all times its voltage will not reduce life expectancy. The mismatch may make it hard to stay in that pack but plenty of people around here use old batteries successfully so thee must be a way.

So don’t worry, at least about the good cells. You can cycle the crap out of these things, they are built for it and after using 100% of capacity everyday for the next 11 yrs you’ll still have 80% capacity! I paid 200 for a starter battery in my truck that will last 5 years working for 20secs a day, while I can rig 12v lifepo with bms for $500 and move over 3kwh in and out every day for over a decade. These batteries rock! Just stay in bounds, that’s all. Maintain 2.5v. or above but especially below 4.2v and/or 55C temperature. These boundaries are very important!

If you have a bad cell you’ll need to fix it, but before you give up know I puffed a cell and antagonized two others but they are still playing well in the pack at the right voltages, without a bms, and Im drawing them down slowly to 2.625v until they are ready to do a 0.2C charge with my inverter, where I’ll test the cell balance as well as possibly the high voltage cutoff. Good luck

 

[DeeK]

I’m going to develop a reputation as a curmudgeon but this issue around low voltage limit keeps coming up and I want to understand why. For EVE LF280 cellls, the big prismatics which are prevalent on this board, the charge cycle for determining a 4000 cycle lifespan is 3.65 to 2.50v at 1C discharge and 25C temp. Full to empty in two hours at room temperature. The manufacturer states that cycle provides the full rated life. It says nothing about what happens if one cell in a pack is low like your case, but if that cell stays at or above 2.5v at all times its voltage will not reduce life expectancy. The mismatch may make it hard to stay in that pack but plenty of people around here use old batteries successfully so thee must be a way.

So don’t worry, at least about the good cells. You can cycle the crap out of these things, they are built for it and after using 100% of capacity everyday for the next 11 yrs you’ll still have 80% capacity! I paid 200 for a starter battery in my truck that will last 5 years working for 20secs a day, while I can rig 12v lifepo with bms for $500 and move over 3kwh in and out every day for over a decade. These batteries rock! Just stay in bounds, that’s all. Maintain 2.5v. or above but especially below 4.2v and/or 55C temperature. These boundaries are very important!

If you have a bad cell you’ll need to fix it, but before you give up know I puffed a cell and antagonized two others but they are still playing well in the pack at the right voltages, without a bms, and Im drawing them down slowly to 2.625v until they are ready to do a 0.2C charge with my inverter, where I’ll test the cell balance as well as possibly the high voltage cutoff. Good luck

one quick clarification, i may be confused but wouldn't 1C discharge indicate "Full to empty" in one, not two hours?

 

[Dzl]

one quick clarification, i may be confused but wouldn't 1C discharge indicate "Full to empty" in one, not two hours?

Correct. 1C = 1hr charge or discharge

I’m going to develop a reputation as a curmudgeon but this issue around low voltage limit keeps coming up and I want to understand why. For EVE LF280 cellls, the big prismatics which are prevalent on this board, the charge cycle for determining a 4000 cycle lifespan is 3.65 to 2.50v at 1C discharge and 25C temp. Full to empty in two hours at room temperature. The manufacturer states that cycle provides the full rated life. It says nothing about what happens if one cell in a pack is low like your case, but if that cell stays at or above 2.5v at all times its voltage will not reduce life expectancy. The mismatch may make it hard to stay in that pack but plenty of people around here use old batteries successfully so thee must be a way.

So don’t worry, at least about the good cells. You can cycle the crap out of these things, they are built for it and after using 100% of capacity everyday for the next 11 yrs you’ll still have 80% capacity! I paid 200 for a starter battery in my truck that will last 5 years working for 20secs a day, while I can rig 12v lifepo with bms for $500 and move over 3kwh in and out every day for over a decade. These batteries rock! Just stay in bounds, that’s all. Maintain 2.5v. or above but especially below 4.2v and/or 55C temperature. These boundaries are very important!

If you have a bad cell you’ll need to fix it, but before you give up know I puffed a cell and antagonized two others but they are still playing well in the pack at the right voltages, without a bms, and Im drawing them down slowly to 2.625v until they are ready to do a 0.2C charge with my inverter, where I’ll test the cell balance as well as possibly the high voltage cutoff. Good luck

A couple clarifications (on standard test conditions), that don't negate your broad point but are worth mentioning:
*4000 cycles = 3500 Cycles (if compressed), 2500 cycles (if not compressed)
** 1C = discharge, 0.5C = charge
***Temperature and calendar aging are wildcards

 

[Steve_S]

Temps, even 2 degrees Celsius can alter charge & discharge characteristics.
Now confirming with my own tests, 4 packs each at different temps 12C, 14C, 16C, 18C temps and I can see the difference between them. I am fortunate in my installation that I can actually do this sort of test, it was not planned or even designed to do this but turns out this occurred because of the powerhouse building & setup I have.

LFP cells can & do contract & expand up to 2mm through "normal" charge & discharge cycling.
Undercharge will not cause bloating. Overcharge however will certainly cause bloating, if too severe, it can affect the "jelly roll" and delaminate internally.

With these cells, the manufacturers indicate the cells are 1C Charge/Discharge "capable" but recommend not to exceed 0.5C to minimize stressing the cells and maximizing cycle life. Discharging up to 1C does not bother them much at all but the charge rate has a significant effect on the cells and how they absorb the charge. The higher the Amperage the more pronounced the Runner Cells will be which will reach HVD before the others. During discharge, the higher the rate, "load" being demanded, will affect the weak runners who will reach LVD (Low Volt Disconnect) before the rest of the cells.

