Rodd -- You've given me a lot to think about, and a big homework assignment. I'll be back to the boat tomorrow and will keep you posted on what I find. I won't be able to do much with the alternator tests until Spring, because I'm prepping to haul out.

I like your thoughts about "dead lead", and might do something about that -- but that also involves bigger diameter cables (or perhaps adding a second set in parallel) to reduce voltage drop. I could leave that bow battery in place and just keep the switch for it in the "combined" position to effectively make it part of the house bank.

That leads to something which I need to think about further: The inaccuracy of the battery monitor's "percent charge" reading might be explained partially by the time when the batteries are in the combined state, under charge. But -- only the positive sides get combined -- the negative sides are still isolated at the point where the shunt is installed. Hmmm.