BMS drained 40% of 105ah battery in 4 hours!

[sunrise]

I am having a bad feeling about this, any advice would be appreciated.

A couple of months ago I purchased 8 105ah (the blue aluminum ones recommended by Will) cells from aliexpress. it arrived a couple of weeks ago. I also purchased 2 JBD BMS with BT modules for the cells. Today I received everything I needed (crimp tools, connectors etc) so I crimped bunch of wires and connected 4 cells to give it a go. After I connect BMS, balance wires and BT module, I installed mobile app and it connected to the BMS and reported 50% charged. I poke around the app and everything seems to be working well. The BT module has LED lit when the app connect to it and once the app closes it goes off. So I just left the cells there.
4 hours later, I opened the app and checked it again, I am surprised by the app showing I only have 9% juice left and that's 40% drop.
I am not sure what is going on here, here are information I can provide so someone with experience can help me diagnose:

1. The battery cells (105ah, blue aluminum shell) is supposed to be new cells. prior to delivery to my house, it was shipped from China and was in transit for over 50 days.
https://www.aliexpress.com/item/4000798882949.html
2. The 120A BMS and BT module:
https://www.aliexpress.com/item/4000425316662.html
3. The BMS mobile app returned 50% charged when I connected BMS, BT module, battery cells, balance leads.
4. 4 hours later when I opened mobile app, only 9% (12.9v) charge left, during the period the BT module was NOT active as the app was closed.
5. I found no component feels hot (BMS, BT module, battery)
6. The battery was not connect to any load.
7. There was a small delta in voltages between cells. min 3.21v, max 3.24v
8. This is the first time I connected the cells in serial.

40ah was a lot energy gone in 4 hours, the fact there was no heat observed makes me concerned that the battery might be defective as it's unlikely that the 24awg wires for BT and balancing could draw this much amps.

I still need to get the a DC source to charge the battery. so for next few days I won't be able to charge it.

Thanks in advance.

 

[GXMnow]

Something does not sound right here. What is the BMS balance threshold set to? How much balance current can the BMS provide?

The cells were probably a lot lower than you think. The resting voltage is not a great indicator of SOC because the discharge curve is so flat. They could have been just above the knee where the voltage starts falling fast. Then even a small draw running the BMS electronics pulled it down. The Bluetooth module will draw power even when it is not connected. It may even draw more if it is searching for a connection. Do you have an amp meter? Measure the current at each balance wire as well as the full pack + and - leads. Even if it is pulling a full amp, 4 hours would only be 4 amp hours. That is not a whole lot to a 105 AH battery.

 

[sunrise]

@GXMnow , I just got started on this, although I have an amp meter, the connectors are all wrong. I also need a good 12v source for charging, the desktop DC unit currently only supply 3A to the 20A R/C balance charger. Looks like I have a lot soldering to do this weekend. Anyway the plan is -
- First I took out the BMS, in case there are something wrong with it.
- I am balance charging the battery @ 2A, This will take forever to bring the battery to a full charger, but hopefully soon the cells are all balanced, just want to see if there are bad cells in them.
- Once the battery are brought to full charge, I will run discharge device on them, that fan/radiator thing which comes with amp/watt meter. This would tell me if the battery is in good shape.
- Once that's done I will add BMS and troubleshoot it again.

At the same time all advice/inputs are welcome and appreciated.

 

[Gazoo]

The only thing I know about that BMS is after going through a few charge/discharge cycles the capacity readings will be accurate. Sounds like you have a ways to go before you get there.

 

[Zil]

First. Connect the cells in parallel, then top balance the cells. There are plenty of instruction on here.

 

[sunrise]

I want to give an update on this, it's very likely the BMS app was reporting wrong SOC since I just connected everything together and there was no charging/discharging cycle for it to give it more information. After I top balance and fully charged it, the cells worked as expected.