I'll attempt to simplify things a bit wrt to series/parallel battery configurations.
It appears that the
battery pack is made up of 8
individual batteries (NOT 8 cells!). Note this difference in terminology. Each of those 8 batteries appears to contains 6 individual cells in parallel. So 3.7V @ ~11 Ah per cell. All of the data (pictures, datasheets) I have seen points to each of the 8 batteries being monitored but not each cell in those batteries; the BMS "sees" qty 8 - 3.7 Volt batteries each with a current rating in the 65 Ah range.
Here is where it is possible for things to get interesting. Lets say a single cell (1/48 of the battery pack; 1/6 of an individual battery) goes bad. It is very difficult for the BMS to detect this because the other 5 cells are still putting out the correct voltage and accepting charge current. This can cause problems both during charging and discharging; however lets focus on charging. The BMS can be doing everything right and limiting charge current/voltage to the battery, however with 1 dead cell the other 5 are actually getting overcharged by 17%. That overcharging / discharging will heavily tax the remaining 5 cells which can induce further cell failures in the same battery. My **guess** (I haven't done math on this) is that 1 dead cell wouldn't cause thermal run-away / venting, however 2 or more dead cells within the same battery...
This scenario also fits with what has been said from the beginning. Several sources were pretty adamant that the batteries hadn't been overcharged. Based on the data they had from the electrical logs, that could well be correct. As a whole, the battery pack never received too much voltage / current. With a series-parallel design lacking individual cell monitoring, it is possible for individual cells to over charge/discharge while the pack as a whole looks fine.
I can see two possible "simple" solutions that wouldn't require much re-engineering beyond the battery itself (because their electrical interface to the aircraft could remain the same). Have a look at the LVP10 (same datasheets). There is a good chance that it is a single cell from the same mfg. Either monitor all 48 cells or monitor 6 strings of 8 series-connected cells. Either option would provide the BMS with a much better overview of individual cell problems and possibly the ability to remove a single string of cells from the circuit without taking the entire battery offline.
On a separate but related note: Unless we don't have the right datasheet (which is possible), how is it that the battery is only rated for -18C? Doesn't the certification process require for extreme cold start (at least -40 F/C) operations?
Vince (1st time poster; long time reader)