I had an interesting battery failure in my Series III a few decades ago.
I had just left the Strezelecki Track aiming for the Birdsville Track when the battery suddenly refused to bring the starter motor to life even though the CB radio was obviously working.
The crank handle easily started the engine and once running, there was no evidence of a problem.
A week or so later I replaced the battery in Broken Hill and was told that the connection between a couple of cells had broken. Assuming that diagnosis was correct, I guess that means the tiny bit of current needed to operate the radio could flow through the electrolyte and that there was enough current flowing to provide spark.
It was a long time ago, so I have no idea what test was done on the battery to diagnose the problem.
It might have been a bit more inconvenient if it weren't for the crank handle on the Series III. :)

