10s3p 30Q Pack Cell or BMS gone bad?

Hey Forum ,

Long time Reader not writer here...
I've got a problem here an need you to help me identifying it.
I've build an E-Bike battery last December. 10s3p Samsung 30Q Cells inside with a Bluetooth BMS
Max Current is 15A but most of the time they have a nice life in my Pack with giving out less than 1A per Cell.
I'm using it near daily for Commuting and hat never an Problem with cell inbalance. Might be because I measured all 3p Groups to be within 5mAh
so yesterday after riding to work I realized one of the 3p Groups was way down 3,5V to 3,7V. normaly my Imbalance is below 5mV.
And on work in dropped even further. see attached pics.

So now I'm in the process of identifying if it was maybe the BMS keeping one Balance Resistor on or if one of my cells has gone bad which I would not like since it's harder to replace

So what I found out so far.
It's all about the 4th Group.
After disconnecting the BMS and reconnecting no Balance Resistor sems to be stuck on. (Checked with Thermal Camera)
So the two remaining possibilities are Software problem of BMS or dodgy Cell.
I tried to rebalance the Cells but the one group seems to always drift away faster than the others, which indicates maybe a self discharging Cell.

This image shows one Cell group a little bit (~1K) warmer than the others. might indicate self discharge but could also be from charging the last.


This shows how the Cell drifted initially.

This one was made after manually rebalancing the first time. The drift is way smaller than before.



a few more attempts later but still it drops lower than the other cells.


It feels like the drift mostly is gone bot still it's dropping lower than the other cells.



two pictures of my battery so you know how I build it. the Cell Group under investigation is the one with the "3" on it.

I hope someone can help me identifying the problem.

Unfortunately this is a big problem with packs whose cells have not been matched. Because of it's very small size, the difference in the cells performance will also show up with a LVC cut as one group sags further than the others.
It's likely that cell group 3 was weak to begin with, but it never really became 'the weakest link' until much later, as the cycles were added on, and what started as one weak cell, gradually became exponentially weaker as time went on.

A poorly designed BMS will typically drain the first or last cell.. you've got a cell in the middle causing trouble, so it's probably not the BMS.

If you discharge graphed that cell group versus say, it's neightbor, you'd probably find a large difference in internal resistance and capacity.

neptronix said:

Unfortunately this is a big problem with packs whose cells have not been matched.

Click to expand...

Which my Pack definitely is/was as I stated in my initial post

HMT_01 said:

I'm using it near daily for Commuting and hat never an Problem with cell inbalance. Might be because I measured all 3p Groups to be within 5mAh

Click to expand...

I have discharged every Cell beforehand with a Junsi 308 duo get their Capacity. additional I noted Max discharge temp and Internal Resistance. Then I did an Excel sheet to match the group to at the end witin 5mAh per Group.
the Matching was done for the Range 4,1V to 3V. The Pack did never see above 4,1V or Below 3,3V while cycling lately I the usage was even lighter between 3,9V and 3,5V the Pack has had around 60cycles. Only for Monthly Capacity Monitoring I' going up th 4,18V.
The last one was done two Weekends ago which schowed ~8,2Ah@3A And all the Cells were perfectly fine and matched.


I'm not saying that it's not possible that group 4 now has a Problem. But I can certainly tell you that it wasn't a weak link from the beginning.
And if it's Group 4 is gone now I would like to know why so I can improve and learn from my mistakes.

EDIT: might be important the BMS is a Bluetooth BMS from GreenBikeKit: https://www.greenbikekit.com/e-bike...y-bms-pcm-with-software-app-programmable.html
Calibrated with a 0,05% Multimeter so It's not way off Calibration. All Voltages measure within 5mV of my Multimeter. Nonlinearitys can't be Calibrated into the BMS.

What controller and motor ?

