Oscillator Ocelot said:
You were 100% correct about the interconnect between the cells failing. After I carefully removed the shrink wrap from the battery, I discovered a full disconnect hidden in one of the tab interconnect folds. I bridged the disconnect with alligator clips and brought the battery pack down to a safe storage voltage. Everything else looked undamaged to my novice eye.
I don't see any other damage, either.
It looks more like a mechanical-force disconnect (as if something were placed between them and pulled outward along the axis of the pack, tearing the tabs apart) than a heat-induced one (from overcurrent/ poor connection/etc). Since the cells aren't swollen, I don't know what could cause that. (swollen cells have certainly done that kind of damage, but they don't unswell, so you'd see this). In this case, I see what looks like torn edges of the tabs, which only happens mechanically.
The tear actually sort of looks like it happened "lengthwise" across the tabs, as if the balance wire on it was pulled hard enough to stress the aluminum tab and start a tear. Then at some point the rest of tab tore thru, and the current path was broken. The wires from the pack don't appear to have any strain relief, so if that wire is shorter than the others even by a little bit, then there'd be more stress on it, potentially allowing this to happen.
A heat-induced disconnect normally leaves arc damage if it happened during a high current instance, or even actually vaporized edges, which I don't see, but would leave some sign of whatever melted from the heat to allow the disconnection. Since there doesn't appear to be any, it probably didn't fail during use.
Now I’m left with the dilemma of how to reconnect the tabs. They’re thick metal tabs (maybe around 0.15mm each tab). I don’t know how they can be reconnected without factory tools.
It looks like the tabs were crimped together, possibly with a current passed thru them during the crimping, which would weld them together. You may actually be able to do the same sort of thing using the Kweld, if you are getting new packs to power it from anyway. If you can use some form of clamping to secure the two tabs such that they overlap, then pass the welding current thru that clamp so it welds the two tabs together, along as much of the contact area as possible.
You may be able to solder them together, but it is likely one of the tabs on each cell is aluminum and the other tinned copper. There are some pretty old threads around here on soldering to A123 tabs, or other pouch tabs, including methods to minimize the heat getting into the cell pouch. I don't have any links ATM, but they're probably from a decade or more ago, especially from when the ebike racing thing was big.
Another possibility is to use some copper (or whatever) strip of appropriate thickness to create a connecting strip between the two tabs, and roll each end of it up with a tab, then crimp that roll as securely as possible, so you have as solid and low-resistance a connection as possible.
Something else I noticed that confused me. The balance wires and power cables were soldered directly onto the battery tabs. I thought the high heat from soldering is supposed to be very destructive to the battery internals, hence the spot welding. Are lithium polymer cells more resilient to soldering heat than lithium ion cells?
They may simply just ignore the potential problems this can cause, and they may be using a high-wattage iron with a very large heat-soaked chisel tip to quickly solder them onto the tabs. Pretinning the wires, and using appropriate flux to ensure fast bonding to the tab material, would minimize the time the heating of tabs occurs.
There've been some discussions over the years about soldering tabs to pouch cells that cover various pluses and minuses and techniques.