battery with one cell down => cell or BMS problem ?

Joined
Feb 20, 2020
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13
Hello!

I have a problem on my ebike: one cells seems not working correctly.

It gets quickly discharged to 0v and does not charge to full 4.2V.

When the other cells are charging this one charge lower.

I have a bluetooth BMS so I can have a good idea of it real time.

It can makes my battery fail and engine not starting when plenty of battery is still there on other cells.

Here are some screenshot and a video.

The bms is https://fr.aliexpress.com/item/4000316595074.html?spm=a2g0o.order_list.0.0.47c45e5bosC6MO&gatewayAdapt=glo2fra

The battery is a LG MJ1 / SANYO GA || BATTERYPACK FULL WELDED bought here:

0506_2022-22-07_11h45.jpg

Screenshot_20220702-152635.png
Screenshot_20220702-173743.png

when charging, BMS app shows this: https://www.youtube.com/watch?v=-R9HoEFA-OA

My question is: is the fault the battery (I think so but not 100% sure) or the BMS ?
The battery does not have many cycles done, so does the BMS.

Thanks in advance.
 
for sure I'll give the reason of the problem here - when and if I find it ;)

Do you think I can invert the BMS wire 8 to another one to see if problems moves to another cells? This will tell if it's the fault of BMS right ? I don't remember if they need to be in specific order or not.
 
I would charge the pack as much as it will take, then disconnect the BMS and let the pack sit overnight. Measure cell voltages before and after. If the cells are bad, the voltage will still drop. If the BMS is bad, the cell voltage will stay steady when it's disconnected.
 
fechter said:
I would charge the pack as much as it will take, then disconnect the BMS and let the pack sit overnight. Measure cell voltages before and after. If the cells are bad, the voltage will still drop. If the BMS is bad, the cell voltage will stay steady when it's disconnected.
Problem solving suggestion is excellent advice ... :thumb: ... only suggestion is not charging pack "as much as it will take" ... 4.15V should suffice for identifying the problem. If the cells are borderline bad it could still take at least 5 days for a significant drop in voltage (e.g. 0.050V). My guess is the initial culprit is the BMS ... could also adversely affect P8 cells.
bruno_mac_douglas said:
The battery does not have many cycles done
What i don't understand is why you are charging p-groups above 4.20V instead of say just to 4.10V. Doing so will extend cycle life longevity of battery. Likewise with average p-group resting bounce back voltage around 3.75V is better than an average resting p-group bounce back voltage of 3.50V for extending cycle life of battery.
 
The BMS needs to be removed in any case.

If you have a bad group, there are likely bad cells also in other groups.

Capacity testing per cell is the only way to sort out go vs no-go, while the pack is being disassembled might as well do the lot.

Please link to the source of the pack so others know to avoid.

Rebuild so that the BMS can more easily be removed and voltages checked / balanced with alternate gear next time.
 
ebuilder said:
\OP, you are the ultimate arbiter of truth based upon what you find by removing the degraded P-group of cells.
You're doing OK yourself in critiquing the previous reply. It's possible he may have missed the following information ...
bruno_mac_douglas said:
The battery does not have many cycles done, so does the BMS.
  • For those that have had an unfortunate experience with a BMS they will tend to put the blame on the BMS.
    For those that believe BMSs are unfairly badmouthed they will tend to blame the lion cells as being inferior.

My rationalizing is that it won't take long for the symptoms of a faulty BMS to show up--premature degraded cells.
The chances of one or more cells degrading a p-group is less likely with a newer pack when other p-groups are ok.
In other words it is likely both the BMS and the p8 cells need to be replaced with a good BMS and OK quality cells.

Thanks for posting that youtube :thumb:
 
ebuilder said:
My personal opinion is....the BMS did not cause the bad cells but rather correctly reported their decline. But we will learn this when Bruno rebuilds his pack.
My personal opinion is we will learn if it's one or both (just P8 or BMS and P8) when he follows through on problem solving procedure by fechter ...
fechter said:
I would charge the pack as much as it will take, then disconnect the BMS and let the pack sit overnight. Measure cell voltages before and after. If the cells are bad, the voltage will still drop. If the BMS is bad, the cell voltage will stay steady when it's disconnected.
May have to let it sit longer than overnight depending on the cells (P8) level of degradation (due to faulty BMS?). The gradual degradation of P8 cells didn't necessarily happen overnight, but likely over several charge/discharge cycles.

Agree that even if P8 discharge is only a few millivolts "overnight" the cells still need to be replaced, but not immediately if BMS is guilty party. If the BMS is the problem replace it, then bottom balance P8 to same as say P4.

Then do a charge/discharge cycle and see if P8 is still off by as many millivolts as before at full charge. If not i'd do a few more charge/discharge cycles just to get an idea of P8 cell degradation. THAT's of course assuming faulty P8 components on the BMS was what caused the degradation of P8 cells.
 
ebuilder said:
We will see ultimately who is right when Bruno's performs his repair.
We both could be right. Either way the odds favor you as P8 cells should most likely be replaced. If it turns out connection P8 on BMS circuit board is cause then remove defective BMS and replace with a good BMS.
 
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