Help one cell charging discharging to fast

[time2roll]

So I dissembled them as I am going to put them in series with bms for capacity test. Disassembled I noticed the voltage of each cell dropping which I understand is normal as it approaches a resting voltage but I am noticing a slight difference in cell voltage.


Cell Voltages:
1) 3.608
2) 3.612
3) 3.610
4) 3.610
5) 3.610
6) 3.610
7) 3.610
8) 3.615

Is this normal?

I would call that done. Ready for series assembly.

 

[koobs]

So I dissembled them as I am going to put them in series with bms for capacity test. Disassembled I noticed the voltage of each cell dropping which I understand is normal as it approaches a resting voltage but I am noticing a slight difference in cell voltage.


Cell Voltages:
1) 3.608
2) 3.612
3) 3.610
4) 3.610
5) 3.610
6) 3.610
7) 3.610
8) 3.615

Is this normal?

Youve got a DeltaV of 0.007V, which is tiny. I wouldn't worry about it. You'll likely diverge during high load testing anyhow due to the unmatched nature of these cells in general.

 

[Solar123456]

Youve got a DeltaV of 0.007V, which is tiny. I wouldn't worry about it. You'll likely diverge during high load testing anyhow due to the unmatched nature of these cells in general.

Thanks for the help. I just assembled the 24v 8s 280ah battery with bms and started capacity testing. I'll be using one of those 180watt heat fan capacity testers so I think its gonna be a while about 45 hours ish if I did my math right. Will report back once complete

 

[Solar123456]

So I started the capacity test a little under 2 hours ago, I believe the battery voltage was around 28.6 when I started.
Cell voltage seems low for only expending about 11.5ah in the approximate 2 hours. When I compare current cell voltages in pic below with SOC chart, that voltage corresponds to between 70 and 80 percent soc. Should I be concerned the voltage dropped so low already?

 

[DerpsyDoodler]

Just let the test run. Voltage is not a good way to determine soc with lfe batteries.

 

[time2roll]

28.6 is more surface charge than resting/start voltage.

 

[Solar123456]

Ok seems like the BMS triggered was triggered by low cell voltage which I have set to 2.5 and to release at 3. Capacity test results in second picture at 237 ah. Does this mean that the capacity of those cells triggered are lower than the 280ah?

the one with the lowest cell voltage is the one I was concerned about when starting this post (the one that was charging faster than the others) and another seems pretty close to it in the picture as well.

 

[MisterSandals]

Does this mean that the capacity of those cells triggered are lower than the 280ah?

Pretty much, assuming test was not flawed.

 

[Solar123456]

Darn! I planned on using these cells for my camper van build and this is more capacity than I will prob need. Would using these lower cells with the others cause any long term issues?

I do plan on filing a dispute but just trying to figure out if I should order replacement cells

 

[MisterSandals]

You may find that you can get by with them just fine. It will help a lot that you are not needing to squeeze every amp hour out of them.
See how high you can charge the pack before those cells start to run away. Set your max charge there. Adjust as necessary.
You will probably get around 85-90% capacity…mostly a guess.

keep us posted on progress, you’re not alone and can share valuable info to the collective.

 

[Solar123456]

You may find that you can get by with them just fine. It will help a lot that you are not needing to squeeze every amp hour out of them.
See how high you can charge the pack before those cells start to run away. Set your max charge there. Adjust as necessary.
You will probably get around 85-90% capacity…mostly a guess.

keep us posted on progress, you’re not alone and can share valuable info to the collective.

ok so I charged and everything stayed pretty balanced and consistent right until around right when I took this screen shot below. Then 2-3 minutes later cell high voltage disconnect was triggered as you can see in next picture. A few questions:

1) would my max battery voltage then be what it was before it triggered?

2) the cell that triggered (cell 8) high voltage disconnect was not one of the two cells with lower capacity that triggered low cell disconnect on original capacity test. Is this normal or an issue? It is one of the cells on the end connected to positive bms lead.

 

[DerpsyDoodler]

ok so I charged and everything stayed pretty balanced and consistent right until around right when I took this screen shot below. Then 2-3 minutes later cell high voltage disconnect was triggered as you can see in next picture. A few questions:

1) would my max battery voltage then be what it was before it triggered?

