Hi,
mm43
...regardless of switches/CBs in the Nav light circuits being isolated, the Nav lights were on and drawing current from the APU battery.
This seems to be a problem not "connected" to the MAIN battery issue.
I did think if the destruction of the MAIN battery ground wire could damage this circuitry or control. Fact (probable) is a high, very high current flowed from battery (MAIN) to ground. Thousand(s) Amps.
They are puzzled and even mentioned lightning strike possibility. And discarded.
A thick ground wire from a battery case doesn´t open without a reason.
