I've recorded voltage on both batteries for the last 2 days without anything connected (1 side of battery disconnected on each). I could post a table, but basically the inboard battery dropped from 12.65v to 12.54v and the outboard dropped from 12.65v to 12.40v. I wasn't convinced this proved anything conclusive, so I dug out my load tester. The inboard battery dropped to 11.5v under heavy load and recovered after removed. The outboard cell dropped to 3.0v under load and returned to 12.12v. I'd say there's an issue with the outboard battery!