Hi David
In the factory std set up battery switch needs to be turned on for charging ie turn on to 1 or 2 start engine then switch to 1+2 to charge both or 1 or 2 to charge them individually. To confirm charging use a meter directly on the batteries, whilst being charged they should be about 14.5V or even higher. When charged but rested they should be about 12.5V. There is a lot of literature on charging marine batteries, written by more experienced people than me , check on it more more precise figures. Note as a rule of thumb do not run the engine with the battery switch turned to 0 as in this case the alternator could be open circuit which would allow the voltage to rise sufficiently to blow up the bridge rectifier.
Jan
325/32 ex 275