I need some help please to identify what went wrong with my 3 month old 12s2p pack (maybe 20 cycles). I used it on my leightweight dirt jump MTB so the impacts/vibrations are quite hard. I charge with 8A and discharge usually between 30A and 60A (sags about 5V) and seldom short 80A peaks (6-7V voltage sag). The battery was spot welded with 0.2mm nickel with kweld at 42.5J. One layer, serial connection width about 30mm. (More info and battery building pics in my board build thread for those who are interested)
Prehistory what happened 1 day before:
The troubles probably started when I lost my battery during jumping although the heat shrink tube of the battery was firmly glued and additionally secured with duct tape. Apparently the heat shrink tube tore during the landing and the battery flew away. I guess the battery landed on the side because the JST port of the BMS is damaged. There are small stones everywhere and hard packed dirt.
At home I checked cell voltages and visual damages. I opened the heat shrink tube of some cells with marks a bit but found minor scratches only.
Most damaged was the BMS
.Then I charged all cells from about 4V to 4.2V with a balance charger and checked cell voltages afterwards. I used a thicker heat shrink tube and a lot of tape and glued it back in place.
Now the main problem:
I was jumping and had fun in dirt park and drained the battery until the end (usually about 42V = 3.5V/cell because of massive sag) but this time it felt a tad shorter.
At home I wanted to charge again but recognized this melted hole. There is fishpaper and several layers of kapton and fabric tape in between so the heat must have been incredibly high to create a hole there. Checked cell voltages, all at 3.7V except group 11 at 2.9V
It took a while until I realized that I could lift the nickel with a knife and that the spot welds on both negative group cells somehow vaporized.
Then I measured the single cells of group 11
What’s going on here ?
Does anyone have any idea what happened here?
I’m quite sure that some damage happened when the battery dropped but what?
The obvious reason would be that the spot welds broke but I am not sure for 2 reasons. First the other welds are still strong, it took some force with the pliers (rolled) to remove the nickel further. On the serial connection on the other side I even damaged a negative pole, too much pulling force, now there is a small buckle (no idea how you salvage cells guys).
The cells are glued with a strong glue similar to sikaflex. In theory it could flex a tad, who knows.
BUT the second reason and what baffles me the most is that all the spotwelds vanished. What I mean is that after removing the nickel there is sharp material left (and sticking out) on the cells and on the nickel strip where the spotwelds were. But on the 2 negative poles of group 11 there is no material, the surfaces are very smooth like polished (also the nickel strip). Here you can see and compare what I mean
Aftermath mockup, the cell with 2.95V (group 11) is not far from the spot where the BMS is damaged so there was definitely some impact.
Thanks for reading
Any ideas?