Marrt,
That original post that you linked to made an impression on me at the time because I was having corrosion/battery/starter problems too w/my Cub, and I was determined to eliminate that problem on my "new" tractor...(whatever it was).
If the Optima is too big, then I'll have to defer to a different brand (but an AGM to be sure).
AGM's have different charging requirements (lower) than lead acid's but the spec eludes me for now...The Robbin engine has a std charging system of 15A(20A opt.) should be plenty for lights, bells and whistles that I want.
The 12.3vdc that you measured sounds a little low for charger o/p, but I don't know Kohlers spec. You ,definately, exercised good "technique" in isolating(from heat) a spill-proof battery in a very tough battery environment...My goals ,too.
Sounds like the "deep-cycle" battery you're using for "starting" will last a LONG time due to low impact starts and almost no "discharge" cycle.../w3tcompact/icons/smile.gif