Does the position of the battery selector switch (1, 2, both, off)
determine which battery is charged by the battery charger or are all batteries in the charging circuit all of the time?
Ron
Most battery chargers are wired directly to the batteries. In order to answer your question, you'll need to trace the wiring on your boat. I'd start with the wiring diagram that came in the boat manual.
Also in many cases the alternator had been wired into the 1-2-B switch, would determine where the alternator output would go (obviously different than the battery charger).
Newer technology, and the idea of reducing the length of electrical cables, has had many owners modify this by wiring their alternators to the house bank with either echo chargers or combiners for the starting battery.
Stu