4 battery 2 switch setup

[mmcons]

I just replaced all four batteries on my boat and one bank is not charging. The boat has two battery switches. It appears to have a seperate port and starboard setup. There is a small can with two battery cables in the center. It has two small curly wires also connected to it. I think it send alternator current to the opposite bank when needed. I have two batteries connected to each switch with the grounds going to their respective engine. The boat grounds seem to be on the starboard engine. I imagine both engines are at the same ground but am not sure. Can I ground both banks to the same engine. I have not traced where the charger is grounded. I'm thinking I am missing a ground. I do have 13.5 v at each charger lead. Anyone familar with this setup?

[CDK]

The marine battery switches I am familiar with have 4 positions: port, stbd, both and none. Two of these in one boat seems a bit too much, unless each engine can again choose between port-1 and port-2, both and none.
The small "can" may be a relay that overrides the switch settings when charging current is available. The only way to find out is follow all heavy battery wires and make a drawing.

Ground in a boat should be what the name implies. Port battery negative to the port engine, starboard battery to starboard engine block and a ground strap between the two blocks. All auxiliary negative wiring connected to the nearest engine block, so all negative wiring is at the same electrical potential.

The battery switches may develop an oxide skin when left in the same position for years. They must be used now and then or taken apart and cleaned. If the "can" really is a cylindrical relay it also may develop a contact problem.