Chris:
The fact that the first cell-failure inevitably leads to the application of a higher charging voltage to the remaining 7 cells - unless the monitoring system detcts the problem - represents a potentially fail-hard situation.
During the charging cycle this may be well managed. But there are threats: The internal resistance of the failing cell during the charge (and discharge) may be situated in a range of values capable to generate high heat (Joule effect, like a resistor). And polarity reversal during the discharge is another threat.
Parallel charging eliminates the above problems.
Not something you want to be happening under the cabin floor...
We engineers MUST imagine everything POSSIBLE (not just probable) and provide means to allow the pilots "manage accordingly". Surprises with these dangerous cells nearby PCB´s inside EE bays nearby electronic modules are potentially threatning issues and
a surprise to me.
A battery like the one we are seeing seems an absurd. Boeing had luck.