48V 17.5Ah battery gives only 5 Ah !!!

vcocciolo

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Hi group.
I hope you can give me some hint because it's driving me crazy!

First of all I purchase a Bafang BBS02 kit for my bike with its 10 Ah battery and everything was fine until the battery has been stolen so I purchase a new one. Still from Aliexpress.

The one in this photo:
Screenshot 2023-12-03 alle 11.38.58.png


This battery immediately gave me problems because its run out after only 30Km of moderate assistance 100 W - 150 W

I measured on a bench, charging and discharging with the "Watt's Up monitor", and in effect the only available energy its around 5 Ah instead of 17.5 Ah
The fully charged battery voltage is 54.6 V but on the discharge the BMS shut down at 48 V (as expected?) giving me only 5 Ah

I changed the BMS (13s7p 30A) but same result!

I teared apart the blue plastic cover to expose the battery cells. It reveals 91 (13x7) 18650 cells 3.7V for 2.5 Ah :
C011695B-7500-4BC5-9A07-00565853CC75.png


With the battery data sheet I discovered an important fact:
A fully charged battery has 4.2V that for a 13s battery pack means 4.2x13=54.6V that is exactly the charging voltage I have
A fully discharged battery has 2.5V that means 2.5Vx13=39V

So, to receive the promised 17.5 Ah, the battery needs to be discharged at 39V but my BMS (correctly?) shuts the current at 48V so it's only 5Ah
Please, be kind, to give me some hints. I'm also afraid to purchase another pack and have the same problem.
I'm quite good with electronics so nothing will be wasted!

Regards

The Battery Pack exposed :
20231128_071001.jpg


The old BMS removed :

20231128_215039.jpg


The new BMS :

Screenshot 2023-12-03 alle 12.04.10.png



The battery at the end of the charging process:

20231202_114309.jpg
 
Breaking News !

I maybe found the problem. I measured the voltage of all the 13 batteries in series and one of them is 2.92 Volts while the others are at 3.87 Volts. So it seems are one or more of the 7 batteries in parallel

20231203_130710.jpg
20231203_130716.jpg


What do you suggest?
1) All the 7 batteries need to be changed
2) Are those iron soldered?
3) Best approach ?

Regards
 
Yes, it appears you have a bad cell, or a bad leg on your BMS "balance charging circuit".
The problem with packs which use "series" connections is that one bad group in the series can limit current delivery to the load, and cause a low voltage cut-off. Having batteries in series are unavoidable with a 48V pack.

Most likely it is just one bad cell which is dragging down all 7 which are in parallel together, but worst case, it could be all 7.
Super worse case is that the BMS balance charge circuit is not providing charging power to one of your 13 parallel groups.
You can determine this by connecting your charger and measuring what the voltage rises to on the 2.92 volt group.
If the voltage climbs up on the bad group, but then starts to plummet faster than the other groups (once the charger is disconnected),
then that is confirmation of bad cell(s) in that group of 7 parallel cells.
Another possibility is that the spot welds on that parallel group is making a poor connection, and resulting in less than the full 7 in that group
working together to carry the load. Inspect the spot welds and make sure they are solid on the 2.92 volt group.
The good news is 18650 2.5 Ah High Current cells are not expensive. Popping loose the spot welds to replace the bad cell(s) and
replacing it will require some skill. Some battery repair shops (which do things like repair laptop batteries) may be able to perform the labor for a fee.

Once repaired, your pack should be a 48v 17.5 Amp hour (Ah) pack.
 
Yes, it appears you have a bad cell, or a bad leg on your BMS "balance charging circuit".
The problem with packs which use "series" connections is that one bad group in the series can limit current delivery to the load, and cause a low voltage cut-off. Having batteries in series are unavoidable with a 48V pack.

Most likely it is just one bad cell which is dragging down all 7 which are in parallel together, but worst case, it could be all 7.
Super worse case is that the BMS balance charge circuit is not providing charging power to one of your 13 parallel groups.
You can determine this by connecting your charger and measuring what the voltage rises to on the 2.92 volt group.
If the voltage climbs up on the bad group, but then starts to plummet faster than the other groups (once the charger is disconnected),
then that is confirmation of bad cell(s) in that group of 7 parallel cells.
Another possibility is that the spot welds on that parallel group is making a poor connection, and resulting in less than the full 7 in that group
working together to carry the load. Inspect the spot welds and make sure they are solid on the 2.92 volt group.
The good news is 18650 2.5 Ah High Current cells are not expensive. Popping loose the spot welds to replace the bad cell(s) and
replacing it will require some skill. Some battery repair shops (which do things like repair laptop batteries) may be able to perform the labor for a fee.

Once repaired, your pack should be a 48v 17.5 Amp hour (Ah) pack.
Hi
Thanks for the answer.

Yes, the voltage of this group go down very fast compared to the other until the BMS cut-off.

Probably is not a BMS fault becouse changed with onother one give exactly the same cut-off voltage.

I ordered 7 new cells with a pre-soldered terminal for 30 € (35 $) so I'm prepared to change all the bank.

I'll let you know!
 

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