HeadingSouth:
They tend to auto balance as long as charge/discharge currents are not too high. If for some reason a sudden current spike occurs,
Yes. But this brings up another failure mode/load incompatibility issue. Current balance and thermal runaway are more or less a steady state issue. They are a function of the thermal characteristics of the battery cells. I'm going to give Boeing and its subcontractors the benefit of the doubt and assume that many combinations of load or charge and ambient temperature were tested for the initial certification of this subsystem.
The 'current spike' brings up another issue which I have found to be often overlooked. That is: the electrical loads on these batteries are not steady state DC. Particularly in the case of the APU, the starter/generator is driven by a controller that draws very high levels of ripple currents from the source.
Batteries, in addition to being an electrochemical voltage source have an equivalent AC model that (overly simplified) can be represented as a series LCR circuit at higher frequencies (tens or hundreds of kHz). If one excites such a circuit near its resonance, it is possible to generate extremely high voltages across the various internal points of this equivalent circuit.
There is the possibility that the various combinations of load ripple and battery AC impedance was not properly characterized when the initial certification analysis was done*. I imagine that subsequent flight tests will be instrumented to capture exactly this kind of data.
*Back in my days at Boeing, I was involved with the 767 static inverter and its adaption to the 747-400. Initially, it had been certified to drive linear AC loads. This was because the smaller loads (typically driven by the standby AC bus) were exempt from limits on harmonic current draw. But, as it turns out, being exempt from a spec requirement doesn't meant that it shouldn't be considered. It turned out that an inverter rated at 1kVA was only capable of delivering about 400 VA to the connected loads before the voltage waveform became so flat-topped that it's output fell out of spec. This new 'all electric' airplane may turn out biting some old school engineers in the a
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