2) the cell that triggered (cell 8) high voltage disconnect was not one of the two cells with lower capacity that triggered low cell disconnect on original capacity test. Is this normal or an issue? It is one of the cells on the end connected to positive bms lead.

When you put the pack together, did you scrub each terminal with sandpaper, clean with isopropyl alcohol, and add noalox or oxguard before assembling the connection?

 

[Solar123456]

Nope, would that affect it that much? I will do all that upon final assembly right now I was just too balancing and testing capacity before Ali express window closes

 

[MisterSandals]

2) the cell that triggered (cell 8) high voltage disconnect was not one of the two cells with lower capacity that triggered low cell disconnect on original capacity test.

Thats actually a good sign: if it were the same cell(s), that would indicate that those cells were lower capacity. If a different cell(s) is running away at the top, that to me anyway, indicates that a good top balance would help.

1) would my max battery voltage then be what it was before it triggered?

Yes, that would be what i would use. And even then, you are just above where i charge all of my batteries (13.8V (2x = 27.6)).
I think if you charged to 27.84V you'd be very happy all around. I'd say you'd get over 95% of your battery rated capacity from this. You're in great shape. I would not top balance unless this is not sustainable after many charge/discharge cycles.

If you set your balance start voltage a little lower so that it balances earlier that might help too. Adjust, observe and adjust again until you get your best results.

 

[Solar123456]

Thats actually a good sign: if it were the same cell(s), that would indicate that those cells were lower capacity. If a different cell(s) is running away at the top, that to me anyway, indicates that a good top balance would help.


Yes, that would be what i would use. And even then, you are just above where i charge all of my batteries (13.8V (2x = 27.6)).
I think if you charged to 27.84V you'd be very happy all around. I'd say you'd get over 95% of your battery rated capacity from this. You're in great shape. I would not top balance unless this is not sustainable after many charge/discharge cycles.

If you set your balance start voltage a little lower so that it balances earlier that might help too. Adjust, observe and adjust again until you get your best results.

Would constantly undercharging the good cells and holding them at a lower voltage (80% of their capacity to match lower capacity ones) negatively effect them or cause any issues down the line? Any other concerns about using the cells with 2 lower capacity in the pack?

 

[MisterSandals]

Would constantly undercharging the good cells and holding them at a lower voltage (80% of their capacity to match lower capacity ones)

What do you consider "undercharging" and 80% capacity? People use these terms over and over yet charge to over 14V so i dunno what numbers you are thinking.

 

[Gazoo]

8s 24v 280 ah Eve cell battery

Those are not EVE cells. They look like 272ah Lishen's to me.

 

[Solar123456]

What do you consider "undercharging" and 80% capacity? People use these terms over and over yet charge to over 14V so i dunno what numbers you are thinking.
View attachment 62047

Perhaps I am thinking about it the wrong way. I just looked at the voltages again I posted before bms triggered high voltage cut off and looks like they are all over 3.4 so does that mean they are all actually charging to approximately 99%.
So I guess that’s less of a concern and what is happening is the lower voltage cells are just cutting off (triggering low voltage disconnect) before other cells are fully depleted which is not necessarily bad for the high capacity cells in this pack because they will just never get fully discharged.
Am I thinking about this correctly?

sorry new to all this just trying to wrap my head around and figure out if I need to be concerned in this scenario long term with mixing different capacity cells

 

[MisterSandals]

looks like they are all over 3.4 so does that mean they are all actually charging to approximately 99%.

Yes

Am I thinking about this correctly?

Pretty much. But are you planning on doing capacity tests repeatedly and discharging until LVD?
You should consider your use scenario and manage your battery so that it does not go into LVD often or at all.

 

[time2roll]

Would constantly undercharging the good cells and holding them at a lower voltage (80% of their capacity to match lower capacity ones) negatively effect them or cause any issues down the line? Any other concerns about using the cells with 2 lower capacity in the pack?

I don't think the better cells will get any worse than the lower capacity cells for limiting the charge etc due to the weak cells.