need help,weird voltage spikes.

[nwillitts]

i have built 2 24 volt packs.when ever i charge them cell 2 in both packs will eventually jump up past 4 volts.i shut down charger at this point.all other cells are in good voltage range,just cell 2 in both packs.
any idea why this would be happening?

 

[Zil]

The cells are not balanced. They need to be carefully top balanced. Check with Will. Worse would be the cell is bad. At 4 volts that is a good bet.

 

[nwillitts]

they started off balanced.

 

[Fred S]

Do they have bms's? Do you have the battery cable (charge cable) attached at cell 2? Are the packs charged separately, or are they paralleled?
Last, what is the charge rate and max voltage?

 

[nwillitts]

packs are separate.charge cable on cell one.chargery bms.i am thinking the bms is just weird,gonna try another bms later.

 

[MisterSandals]

Can you post a pic of how you have your batteries wired?
Have you checked your connector wires for resistance? I would be suspicious of the ones that service cell 2.

You could try rearranging your cells in the battery to see if the problem comes back to the cell position or if it follows that particular cell.

Do yourself a favor and label your cells. I like using letters because they are less likely to be confused with the cell position (... cell 6 in the cell 2 position, less clear than cell B in the cell 2 position).

 

[Gazoo]

The cells are not balanced. They need to be carefully top balanced. Check with Will. Worse would be the cell is bad. At 4 volts that is a good bet.

If the BMS is working properly and the cells are matched then the lack of top balancing would not cause the batteries to go out of balance. 4 volts is too high.....there is something else going on. If the BMS is working properly the voltage should not have gone that high.

I will be ordering batteries soon and may not top balance. Nothing against Will. But I have not seen any evidence that top balancing is necessary on matched cells. Others will differ and that's fine.

 

[Zil]

Cells need to be balanced before put in service. Bottom balanced for deep draw uses such as EV. Top balanced for fractional C as in solar storage. You need more experience if you have 'not seen any evidence'.
Agreed the BMS should have stopped that charge. But I don't know the settings in that BMS.

 

[nwillitts]

just so weird.here is a pic of how they are wired.numbers are correct on batteries.same as pack 2.

 

[John Frum]

Two issues as I see it
1 A cell or the pack configuration is misbehaving
2 Bms not doing it's job

What BMS are you using?
What is the high cell disconnect voltage value on the bms?
What is the voltage for the battery as a whole?

 

[nwillitts]

bms is chargery.
disconnect is 3.7.
26.82 is total for pack.
this will be for solar system.

 

[John Frum]

bms is chargery.
disconnect is 3.7.
26.82 is total for pack.

26.82 / 8 = 3.3525 volts
That one cell is way out of balance.
Whatever else is going on it would appear that your bms is not doing its job.
Did the bms trip on the cell going out of spec high?

 

[nwillitts]

no,but it beeped.

 

[John Frum]

I guess you should verify the cell voltages with your dvom if you havent already and you should discharge that cell.
Is the bms trying to bring the voltage down on that cell?

 

[nwillitts]

cell comes down immediately when charger is disconnected.bms seems to try while charging.@27.1 across the pack at the moment.the #2 cell is @3.49v.

 

[John Frum]

Do the chargery and the dvom agree on the cell voltages?

 

[Gazoo]

Cells need to be balanced before put in service. Bottom balanced for deep draw uses such as EV. Top balanced for fractional C as in solar storage. You need more experience if you have 'not seen any evidence'.
Agreed the BMS should have stopped that charge. But I don't know the settings in that BMS.

I am very well aware of all the recommendations. Experience is not the same as evidence. But yes, I will need to experience this for myself.

If I decide not to top balance and my battery cells are in balance and stay in balance, and have the full capacity after a few cycles then I will not understand the benefit of top balancing.

On the other hand if my batteries don't stay balanced and capacity is lost due to a stray cell, only then I will understand the need to top balance. This will be an interesting experiment and experience.

IF I end up having to top balance I will most likely purchase a single cell Lifepo4 charger. I have plenty of time to experiment and experience before I make my battery pack final. I will be happy to post my results although it will be awhile as I haven't ordered the batteries yet but will soon. My findings will be my evidence one way or the other.

 

[nwillitts]

Do the chargery and the dvom agree on the cell voltages?

yes.
i ordered this to charge cells and balance-

https://www.amazon.com/gp/product/B01D4PORJW/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

takes me back to my RC Drone days.will see what happens.will post results.

 

[nwillitts]

ok,charging up using my devices.cell 1 being weird.

 

[nosys70]

i got the same problem with the 280Ah xuba LFP.
i got two that are raising voltage a lot faster than the others.
these batteries have unusual specs.
they are running fine fully charged at 3.3V (i charged at 3.3 then discharged over 250W/24H, and get the full capacity)
i have 2 cells going at 3.7 very fast, while all the other seems to stay at 3.4 max
first time , i charged without BMS and the 2 cells goes very high (over 3.8 while others were at 3.3)
Then after discharging , i installed the BMS, all were at 3.0 and recharged.
Again all were at 3.3/3.4 but the 2 same ones goes to 3.6.
So these Xuba batteries are fine, but the curve is very flat (you have full capacity between 3.4 and 3.0)
if you order some, better to take some spare cells. Will do next time, as i plan to build a second 8s pack.