Oh, please folks, temper your enthusiasm to hang these guys. For all we know it might very well be grotesquely misplaced.

As I stated in another thread, I'd prefer to read commentary from someone who has actually had first-level access to the relevant design and test documentation. Everything else is nothing more than conjecture.

What are some of the failure modes of battery systems, including Li-based systems?

OK, here's an incomplete list that has nothing whatsoever to do with the 787 case because, well, no design and test data has been released. Again, this has nothing to do with the 787. Think of it as the start of a list of things to watch out for if you were going to set out to design a battery pack and related systems.

    Sourcing
        - Latent defects due to manufacturing QA issues
        - With Li-based cells such things as humidity content can
          be critical as they might lead to the generation of methane
          gas during the service life of the cell.

    Assembly
        - Handling and storage problems
        - Mechanical damage, from dropping a cell to any number of 
          imaginable issues.
        - Assembly defects that can lead to (I**2)*R heating issues

    Testing (both at by supplier and OEM)
        - Failure to detect defects (sometimes impossible if internal)
        - A faulty testing process that actually causes or accelerates damage
          to cells, perhaps only cells that have specific problems
        - Inadequate issue escalation workflow

    Electrical
        - (I**2)*R heating of faulty interconnects (inter-cell and elsewhere)
        - Component defects that can lead to (I**2)*R heating issues
        - Oxide formation that can lead to (I**2)*R heating issues
        - Interconnect crimp quality problems that can lead to (I**2)*R 
          heating issues
        - Interconnect crimp quality problems that can lead to other issues
        - Main pack electronics problems
            This can include chargers and monitoring electronics
            - Charge termination failure or inadequacy
            - Thermal sensing failure or inadequacy
            - Pressure sensing failure or inadequacy
            - Fluid sensing failure or inadequacy
            - Inadequate semiconductor rating (for example: operating temperature)
            - Inadequate power semiconductor heat dissipation
                This doesn't necessarily mean bath thermal design.  
                It could be caused by inadequate or faulty mounting of TIL 
                (thermal interface materials).
                It could also be caused by mechanical and environmental issues 
                affecting the TIL or the mating of the semiconductor to the heat
                dissipation mechanism.  For example, vibration.
            - Inadequate or faulty failover modes
            - Static damage to semiconductors during electronics assembly, testing 
              or general handing
            - PCB via or trace fractures, contamination or over-etching
            - BGA ball soldering problems 
                (mostly only visible via x-ray inspection)
            - Bad, cold, fractured or chemically compromised solder joints
            - Dendrite buildup in lead-free solder chemistry leading to shorts 
              in small pitch components
            - Unaccounted race conditions
                Example: pull-pull technologies without adequate dead-band
            - Thermally-induced failures due to changes in operating 
              characteristics of components as temperature fluctuates.  
                Example: The aforementioned push-pull topology where the deadband
                might have been adequate within a certain temperature range but 
                causing shot-through shorts outside of this range.
            - Unaccounted for effects of such environmental issues as humidity.
                Example: Many VCO chips do not deal with humidity very well,
                particularly if it can condensate.
            - Faulty or non-existing conformal coating
            - Inadequate static damage counter measures 
                (inputs, anything expose to handling)
        - Sub-pack electronics problems
            This is applicable if a large pack is made from a multitude of 
            smaller packs, each with it's own control and monitoring electronics.
            Sub-pack electronics can suffer from the same issues as the main pack
            electronics above.

    Software
        - Myriad of code-based issues 
           (lock-ups, endless loops, pointer problems, uncaught exceptions, etc.)
        - Software problems caused by hardware issues
           Example: Static damage to a chip causing a bit to unexpectedly flip.
           If not caught in software this can lead to catastrophic failure

    Mechanical
        - Faulty inspection and certification process
        - Inadequate design materials
        - Adequate materials but supplier quality issues resulted in unexpected 
          materials issues
        - Inadequate design tolerances 
        - Adequate design tolerances but faulty assembly, subcontractor or testing
          process caused them to be violated
        - Faulty fasteners
        - Faulty fastener application
        - Unexpected interference between components
        - Mechanical pinching or chafing of wires
        - Mechanical puncture damage to cells during installation or operation
        - Mechanical abrasion to protective cell wall or packaging
        - Insufficient mechanical rigidity
        - Insufficient venting mechanism
        - Insufficient containment or venting of gasses (methane), 
          flames or heat
        - Mechanical issues once pack is installed
            - Thermal
            - Mechanical (pinching, squeezing, twisting, cutting)
            - Resonance

    Thermal
        - Inadequate thermal management
        - Lack of consideration of thermal runaway problem
        - Lack of thermal isolation between individual cells or sub-packs
        - Insufficient thermal sensing
        - Insufficient thermal overload coping mechanism 
          (both mechanical and electronic)
        - A design that might cause severe differential heating of cells deep
          inside the pack
        - Inadequate electronic and mechanical fail-over measures in the case
          of thermal runaway
        - Inadequate thermal runaway detection
        - Cell supplier issues (quality control, testing, etc.)
        - Thermal issues once installed
            - Problematic heat exchange
            - Heat input from adjoining assemblies
            - Accidental plugging of filters, inlets, etc.

    Final release
        - Electrical testing
        - Mechanical testing
        - Environmental testing
        - Random destructive testing at the cell level

    Operational
        - User fails to follow published procedures
        - Alarms are ignored
        - Mechanical issues are ignored
        - Inadequate maintenance
        - Handling issues

Anyhow, this is not, by a long-shot, an exhaustive list. There's a lot I am ignorant about and, of course, that stuff is missing. Feel free to expand.

The point is that a proper investigation would use --at a bare minimum-- something like this and, more than likely, a book full of specs and procedures before reaching a verifiable and reproducible conclusion.

Maybe it's just me. I don't know. I just don't like it when people do some of what is being done to Boeing. Particularly when they don't have access to the reams of FAA, Boeing and airline data that is required to really understand the problem.

EDIT: Formatting