On or way back it seemed as if the N:L battery controller is starting to beep and beep.
Usually this is due to a contact fault and by shifting the wire around a bit it stops (after some corrugated road driving this may have caused it).
But then it persisted. 1 km away from home, the car stalled. The National Lunar controller reads that the crank battery is flat, but not the auxiliary battery.
So by clicking on the 'override' button both batteries work in series and presto, the car starts again. So got home.

Now for the mystery: Now at home, when I plug my trickle-charger in, the crank battery actually reads that it is fairly well charged (this is reading it on the charger, but not on the NL controller system inside the car)?

In this state, when trying to start the car, it sounds too flat and cranks over slowly once or twice. But when switching to override, it starts fine again.
Can this be the alternator? I thought if the alternator is gone it would not drive, not even if set to 'override'?