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sunrise

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A couple of months ago I ordered 8 105ah prismatic cells from aliexpress (BLS), advertised as new 2020 cells. It arrive after long wait for them to be shipped from China. Today I finally got everything (JBD smart BMS, 20A dc charger, lugs, etc) needed to test them. and I was really disappointed.

Long story short, after I top balanced the cells in parallel with 20A charging current, I rewired them in serial with JBD BMS and used the battery tester (fan+heat sink)
and found cell #3 is at 2.65v at 14A discharging rate. This seems indicates cell 3 has vast different internal resistance from the rest 3 cells. it's so bad that if I used one of the "shadow charge/discharge" profile, the BMS would actually cut it off right off it's fully charged :(

I'd appreciate any advice on how I should further investigate this before contact seller or file dispute.


IMG_0820.PNG
 
As @Craig said definitely confirm with a multimeter and isolate the problem. There have been a number of posts recently where people have jumped to conclusions without checking to make sure the BMS was accurate and it ended up being a faulty BMS or a bad connection.

edit: crosspost
 
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Ok then can you recharge that cell individually just to make sure it was fully charged

Prior to the load test, I put all 4 in parallel and charged them full, and I used multimeter and tested all 4 cells and they were around the same voltage (3.54 or so) is that still not enough?

I could try the multi-stage top balancing after the capacity test, but it seems obvious the internal resistance of cell 3 is much lower than the rest 3 cells.

This is very disappointing, the cells I purchased from aliexpress vendor(BLS) are one of the more expensive ones, I was trying to avoid this kind of hassle by passing on those bargain ones. but still. :(
 
The discharge test is still on going, I have noticed the BMS already registered 3 occurrences of single cell undervoltage events, I guess it has not trigger cut-off protection because the low voltage 2.6 didn't last long enough (5sec?) for it to kick in, But it's definitely not looking good. :(
 
I guess it has not trigger cut-off protection because the low voltage 2.6 didn't last long enough (5sec?) for it to kick in,

I didn't know the BMS required 5 seconds of LVD to activate? I thought it was instant.

I would still take Craig's suggestion and charge that one cell all the way up and then capacity test it.
 
I didn't know the BMS required 5 seconds of LVD to activate? I thought it was instant.

I would still take Craig's suggestion and charge that one cell all the way up and then capacity test it.
actually I was wrong about the 5s delay, there is a delay for releasing from it, not activating it.
So in my case, it must have triggered it , then the current stopped, voltage going back up, after 2 seconds, it's released and discharge starts again
 
Did you use your multi meter at the cell terminals themselves, or at the busbar or wire or other?

If at the cell terminal itself, it might be the cell. If at the wires/busbars, it could just be at the connections (screw not tight, etc).

Hope you are able to resolve this. Good luck!
 
Did you use your multi meter at the cell terminals themselves, or at the busbar or wire or other?

If at the cell terminal itself, it might be the cell. If at the wires/busbars, it could just be at the connections (screw not tight, etc).

Hope you are able to resolve this. Good luck!
Wow, this is it! all make sense now. After I read your comment I went to check my connection (previously I did use multimeter on busbars) and I discovered one of the terminal has a small piece insulation squeezed by crimper got between washer and bus bar. after I fixed this everything seems working as expected.

I love this forum.

Thank you all!
 
I just love happy endings

Note: when this occurs on a ready made, sealed consumer battery .....you can't do this because you cannot get to the individual cells

BIG covert but shameless plug for DIY batteries
 
Great to hear this actually worked out.

Assuming you are satisfied with a good product, please provide a link to the store.
 
I just love happy endings

Note: when this occurs on a ready made, sealed consumer battery .....you can't do this because you cannot get to the individual cells

BIG covert but shameless plug for DIY batteries
SOK and a few other LFP batteries have openable cases.
How is your comment covert or a secret?
Who doesn't like happy endings except for the Pope?
 
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