speicher lane

OP, By your diagram the switch is between the banks of batteries and not between the load you would be intending to eliminate power to. The only selection that your wiring diagram is controlling is if the batteries on bank #2 would be getting a charge from your charger when the switch was set to both... everything connected to the batteries is "live" constant power

Are you running 3 batteries or more as the drawing is showing 2 groups of 2 batteries and yet mentions 3 banks, so it may help to now exactly what you are trying to have for circuits banks ( starter/ stereo etc), # of batteries dedicated to each circuit and if batteries are left in series or if he wants each isolated.... it could be you need to add switches for full battery supply control and maintain charging from the alternator.

Ultraboy

The boat has four batteries total. The two batteries on the right hand side of the diagram are connected in parallel, and they power only two amplifiers—through a common positive lead, just like is shown in the diagram.

then the two batteries on the left-hand side of the diagram are not connected to each other besides when they connect to the switch. Hence I considered this three different “banks“, so sorry if that was confusing.

BillK

Ok, Just so you know it does not matter if you connect the two batteries together at the switch like on the left bank or at the batteries like on the right bank. Electrically it is exactly the same.

It is going to be very hard to diagnose this without being present. If I was there at the boat I would probably figure it out in about ten minutes as would anybody else with good electrical skills. Did you wire the switch or somebody else ? I would think the two wires from the charger should go to the two battery banks, one to post 1 and one to post 2 otherwise the only time the right hand bank will be charged is if the switch is in the "both" position. Like has been mentioned a few times, a picture of the back of the switch would be worth a thousand words.

Where are you located ? Maybe there is someone close that could look at it for you.

Ultraboy

I looked at the battery switch again tonight, but forgot to take a picture, my bad. The back of the battery switch has the wires connected just like in the original diagram, with the addition of one “mystery“ red wire connected to the #1 terminal. This mystery wire runs back to the transom of the boat, then dives behind the engine and is obscured from view by a bunch of other loomed wires around the engine and transom.

Also the switch obviously has a positive wire coming off the “common“ terminal, which I believe probably goes to the alternator??? Does that sound correct?

speicher lane

if the mystery wire is connected directly to the alternator, whoever wired the boat is possibly having the alternator charge the batteries through that wire to the battery switch connection vs. through the starter lead... best to find out where the wire run to...

What boat is it?

Ultraboy

Boat is a powerquest 280. It has a large aftermarket stereo that was installed by one of the previous owners, hence the additional batteries. I’m assuming this boat came with two batteries from the factory, and the other two were added after the fact.

Ultraboy

Is the alternator wire and the starter lead both supposed to connect to the “common“ terminal on the back of the battery switch?

speicher lane

In a factory engine harness, the "alternator wire" feeds power back to the batteries via the starter lead to the battery switch common completing the loop