Apple’s original patent application which was published touched on a battery pack that is capable of informing the user of whatever faults that is happening to the battery pack. The battery pack also comprises of a battery cell as well as enclosure material which keeps the battery cell in place. This fault might have somethin to do with a puncture in the enclosure material, a short circuit within the battery cell, and/or a change in pressure within the battery pack. In order to figure out the fault, this fault-monitoring mechanism might be able to make use of a conductive mechanism, a color-changing mechanism, and/or a sensor mechanism in the battery pack itself.
No idea on whether this patent will be applied eventually, if ever, but it is definitely worth taking a look at.