* LVD can also be triggered by one cell reaching LVD threshold even for a moment. Example: Cells are at 3.1V ea +/-10mv with 5A discharge ongoing. All of a sudden, you turn on a "coffee maker" that draws 60A off a 24V bank, that "demand" for a moment cause the cells to deviate (this is normal) but IF there is a weak runner it could momentarily drop to 2.55V, trigger LVD which cuts off the battery till the BMS cycles and reconnects. THERE IS A WORKAROUND of sorts. Depending on BMS, some allow for setting the disconnect delay time / Cutoff Delay Time (or similar, depends on BMS) which is usually quite short - extending that can usually solve this as that "initial" drop for the weaklings are short. Don't go too far, try incrementing by 2-5 seconds at a time, be "VERY conservative".

More details are available in my signature, see ESS and LFP Voltage Chart links.

 

[DeeK]

Temps, even 2 degrees Celsius can alter charge & discharge characteristics.
Now confirming with my own tests, 4 packs each at different temps 12C, 14C, 16C, 18C temps and I can see the difference between them. I am fortunate in my installation that I can actually do this sort of test, it was not planned or even designed to do this but turns out this occurred because of the powerhouse building & setup I have.

LFP cells can & do contract & expand up to 2mm through "normal" charge & discharge cycling.
Undercharge will not cause bloating. Overcharge however will certainly cause bloating, if too severe, it can affect the "jelly roll" and delaminate internally.

With these cells, the manufacturers indicate the cells are 1C Charge/Discharge "capable" but recommend not to exceed 0.5C to minimize stressing the cells and maximizing cycle life. Discharging up to 1C does not bother them much at all but the charge rate has a significant effect on the cells and how they absorb the charge. The higher the Amperage the more pronounced the Runner Cells will be which will reach HVD before the others. During discharge, the higher the rate, "load" being demanded, will affect the weak runners who will reach LVD (Low Volt Disconnect) before the rest of the cells.

* LVD can also be triggered by one cell reaching LVD threshold even for a moment. Example: Cells are at 3.1V ea +/-10mv with 5A discharge ongoing. All of a sudden, you turn on a "coffee maker" that draws 60A off a 24V bank, that "demand" for a moment cause the cells to deviate (this is normal) but IF there is a weak runner it could momentarily drop to 2.55V, trigger LVD which cuts off the battery till the BMS cycles and reconnects. THERE IS A WORKAROUND of sorts. Depending on BMS, some allow for setting the disconnect delay time / Cutoff Delay Time (or similar, depends on BMS) which is usually quite short - extending that can usually solve this as that "initial" drop for the weaklings are short. Don't go too far, try incrementing by 2-5 seconds at a time, be "VERY conservative".

More details are available in my signature, see ESS and LFP Voltage Chart links.

Would you be willing to share the results of your tests? I am very interested in seeing the temperature related differences for capacity. I've found generalized temp vs capacity curves but specific, real-world test data would be terrific.

 

[Steve_S]

These are observations I am making through my extended Thrash Testing cycle which comes to an end this week Thank Goodness... An obscene amount of cash has been dumped into Gasoline to run the Genset for these tests over the past 3 weeks... 4 hours run time = $25, by end of this week that will be 3 oil changes (0W30 Synthetic) and 120 hours runtime. Yes, just about $750 in gas... DISGUSTING ! Actually running a 100A Charge run ATM plus what the SCC adds in.

The colder pack takes charge a bit slower than the warmest pack. Even 4 degrees Celsius which really isn't much makes a difference... BUT it is -16C outside and the powerhouse is heated to 15C now as opposed to the previous 10C limit when I was running Lead Acid Only. I have not been logging the info technically, just making notes for myself along the process, as this was not a part of my test cycle process, it's popped out incidentally.

 

[Capt Bill]

* 1C = discharge, 0.5C = charge

What? ... plus my reaction to a near previous post saying: ** 1C = discharge, 0.5C = charge ???

... maybe I am scann reading out of context ???

From my studying what 1.0C or .0.5C means ... I got down 1C = 1 x Ah rating of battery, ... and for a 100 Ah battery, that would mean 100 Amps Charge or Discharge; and for 280 Ah LiFePO4 ... 1C means 280 Amps Charge or Discharge. and 0.5 C means half those amount of Amps IN or Out for Charge or Discharge ... Please correct me if I an way off base :+)

 

[Dzl]

What? ... plus my reaction to a near previous post saying: ** 1C = discharge, 0.5C = charge ???

... maybe I am scan reading out of context ???

Yes, you are missing the context. The comment I am responding to is quoted above the response.

From my studying what 1.0C or .0.5C means ... I got down 1C = 1 x Ah rating of battery, ... and for a 100 Ah battery, that would mean 100 Amps Charge or Discharge; and for 280 Ah LiFePO4 ... 1C means 280 Amps Charge or Discharge. and 0.5 C means half those amount of Amps IN or Out for Charge or Discharge ... Please correct me if I an way off base :+)

Your understanding is correct, you just missed the context of the statement (Cycle life test conditions for EVE 280 cells = 1C discharge 0.5C charge)