First Time Battery Surgeon - Help Needed on 10s Li-Ion Pouch Cells

Hi All, First time battery surgeon here. Purchased this 36v "30ah" pack used. Previous owner says it used to work fine, but has been sitting for 4+ months. Battery appears to come from "BTRPower" on ebay / amazon.



Upon initial inspection, voltage reads ~14V



However, once I put it on the charger, voltage jumps to ~41V...then charger shuts off. Pack now reads ~40V. I decide to give it a try on my bike...bike powers up fine, give it some throttle and it starts moving for a few seconds...then dies. Pack is now back to reading ~14V. So I decide it's time to suit up for my first surgery.



To my surprise...I find 10s 1p pouch cells, not the expected 18650s. I disconnect the BMS input wires (1 red, 1 black, and 8 white), and find the pack now reads 37.5 volts on the charge port (still 14V on the discharge port).



With my negative probe on the battery negative terminal, and positive probe at the lower most BMS pin (empty slot beneath the black wire) I get:

[EDIT: Updated with corrected measurements]

0 (no wire here)
...then moving up one pin at at time:
4.19 (the black wire...4.19 difference from previous)
7.0 (2.81)
11.19 (4.19)
15.38 (4.19)
15.38 (no sense wire here)
15.38 (no sense wire here)
19.5 (4.12)
23.7 (4.1)
27.8 (4.2)
32 (4.2)
36.2 (4.2)
40.4 (4.2)
40.4 (no sense wire here)

So obviously the 2.81v cell needs help. This would be the second pouch from the bottom right?

So now I am at the end of my knowledge. What would your next steps be?

Does anyone know anything about these pouch cells? Should I look for a replacement for the 2.8v cell? Any idea where to get them?

Or should I try charging the low cell up to ~4v and letting the BMS balance from there? I have not ever charged cells like this before...do I need to disconnect them from the rest of the pack? Or can I just charge directly on the pouch tabs (wherever they are...I haven't found them yet)

PS after reconnecting the BMS sense wires to the BMS, it took a charge for awhile, then the charger turned off again (as expected, as most of the cells were at ~4.2v already.

are you sure you have 9 pouches. i dont know pouch cells or the bms but arent you missing a wire?
36.9/9=4.1v

If you can see 10 pouches, you should be able to measure 10 cell voltages......but you only show 9 ?
Maybe that “dead led” is actually a dead cell ?
Sometimes , the bms is powered by just one cell and over time, without charging, that one cell can be drained and killed !
Check cell count again and look for those bms wires.
If yuo need to, you can charge each cell individually via those bms wires using a single cell charger.

Hillhater said:

If you can see 10 pouches, you should be able to measure 10 cell voltages......but you only show 9 ?

Click to expand...

Ha...duh. Yes, I think I didn't measure the last one. I will run through them again later and update.

To charge the cell individually through the BMS...I will connect the positive from the single cell charger to the cell's BMS wire, and negative to where? Battery negative?

I measured again today...this time going from battery negative to each BMS wire. I updated the original post with corrected voltages. Summary...I low cell at 2.8v, which makes sense...I'm sure as I put any load on that the BMS will cut out.

Appreciate advice for moving forward. Can I charge that cell through the BMS sense wire? I have a single cell 4.2v charger I can wire up...would I put the positive charger output on the BMS sense wire for the low cell? and negative to Battery negative? Or would it have to connect to the next BMS wire? (so in this case charger positive to the 7V BMS wire, and charger negative to the 4.19v BMS wire next to it?)

Yes, to cHarge the single cell, connect to those bms wires that go to the low cell and the adjoining cell.
Be careful.....double check....that you get the polarity correct.

Just a quick update...i was able to wire up a 1A 4.2v single cell charger to the low cell. It is slowly but surely charging up...I'm not leaving it unattended, so it's been slow going, but have it up over 3.4v now and it is holding it's voltage fine when off the charger (not plugged in to the BMS...I suspect the BMS may be draining this cell, will test that after rebalancing the pack). Of course the real test will be to see if it can stay balanced after it's back up and running...