APU Shutdown...
The fact that the APU shutdown because a battery had failed seems odd. The APU is there to generate power and was running... Clearly the expectation of battery failure was low in the engineer's mind when the systems were designed. Later on, in testing and certification, the same assumptions must have been made, or this undesireable behaviour would have been caught before.
Now that we know that these batteries can and have failed... it might be an idea to think about and test, what other system are effected by the batteries going off-line...