This Controller:https://www.ebike-solutions.com/de/...toren/bafang-cst-hinterradmotor-36v-250w.html
And Motor:https://www.ebike-solutions.com/de/...controller/ebs-v2-controller-foc-36v-15a.html

But I don't know why that matters.
Like I said. Battery never saw over 15A which is 5A per Cell and thats nothing for 30Qs.
I'm using about 3-5Wh/km so I'm not letting the Motor do all the work and the battery never had a hard life. Everything was specd. to last long.

Yes it matters how you discharge and charge the battery and at what levels.

Yes I know that it matters how I charge/discharge.
But I already stated in the first post

HMT_01 said:

Max Current is 15A but most of the time they have a nice life in my Pack with giving out less than 1A per Cell.

Click to expand...

So no difference with which Controller I take the 15A.

Charging current differs on how mich Time I have. between 1A and 4,5A if It has to be fast, mostly I charge the pack at 2A. That is max Charge of 1,5A or 0,5C per Cell as stated in the Datasheet.

Wasn't aware that you built this pack yourself and carefully matched it. A discharge test answers your initial question. Fully balancing the pack and running it on the bike without the BMS to say, a safe 33% SOC... and measuring the resulting voltages would 100% confirm whether it was the cells or the BMS.

If the pack was closely matched in capacity and internal resistance in the groups, the reason could be likely..
1) power bus problem ( one cell in that group is not fully connected )
2) random weirdo failure of a cell which took out the other parallel cells over time.

Just replace the three bad cells.

So well. unexpected change of things...
As I said initially it seemed to me that the drift was way slower afterI disconnected the BMS once to check for Voltages as opposed to when I first found of the disbalance.

What I think I found out now is that all this voltage driftig was due to the first heavy disbalance and a not good enough balancing, leaving the Cell strings at different SOCs.

So what I did yesterday was a Capacitiy check from 4,08V to 3V per Cell.

So explaining the graph the blue line is Series Pack 8 which has the Pack fuse in series with it so that explains the lower voltage compared to the others. As you will see later it's also the highest one during charging.
Series string 4 ist the yellowish line dropping the fastest but not having heaviliy less Capacity. Still having the characteristic discharge behaviour like the other 30Q cells.

So then I Charged the Cells back to storage Voltage 3,75V and let them sit until I decide what to do next. After about an Hour I saw that the cells semed not to drift further apart anymore. So to be shure the Cells are having no unnormal leakage current I charged the Pack up to 4,08V per Cell and balanced al the Cells with a Cutoff current of 0,05A to be shure they are really balanced at the end.
So letting them sit for about an hour again still no noticeable drift.
So I measeured all Voltages with my DMM and rechecked them today after letting the battery sit for 10 hours.
Voltages:
yesterday today
4,0797V 4,0783V
4,0797V 4,0783V
4,0800V 4,0876V
4,0782V 4,0762V
4,0801V 4,0787V
4,0801V 4,0787V
4,0807V 4,0793V
4,0979V 4,0783V
4,0804V 4,0794V
4,0811V 4,0797V

No drifting apart anymore.

So after that I charged them up to 4,18V again and now I'm doing again a Capacity Check but I'm pretty shure they will come all out bretty close to each other.

If that is the case than the question remains: What was the initial Cause for the disbalance?

Have you ever heared of a "sticky" balance mosfet on a BMS?

Good job ! Just monitor. The lower you discharge the more likely to go on balance. I've only had a BMS balance Channel get stuck open and drain and drain. But it was with a singalab bms with led for each channel. Or else I would have never known.

Yeah I'm quite happy that I don't need to replace any Cells

Discharge after the Balance from two Days ago. I don't think that there was done any harm to any Cell. Just remember Cell4 started a few mV below the others due to not perfect Calibration of my Junsi. So I'm very Happy that everything is working again. But I need to have a closer look on my BMS and think of disconnecting it when I'm not using the battery for longer time and won't look after it. When I'm using it I check the Voltage at least 3 times a day so not much harm can be done by a sticky Balance Channel