Lowering the low cell-voltage cut-off?

[ttul]

I have a 24-cell LiFePO4 pack with about 12 damaged cells from a previous owner who allowed the cells to discharge over the winter. The bad cells still hold charge, but when discharging, their voltages drop below 2.5V under heavy load, which is the cut-off setting in my Ant BMS.

Because the cells are already damaged and on their way to the recycling depot anyway within the next six months, would it be "safe" for me to reduce the low-voltage cut-off in the BMS so that I can make use of these duds while I wait for their replacements to arrive?

 

[Hedges]

Do you think those cells have any Ah capacity below 2.5V?

Heavy load means additional voltage drop due to internal resistance, but if capacity of the cells is reduced, you can only get as many Ah from the pack as the smallest capacity cell. As indicated by low voltage.

Are the bad cells getting fully charged, or did they self-discharge to lower SoC so pack is no longer balanced?

 

[ttul]

The cells probably have plenty of capacity. I charged all of them to 3.65V individually and they went down to 3.2 or 3.3 when left alone for weeks. So there is capacity in the cells; just when under load, obviously they cannot maintain the same potential.

 

[ghostwriter66]

if they dropped that much in just that short amount of time then you definitely have bad cells ... assuming you don't have a good cell tester there ... however i would still hld onto those cells ... you never know when you will just need a couple extra batteries to do something

 

[ttul]

Thanks. Yeah I’ll hang onto them. Just wondering if it’s _safe_ to drop the low voltage protection on the BMS so I can drive around while waiting for the replacements to arrive.

 

[fpgt72]

I am really new here, so my question is really coming to my head as I am learning this stuff.

I remember we always are told, when putting batteries in a new device don't ever mix and match "old" batteries and "new" batteries. The thought was you will drain the new batteries quicker as they have to "carry" the older weaker ones.

I am not sure that is even true or not, I just remember it for the past 50 years.

Will the same thing happen with these batteries, will the OP's "good" batteries have to carry the "bad" ones.

Sorry for side tracking your thread, I am trying to learn and don't really want to start a new thread for every question that pops into my little peanut brain.

 

[Steve_S]

Charger your battery pack to NO HIGHER than 3.450 Volts per cell.
Allow the charge amperage to drop to EndAmps/Tailcurrent value then switch to float at 3.425 Volts per cell.
Low Voltage Cutoff should actually be no lower than 2.650 Volts per cell. This allows for deeper discharge buffering while not allowing the cells to go too low causing a Low Volt Disconnect which it cannot recover from. *LFP always recovers a bit after a deep discharge.

FYI:
LFP Always settles after a charge, it is a chemistry FACT ! When looking at Total Pack Voltage, that may represents as a 1.0-1.5 Volt drop after charging, without load or float charge applied.
LFP also will recover voltage post deep discharge, it is typically no more than 0.050 volts per cell. Again a chemistry fact.

EndAmps/TailCurrent is calculated as follows.
Using a 100AH Battery as example: 100AH X 0.05 = 5A EndAmps/Tailcurrent.
EndAmps is the point that the amps taken drops to as the resistance within the cells increases. This for all intents & purposes is 100%. By using FLOAT afterwards, to allows the cells to balance internally & between packs if you have multiple packs in Parallel. THERE IS NO GAIN by pushing more charge beyond EndAmps and IN FACT if done too much, can result in harm.

Battery Packs in Parallel are NOT the same as Battery Cells in Parallel within a pack. Parallel cells within a pack can be problematic at best and most especially if using unmatched cells. Parallel Packs add redundancy & fall back should any single pack fail and they also divide & split Load & Charge proprotionately.

There are several useful links in my signature, including voltage charts and guides which would answer many of your questions and provide reference info in a handy format for quick reference.

Lastly I HAVE TO POINT OUT that LFP is actually a pretty tough chemistry and not that easy to harm unless you get seriously extreme. Even if fully discharged to below 2.500Vpc and frozen solid in our cold temps (I'm up in Algonquin so I know) can sit for days/weeks and if thawed gently will not be affected. * provided no damage to casing etc from swelling or other.

Mixing Batteries !
Unlike many other chemistries, LFP can be mixed, it is NOT like Lead Acid where the lowest common denominator cell rules them all. All cells within a specific "battery pack" MUST be the same otherwise the BMS will get confused and cause errors. But you can have different "complete packs" in a bank with different aging & wear and that is not a problem. You can even put 100AH, 200AH & 300AH Battery Packs together in parallel without issue, just gotta do the math & ensure that the system is properly calibrated for the voltage reading.
FYI: I run 1x105AH, 2x175AH, 3x280AH in my House Bank.