How does the battery master switch activate the battery contactor in Cessna 152?
I think two of my premises were wrong.


The battery contactor is activated by connecting the left contact to ground (instead of 24 V as I thought). It works different from the starter contactor (which has a permanent ground connection).
The battery master switch toggles between ground [on] and floating [off] (instead of primary bus [on] and ground [off] as I thought).


Like this, the plane will be able to start!